Nitrogen is essential to nearly every aspect of plant growth, but it is one of the most difficult nutrients to manage. When plant available N exceeds crop demand, nitrate (NO₃) accumulates in soil, increasing the risk of N loss to the environment. Excessive levels of available N can also produce succulent plants that are more susceptible to environmental stress and pest pressure. When plant-available N is too low, crop health and productivity suffer. Understanding the forms of N in the soil and the factors that influence them will help improve management of this dynamic nutrient.

The Nitrogen Cycle

Practical knowledge of the N cycle is key to effective and efficient N management. The N cycle is extremely dynamic and its behavior in soil is complex. Nitrogen transformations and losses are affected by the form of N added, soil characteristics and conditions, and the vagaries of the weather. The rate and magnitude of N transformations and losses are difficult to accurately predict.

Figure 1. The Nitrogen Cycle

Nitrogen Inputs

As shown in Figure 1, there are two forms of the N used by plants: ammonium (NH₄) and nitrate (NO₃). In addition to commercial fertilizer sources, plant-available N in the soil may increase through mineralization (the microbial conversion of organic N to ammonium and then nitrate) of soil organic matter, manure and other organic residuals. See also Fertilizers and Soil Amendments. 

Nitrogen in soil organic matter: Organic matter contains the largest pool of soil N, usually comprising more than 90% of total soil N. The total amount of organic matter N in the plow layer of agricultural soils is impressively large. As a rule of thumb, you can assume that for each 1% of organic matter in the surface 6 inches of soil, there are 1000 lbs of N per acre. Thus, a soil with 3% organic matter contains about 3000 lbs of N per acre.

The amount of total organic matter N that becomes available to plants in any one year, is relatively small as a percentage of the total organic matter. For most soils, 2%-4% of the total organic matter N is mineralized, or converted to forms plants can use, annually. This is roughly equivalent to 20-40 lb of available N per acre for each 1% of organic matter in the surface 6 inches of soil. This mineralization is not constant throughout the growing season, however. A flush of available N is normally mineralized in late spring with lower rates of mineralization occurring during the growing season. Moisture conditions will greatly influence mineralization, with the highest rates when the soil is well-aerated and near water holding capacity. Small flushes of N will be released when dry soils are re-wetted during the season. The rate of mineralization is dependent on the activity of microbes, especially bacteria. Such activity is favored by warm, well-aerated soils with adequate, but not excessive, moisture and a pH above 6.0. These conditions are also favorable for the growth of most vegetables. Excessively wet conditions can greatly limit bacterial activity, and therefore N mineralization. For these reasons, the N contribution of mineralizing soil organic matter is frequently estimated at a conservative rate of 10 lb of available N per acre for each 1% of organic matter in the top 6 inches of soil.

Nitrogen found in manures and other waste products is discussed extensively in Fertilizers and Soil Amendments.

Previous crops: Many vegetables leave little residue in the field and thus they provide little N benefit to subsequent crops. However, previous forage or cover crops can supply large amounts of N to succeeding crops. Legumes, such as alfalfa and red clover, can furnish 100 lb or more of N to crops that follow (Table 1). Other legumes, mixed grass-legume stands and grass sods supply less N to succeeding crops. Keep in mind that most of the N is in the leaves, not the roots. If a legume hay crop is harvested, most of the N is removed from the field along with the hay.

Table 1: Nitrogen Credits from Previous Crops

Nitrogen Losses

Nitrogen losses occur in several ways. The loss of available soil N not only costs growers money, it has the potential to negatively impact both air and water quality. Understanding the cause of N losses can help growers make management decisions to improve N use efficiency and minimize negative environmental impact. 

Volatilization Losses: These losses occur mainly from surface applied manures and urea. The losses can be substantial; more than 30% of the N in topdressed urea can be volatilized if there is no rain or incorporation within two or three days of application. Losses are greatest on warm, breezy days. Volatilization losses tend to be greater from sandy soils and when pH values are above 7.0. Delaying the incorporation of manures after they are spread also leads to volatilization losses of N. Under the right conditions more than 50% of the ammonium N may be volatilized within the first 48 hours following surface application of manure without incorporation. 

Not only does volatilization reduce the fertilizer value of manure and urea, it can degrade air and water quality. Ammonia in the atmosphere can form particulates that contribute to smog. Ammonia emissions can also contribute to eutrophication of surface waters via atmospheric deposition.

Leaching Losses: The nitrate form of N is especially mobile in soil and is prone to leaching losses. Leaching losses are greatest on permeable, well-drained to excessively-drained soils underlain by sands or sands and gravel when water percolates through the soil. Percolation rates are generally highest when the soil surface is not frozen and evapotranspiration rates are low. Thus, October, November, early December, late March and April are times percolation rates are highest and leaching potential is greatest. This is why nitrate remaining in the soil after the harvest of annual crops such as corn in September is particularly susceptible to leaching. The use of fall cover crops can take up this residual N and prevent it from leaching. The N will then be released for crop use after the cover crop is plowed down in the spring. Of course, leaching can occur any time there is sufficient rainfall or irrigation to saturate the soil. This is why it is important to attempt to match fertilizer N application rates with crop N needs.  

Nitrate leaching accounts for the vast majority of N losses from cropland. Nitrate leaching can have a direct impact on water quality. When nitrate leaching contaminates groundwater serving drinking water supplies, human health can be impacted. The greatest concern is for infants; high levels of nitrate can be toxic to newborns, causing anoxia, also known as “blue-baby” syndrome. High nitrate levels in drinking water are also harmful to young or pregnant livestock. Depending on regional hydrology, leaching losses of nitrate can also contaminate surface waters.  

Denitrification Losses: These losses occur when nitrate is converted to gases such as nitrous oxide (N₂O) and nitrogen gas (N₂). The conversions occur when the soil becomes saturated with water. Poorly drained soils are particularly susceptible to such losses. In especially wet years on some soils, more than half the fertilizer N applied can be lost through denitrification. The most favorable conditions for denitrification occur in early spring and late fall. Minimizing the concentration of nitrate in the soil during these periods by delaying N application in the spring and planting cover crops in the fall will help reduce denitrification losses.

Most of the N lost during denitrification is in the form of inert nitrogen gas (N₂) which has no negative impact on the environment (our atmosphere is approximately 78% N₂). Only a small percentage of denitrified N is lost as nitrous oxide (N₂O); however, this is a powerful greenhouse gas. The impact of 1 lb of nitrous oxide on atmospheric warming is over 300 times greater than 1 lb of carbon dioxide. Agricultural activities account for over 70% of nitrous oxide emissions in the US. 

Immobilization: Immobilization occurs when soil microorganisms absorb plant-available forms of N. The N is not really lost from the soil because it is held in the bodies of the microorganisms. Eventually, this N will be converted back to plant-available forms. In the meantime, however, plants are deprived of this N, and N shortages in the plants may develop. Immobilization takes place when highly carbonaceous materials such as straw, sawdust or woodchips are incorporated into the soil. Manure with large amounts of bedding and compost with C:N ratios greater than 30:1 may cause some immobilization.

Crop Removal of Nitrogen: A significant quantity of N is removed from soil via crop harvest. For example, good sweet corn crops may remove over 150 lb N per acre annually. Anticipated crop removal of N is one of the factors used in making N fertilizer recommendations. Depending on the crop, variable amounts of the nitrogen taken up by the crop are returned to the soil after harvest in nonharvested plant parts. With sweet corn this can be as much as 100 lb N per acre. As these leaves and stalks decompose, the N is released into the soil for use by a subsequent crop. Cover crops can take up much of this N to prevent losses by leaching or denitrification.

Topdressing and Sidedressing Nitrogen

Topdressing is defined as a fertilizer application to a crop any time after planting. In popular usage, topdressing sometimes refers to a broadcast application of fertilizer made after planting. Alternatively, fertilizer can be sidedressed as a band along the side of the row of a growing crop. Sidedressing is commonly done immediately before or during cultivation. When urea-containing fertilizers are used, cultivation helps reduce volatilization losses.

Sidedressing of relatively soluble N fertilizer is an important component of efficient nitrogen management. The N accumulation pattern for annual crops is very similar to biomass accumulation (Figure 2). Early in the season, when crop growth is slow, crop N needs are very small. A starter fertilizer is generally sufficient to satisfy those needs. Any soil nitrate in excess of crop N needs during this period is prone to leaching and/or denitrification losses. The next phase of crop development is characterized by rapid vegetative growth. The N demand during this phase is the highest of the growing season. As much as 85% of the total N uptake occurs during this period. Efficient recovery of the fertilizer portion of N can be achieved by sidedressing fertilizer N immediately before this phase. Delaying application of a large portion of N fertilizer until sidedress also allows growers to use the Pre-sidedress Soil Nitrate Test (PSNT) to help determine N needs.

Figure 2. Generalized nitrogen accumulation curve for annual crops

Pre-sidedress Soil Nitrate Test (PSNT)

The dynamic nature of the N cycle and its sensitivity to weather limits the value of routine, pre-season soil testing for predicting N availability during the season in our humid environment. However, under certain circumstances, in-season soil testing has proven useful. The PSNT, developed by Dr. Fred Magdoff at the University of Vermont in the early 1980’s, was originally intended to help estimate the amount of available N for field corn in fields where manure had been applied and/or forage legumes were grown in rotation. Over the last thirty years, research conducted in the Northeast has found the PSNT useful for improving N management of several vegetable crops including sweet corn, peppers, pumpkin, winter squash, and cabbage. The PSNT is most suitable for use with annual crops, which accumulate N rapidly within a single growing season.

The PSNT is especially useful where large amounts of N from mineralization are expected, and the test works best when pre-plant and starter fertilizer N rates are less than about 50 lbs N per acre. PSNT samples are collected about a week before the rapid growth phase (see Figure 2), to provide an indication of how much N has been made available from mineralization. During wet springs with heavy leaching rains, or in sandy soils with rapid losses, the PSNT will also provide some indication of how much N remains in the root zone.

As with all soil testing, information from a PSNT should be used along with the grower’s experience and knowledge of the field. Interpretation of the PSNT is also crop-specific. Research in the Northeast has shown that when the soil nitrate N level is above 20-25 ppm there is rarely an economic response to the application of sidedress fertilizer N for sweet corn. Based on research and experience in New England, New Jersey, and New York, a threshold of 25-30 ppm seems appropriate for peppers, tomatoes, butternut squash, cabbage, pumpkin, and probably other long-season vegetable crops. When PSNT values are below threshold levels, the appropriate rate of sidedress N should be determined based on the level of nitrate N reported, previous N application, realistic yield expectation, the field’s management history, and growing season conditions. See Table 2 for recommendations on timing of sampling and making sidedressing applications of N based on PSNT for many vegetable crops. 

Table 2. Timing of PSNT and sidedress nitrogen needs of crops¹

¹ Adapted from: Rutgers Cooperative Extension Bulletin by J. Heckman, “Soil Nitrate Testing as a Guide to Nitrogen Management for Vegetable Crops”

² If soils have 0-30 ppm nitrate, apply the full sidedress amount recommended. For sweet corn, the threshold is 25 ppm nitrate. Above 30 ppm no additional N is needed and could hurt yields.

Samples for the PSNT should consist of a well-mixed composite of 10-20 cores or slices of soil to a depth of 12 inches. This is a deeper sample than what is recommended for routine soil sampling. A deeper sample is required for nitrate testing to accurately reflect the concentration in the effective root zone due to its mobility in soil. Avoid sampling fertilizer bands or areas that may have received extra N. About one cup of the composite should be dried to stabilize the nitrate. A good method is to spread a thin layer of the soil on a cookie sheet or aluminum foil to air dry. Use a fan to reduce drying time. Do not place damp samples on absorbent material because it can absorb some of the nitrate. You can skip the drying step if you can deliver the samples to the soil testing lab in less than 24 hours; however, samples should be kept cool. Fields should be sampled for the PSNT about a week before the time when sidedressing is normally done. This should allow adequate time for drying, shipping, and testing (lab results are generally available within 24 to 48 hours) and for you to plan your fertilizer program.

For the purposes of fertilizer grades and recommendations, phosphorus (P) is measured as phosphate, or P₂O₅. 

The amount of extractable P in a soil should not exceed the optimum soil test range to obtain the most economic return from P applications and to avoid negatively impacting water quality.  When extractable P exceeds the environmental critical concentration, which is much higher than the soil test optimum range, the risk of dissolved P loss in subsurface water flow or runoff is significantly increased. This P pollution can stimulate excessive growth of algae in lakes and ponds. When the algae die and their biomass is rapidly decomposed by microorganisms, oxygen levels are reduced below the level needed by fish and shellfish, resulting in large die-offs of aquatic life.

Excessive P amounts in soils are difficult to reduce because vegetable crops remove little P from the soil compared to N or K. For example, Table 4 shows that sweet corn takes up about 155 lb N per acre and about 105 lb K per acre, but only about 20 lb P per acre. However, many growers apply about 100 lb P per acre annually. This is justified only if soil test P levels are below optimum. If the soil test level for P is above optimum, there is little if any crop response to additional P applications.

Plant uptake of P is extremely slow in cold soils. For this reason, when planting early into soils testing Optimum or lower, it is often advisable to apply up to 30 pounds of P₂O₅ as starter fertilizer in a band about 2" below and 2" to the side of the seed when planting, or as a liquid around transplants. Keep in mind that P availability is reduced in alkaline soils (pH >7.3) as it will bind with Ca, and in acidic soils (pH <5.5) when it binds with Al, in both cases becoming unavailable to plants.  Therefore, it is important to first balance the soil’s pH with lime applications into the range of pH 6-7 before making P₂O₅ applications.

Potassium (K) is measured as potash (K₂O), similar to the way P is measured as P₂O₅. Crop need for K varies considerably as can be seen in Table 4. It is important that the soil K plus the applied K is enough to meet crop needs. However, excessive levels should be avoided because K can interfere with the uptake of Ca and Mg (see Cation Exchange Capacity and Base Saturation in the next section). K is subject to leaching on sandy soils low in organic matter, so if high amounts are needed, split applications should be used.  Very high application rates of K are also known to suppress Mg uptake, and when soil test Mg levels are low, may cause Mg deficiency.

Calcium (Ca) is usually supplied in sufficient quantities by liming if appropriate liming materials are chosen (see Soil Acidity, pH, and Liming in the next section). If soil pH is high and Ca is needed, it can be supplied without affecting pH by applying calcium sulfate (gypsum) which contains 19%-23% Ca. Small amounts can also be applied as calcium nitrate fertilizer (19% Ca) or superphosphate (18%-21% Ca). See Table 5.

Magnesium (Mg) is most economically applied as dolomitic or high-mag limestone (see Soil Acidity, pH, and Liming). If liming is not needed, Sul-Po-Mag (11% Mg, 22% K) can be used. Blended fertilizer containing Mg can also be ordered.

Micronutrient deficiencies are rarely observed in New England soils, especially on soils that have a history of compost or manure applications. For this reason little research has been done to calibrate soil tests for micronutrients, and there are no reliable soil test calibration data to interpret soil micronutrient levels and make recommendations for amounts of micronutrients to apply. Most New England labs offer micronutrient soil testing and present results relative to values typically observed in soils analyzed in their laboratory for comparison.

Calibration data for plant tissue analysis are also limited. When a grower suspects a micronutrient deficiency, the recommended procedure is to collect soil and plant tissue samples from areas in the field with good and poor plant growth, and have the samples analyzed using standard methods. A relative comparison of the concentrations of micronutrient in the soil and plant tissue samples will usually allow a diagnosis to be made. Foliar applications of micronutrients may alleviate nutrient deficiencies observed during the season. However, applications of micronutrients to the soil are the better long-term approach to address the rare occurrence of micronutrient deficiencies. 

Micronutrient deficiencies are most likely to occur in sandy soils with low organic matter. High soil pH may also bring about micronutrient deficiencies, especially in sandy soils. It may be necessary to maintain pH between 6 and 6.5 to avoid problems in coarse-textured soils with low organic matter. Of all the micronutrients, boron (B) is most likely to be needed in New England to supplement soil levels for vegetable production. Cauliflower, broccoli, cabbage and beets are most susceptible to “hollow heart” caused by boron deficiency. Boron, copper, zinc and molybdenum have small ranges for optimum soil test values, which means the difference between deficient levels and toxic levels in the soil are small. Be careful with applications of these nutrients to avoid toxic quantities. Some vegetable crops are particularly sensitive to high levels of boron. Sensitive crops should not be planted on fields following crops that have received boron application. Table 3 lists crops according to their sensitivity to boron.

Table 3: Relative Tolerance of Vegetables to Boron

Adapted from L.V. Wilcox, Determining the quality of irrigation water, USDA Agricultural Information Bulletin 197 (1958) and information from Robert Becker, Cornell University.

A number of individual soil tests may be used to assess aspects of soil health, including those obtained with routine soil analysis. However, a comprehensive evaluation includes a suite of complementary tests to measure chemical, physical, and biological soil properties. Researchers at Cornell University evaluated a large number of these measurements for use in Northeastern cropping systems and now offer the most meaningful measurements as part of the Cornell University Soil Health Test. This suite of soil health indicators is designed to provide growers with helpful information about problems that may limit crop productivity and/or soil performance. Recommendations are provided with the results to help growers address problems that may be identified. More information about the Cornell University Soil Health Test (including sampling protocol, current prices, and submission forms) can be found at http://soilhealth.cals.cornell.edu/. Other soil testing laboratories in New England now offer soil health tests as well; check with your local lab.

Soil organic matter (SOM) is critical for soil health because of the beneficial chemical, physical and biological properties it imparts. SOM increases soil nutrient availability, being a source of nutrients itself as well as by increasing cation exchange capacity. SOM also benefits the physical properties of soil by contributing to soil structure and increasing water holding capacity. Finally, SOM supports soil biology as a substrate for the growth of soil microbial communities. In turn, these soil microbes mineralize SOM into plant-available nutrients, develop symbiotic mycorrhizal relationships between soil fungi and plant roots, and produce sticky substances responsible for binding soil aggregates.

SOM is composed of materials containing carbon that came from living organisms, including plant and animal residues, organic-based amendments, and soil bacteria and fungi, all in various stages of decomposition. SOM is often divided into two categories. The more stable component is humus, which is primarily comprised of long-dead material that is highly decomposed and beneficial for soil structure and carbon storage. Active SOM, on the other hand, is relatively undecomposed and accessible to soil microbes for mineralization. It is important to maintain both types of SOM (see Building Soil Organic Matter). In any case, it is important to maintain SOM because it is typically the most important component of soil for nutrient supply, water holding capacity, cation exchange capacity, and soil structure. 

Native total SOM content in soils used for vegetable production in New England is almost always less than 10% and typically in the 2-6% range. SOM accumulation is generally limited by several abiotic factors, including temperature, moisture, and soil texture. Well-drained, coarsely textured soils tend to have lower levels of SOM, due in part to the rapid microbial decomposition rates favored by these soil conditions. In contrast, loamy soils often have 3-6% SOM. 

SOM supplies nutrients through the process of mineralization, which is the microbial decomposition of organic compounds into carbon dioxide and their mineral constituents. Soil microbes are most active in warm soils (over 70°F) that are moist, well-aerated, and have a pH between 6 and 7 (also ideal conditions for most vegetable crops). Mineralization of nutrients will proceed rapidly under these conditions, provided there is an adequate supply of SOM and an abundant soil microbial community.

SOM directly influences water holding capacity through its capacity to absorb large amounts of water. It also indirectly boosts water holding capacity by improving soil structure, creating more pore space for water storage and larger pores for air. Soil structure is enhanced by SOM because, as it decomposes, sticky compounds like gums, carbohydrates, and resins are produced by microorganisms. These gums bind soil particles together into secondary aggregates. This in turn bolsters cation exchange capacity, which is influenced by both clay and SOM content (both supply negatively charged sites that hold cations). In most New England soils, the stable humus portion of SOM accounts for the vast majority of the cation exchange capacity, as these soils are typically low in clay content. See also Cation Exchange Capacity and Base Saturation. 

Soil organic matter (SOM) is in a constant state of flux, with additions and losses simultaneously occurring. To maintain SOM levels, one must ensure that losses do not exceed additions. 

SOM can be lost from soil through both wind and water erosion. However, the primary mechanism for SOM loss is microbial decomposition. Soil microbes use SOM as a source of energy and nutrition, converting SOM into carbon dioxide and its constituent mineral elements. The rate of SOM decomposition is controlled by a number of factors including soil temperature, moisture, aeration (oxygen), and the quality or characteristics of the SOM. In agricultural soils, these factors are all greatly influenced by soil management. For example, aggressive tillage and cultivation increases aeration and breaks apart soil aggregates that protect SOM from microbial decomposition. Reducing tillage and cultivation is an effective management strategy to maintain, or even increase, SOM content  (see Reduced Tillage).

There are a number of ways to increase or maintain SOM. Increasing the quantity of plant residues returned to the soil is one of the most sustainable strategies for maintaining SOM. Most vegetables leave little residue in the field, and SOM will usually decrease if these are the only residues provided to the system.  Although cover crops can add enough biomass to maintain SOM, it can be difficult to increase SOM with some cover crop species or mixes. Including sod-forming crops in the rotation, however, can increase SOM. A more rapid and direct method of increasing SOM is by adding organic amendments, such as organic mulches and compost. While the application of organic amendments can rapidly increase SOM, extractable soil phosphorous (P) concentrations must be monitored to avoid excessive applications (see Fertilizers and Soil Amendments).

Cation exchange capacity (CEC) is a measure of the soil’s ability to retain and supply nutrients, specifically the positively charged nutrients called cations. These include calcium (Ca⁺⁺), magnesium (Mg⁺⁺), potassium (K⁺), ammonium (NH₄⁺), and many of the micronutrients. Cations are attracted to negatively charged surfaces of clay and organic particles called colloids. CEC is reported as milli-equivalents per 100 grams of soil (meq/100g) or as centimoles of charge per kilogram (cmole⁺/kg). CEC can range from below 5 meq/100g in sandy soils low in organic matter to over 20 meq/100g in finer textured soils and those high in organic matter. Low CEC soils are more susceptible to cation nutrient loss through leaching, and may not be able to hold enough nutrient cations for a whole season of crop production.

The cations Ca⁺⁺, Mg⁺⁺, K⁺, hydrogen (H⁺) and aluminum (Al⁺⁺⁺) account for the vast majority of cations adsorbed on the soil colloids in New England soils. It is important to note that H⁺ and Al⁺⁺⁺ are not plant nutrients. Both H⁺ and Al⁺⁺⁺ are considered acidic cations because they tend to lower soil pH while Ca⁺⁺, Mg⁺⁺, and K⁺ are considered basic cations and have little to no influence on soil pH. If all the cations are basic and none are acidic, there would be a 100% base saturation and the soil pH would be close to 7 or neutral. In acid soils there are acidic cations adsorbed on the soil colloids (called exchangeable acidity) and the percent base saturation is less than 100. A soil with a pH between 6.5 and 6.8 will typically have a base saturation of 80%-90%.

One of the most important aspects of nutrient management is maintaining proper soil pH, a measure of soil acidity. A pH of 7.0 is neutral, less than 7.0 is acidic, and greater than 7.0 is alkaline. Most New England soils are naturally acidic and need to be limed periodically to keep the pH in the range of 6.5 to 6.8 for most vegetable crops. Scab-susceptible potato varieties are an exception, but some lime may still be needed to maintain the recommended pH of 5.0-5.2. When the soil is acidic, the plant availability of nitrogen (N), phosphorus (P), and potassium (K) is reduced and there are usually low amounts of calcium (Ca) and magnesium (Mg) in the soil. In contrast, most micronutrients are more soluble and are therefore more available to plants. Under very acidic conditions aluminum (Al), iron (Fe), and manganese (Mn) may be so soluble they can reach toxic levels. Soil acidity also influences soil microbes, which decompose organic matter and recycle crop nutrients. For example, when soil pH is low (below 6.0), bacterial activity is reduced and fungal activity increases. Acidic soil conditions also reduce the effectiveness of some pesticides. These conditions also limit the ability of cover crops like legumes to fix nitrogen. 

The most effective way to manage soil acidity is to apply agricultural limestone. The quantity of lime required is determined by the target pH (based on crops to be grown) and the soil's buffering capacity, measured in a soil test. Buffering capacity refers to the soil’s tendency to resist change in pH. Soil pH is only a measure of active acidity, the concentration of hydrogen ions (H+) in soil solution. When lime is added to a soil, active acidity is neutralized by chemical reactions that remove hydrogen ions from the soil solution. However, there are also acidic cations (H+ and Al+) adsorbed on soil colloids (see Cation Exchange Capacity and Base Saturation, previous section) that can be released into the soil solution to replace those neutralized by the lime. This is called reserve acidity. Clays and soils high in organic matter have the potential for large amounts of reserve acidity. These soils are said to be well-buffered. To effectively raise the soil pH, both active and reserve acidity must be neutralized. Soil test labs determine buffering capacity and lime requirement by measuring or estimating the reserve acidity. 

The neutralizing power of lime is determined by its calcium carbonate equivalence. Suppliers can tell you the calcium carbonate equivalence of the lime you are purchasing. Recommendations are based on an assumed calcium carbonate equivalence of 100. If your lime is lower than 100, you will need to apply more than the recommended amount, and if it is higher, you will need less. To determine the amount of lime to apply, divide the recommended amount by the percent calcium carbonate equivalence of your lime and multiply by 100. Wood ash is another amendment that may be used to manage soil acidity. The calcium carbonate equivalence of wood ash is typically between 30-50%, but it can vary widely. If purchasing wood ash from a supplier, they will provide a recent analysis. Otherwise, the wood ash should be submitted to a lab offering lime analysis to determine the calcium carbonate equivalence.

The speed with which lime reacts in the soil is dependent on particle size and distribution in the soil. To determine fineness, lime particles are passed through sieves of various mesh sizes. A U.S. Standard 10-mesh sieve has 100 openings per square inch while a 100-mesh sieve has 10,000 openings per square inch. Lime particles that pass through a 100-mesh sieve are very fine and will dissolve and react rapidly (within a few weeks). Coarser material in the 20-30-mesh range will react over a longer period, one to two years or more. Agricultural ground limestone contains both coarse and fine particles. About half of a typical ground limestone consists of particles fine enough to react within a few months, but to be certain you should obtain a physical analysis from your supplier. Super fine or pulverized lime is sometimes used for a “quick fix” because all of the particles are fine enough to react rapidly.

Lime will react most rapidly if it is thoroughly incorporated to achieve intimate contact with soil particles. This is best accomplished when lime is applied to a fairly dry soil and disked in (preferably twice). When spread on a damp soil, lime tends to cake up and doesn’t mix well. A moldboard plow has little mixing action, therefore, disking is preferred. If growing vegetables in no-till fields, it is common to apply needed lime and work it in before limiting tillage in the field, then to apply a smaller amount of lime more frequently to the surface to maintain adequate pH. Lime moves slowly through the soil profile without incorportation. If lime is applied and not incorporated the material likely won’t be spread throughout the entire rootzone, reducing the neutralizing ability.  

Besides neutralizing acidity and raising soil pH, lime is also an important source of Ca and Mg for crop nutrition. It is important to select liming materials based on Ca and Mg soil content with the aim of achieving sufficient levels of each for crop nutrition. If the Mg level is low, a dolomitic lime (high magnesium lime) should be used; if Ca is below optimum, a calcitic (low-Mg lime) should be used.
 

Normally it is not necessary to supply all the nutrient needs of a crop from fertilizer alone, because the soil already contains some quantity of nutrient elements available for crop growth. It is necessary to test the soil to determine its nutrient status, because only then can you determine the additional amounts of appropriate fertilizer materials to apply. Frequently, growers apply more fertilizer than the crop can use and the excess, especially nitrogen, is leached from the root zone into groundwater. 

The Plant Nutrient Recommendations tables in each crop section can be used to determine nutrient needs based on soil test results. Read the section on Soil Testing to better understand nutrient recommendations.

In general, the goal should be to maintain nutrient elements within the optimum range as reported on the soil test. When nutrient levels are within this range, the needs of most crops will be met. If levels are below optimum (low or medium), most crops would benefit by adding the appropriate nutrient(s) to increase levels to optimum. However, if levels are at above optimum (very high) levels, there will be no additional benefit and excess levels may reduce crop yield or quality and may cause environmental harm. This happens in fields where soil testing was not used to monitor fertility levels or when nutrients are applied even when soil levels are sufficient. When a nutrient is above optimum levels it should not be included in any amendments until the excess is taken up by crops. In this case, it is wise to temporarily stop applying compost or manures until nutrient levels are in the desired range because the addition of these amendments can add high levels of nutrients, especially phosphorus. This is a practical way to manage nutrient levels if small to moderate amounts of mixed crops are to be grown.

Table 4 lists amounts of certain nutrient elements that are removed by vegetable crops. It includes both the amounts removed by harvest and those that remain in crop residue and are returned to the soil. These figures for crop removal should be considered approximate. They can be used as a basis for adjusting your own application rates up or down on a trial basis. Keep in mind that nutrient removal varies with factors such as soil moisture, temperature, and pH. Plants can absorb large amounts of a nutrient if it is in abundance; but this may not increase yield. Excess levels of some nutrients can reduce the yield and/or quality of some crops.

Nutrients that are relatively immobile in the soil, such as phosphorus, calcium, magnesium and the micronutrients, are not all extracted by plants because the roots do not come in contact with all of the nutrients. This is especially true of certain vegetables that have small or sparse root systems. Most of the phosphorous applied to the soil becomes fixed in a form unavailable to plants. Thus, it is necessary to provide some excess amounts of nutrients when soil test levels are below the optimum range, and this is why rates sufficient to replace the amount removed by crops are sometimes recommended for soils that test in the optimum range.

Table 4: Approximate Nutrient Removal by Selected Vegetable Crops. 

¹ These are assumed yields. Actual yields may vary depending on weather and cultural practices. Adjust nutrient removal rates accordingly. To convert to volume or count yield units see Table 15: Approximate Yields.

Several states in New England have nutrient management regulations that impact vegetable production practices. The EPA began regulating municipal waste water treatment facilities to manage water pollution, and more recently, regulations have gone in place to regulate non-point source pollution such as that coming from fertilizer or manure applications on agricultural fields. Below is a list of states with regulations and recommendations on how to comply.

Maine: Regulations for crop farms require the development of a nutrient management plan (NMP) if a farm is importing more than 100 tons of manure or regulated residuals annually. The nutrient management plan must address storage and utilization of manure and off-farm nutrients on land to which the regulated residuals or manure are applied. The NMP must include or provide for minimum distances between manure storage, stacking and spreading areas and property lines and surface water based on site-specific factors determined to be effective for controlling runoff and for preventing contamination of surface water. The NMP must include soil test reports for each field where manure or other crop nutrients will be applied, and soils must be tested for each field at least every 5 years. For each field, the NMP must show the calculation of nutrients required to grow the specific crop. The producer may write their own plan, but it must be approved by a Maine certified nutrient management planner licensed through the Nutrient Management Office. To find a licensed nutrient management planner, contact:

Mark F. Hedrich, Nutrient Management Program Manager. Maine Department of Agriculture, Conservation and Forestry, Division of Animal & Plant Health, 28 State House Station, Augusta, Maine 04333; 207-287-7608; mark.hedrich@maine.gov

New Hampshire: Best Management Practices (BMPs) for Agriculture are prepared by the Agricultural Best Management Practices Task Force and the USDA Natural Resources Conservation Service (NRCS) in Durham, NH and provide a guide for growers for handling manure, agricultural compost and chemical fertilizer. There is not a law that explicitly requires growers to follow them; however, it is in the producers’ best interest to follow these BMPs as they provide protection from nuisance allegations. State law under RSA 431:35 requires the New Hampshire Department of Agriculture, Markets & Food to respond to complaints involving the mismanagement of manure, agricultural compost and chemical fertilizer. Copies of the BMP manual are available to producers as a tool, but the NH Department of Agriculture, Markets & Food does not have the authority to enforce the implementation. A copy of the manual can be found here: http://agriculture.nh.gov/divisions/regulatory-services/nutrient-management.htm

Vermont: All farms must comply with the Required Agricultural Practices (RAPs) effective since July, 2017. The RAPs are practices and management strategies by which all types of farms must be managed to reduce the impact of agricultural activities on water quality. These standards are intended to improve the quality of Vermont’s waters by reducing and eliminating cropland erosion, sediment losses, and nutrient losses through improved farm management techniques, technical and compliance assistance, and where appropriate, enforcement. The RAPs establish nutrient, manure, and waste storage standards, make recommendations for soil health and establish requirements for vegetated buffer zones and livestock exclusion from surface water. The RAPs also establish standards for nutrient management planning and soil conservation. Full text of the RAPs regulations may be found on the Vermont Agency of Agriculture, Food and Markets website here: https://agriculture.vermont.gov/rap. Questions regarding these regulations should be directed to the Vermont Agency of Agriculture, Food and Markets, Water Quality Division, AGR.WaterQuality@Vermont.gov, (802) 828-2431. A factsheet specific to the RAP applicability to vegetable producers can be found here: https://agriculture.vermont.gov/sites/agriculture/files/documents/VeggieFactsheet.pdf

Massachusetts: In 2012, the Massachusetts Legislature passed Chapter 262, An Act Relative to the Regulation of Plant Nutrients. The text of the enabling legislation can be found at: https://malegislature.gov/Laws/SessionLaws/Acts/2012/Chapter262. The Act requires the Department of Agricultural Resources (MDAR) to promulgate state-wide regulations to ensure that plant nutrients are applied in an effective manner to provide sufficient nutrients for plant growth while minimizing the impacts of the nutrients on water resources in order to protect human health and the environment. The Act also requires that these regulations are consistent with UMass Extension’s educational and outreach materials relative to nutrient management and fertilizer, which may be found here: https://ag.umass.edu/resources/agriculture-resources/umass-extension-nutrient-management. In response to the Act, MDAR developed regulations entitled “330 CMR 31.00: Plant Nutrient Application Requirements for Agricultural Land and Land Not Used for Agricultural Purposes”, found here: https://www.mass.gov/service-details/plant-nutrient-management. A detailed factsheet specific to nutrient regulations on agricultural land can found here https://www.mass.gov/doc/plant-nutrient-regulations-fact-sheet-for-agricultural-land/download. The regulation gives MDAR state-wide authority to regulate and enforce the registration and application of plant nutrients including, but not limited to, fertilizer, manure and micronutrients.  The Cape Cod Commission, Martha’s Vineyard Commission, and Nantucket Commission have the option to adopt their own ordinances in regard to nutrients and fertilizers, but they cannot be less restrictive than the state regulation.

Connecticut:  Currently there are no nutrient management regulations affecting vegetable farmers in CT. There is a Phosphorous Law (CT PA-1255) but it primarily affects wastewater treatment facilities and homeowners who may only apply P if a soil test shows a need and not during winter months.

Rhode Island: No regulations of nutrient applications are in place in RI, however, on a case-by-case basis (and rarely at that) regulation may occur as part of a response to an environmental violation, for example, a consent agreement resulting from water quality impairment.

Soil test methods used for vegetable production are designed to provide a measurement of soil pH and nutrient availability. Soil testing labs use different methods, and it is best to select a lab using analytical methods appropriate for New England soils that provide soil test interpretations based on field correlation and calibration under local conditions. A list of New England soil test laboratories is provided at the end of this section. In New England, routine soil analysis typically includes a measure of extractable phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S), plus several of the micronutrients (e.g., boron, manganese, zinc, copper, and iron). In addition to routine soil analysis, other tests and analytical services are available including: soil organic matter, soluble salts (conductivity), Pre-sidedress Soil Nitrate Test (PSNT), plant tissue analysis, manure and compost analysis, and irrigation water testing.

Several labs have recently adopted Soil Quality, or Soil Health test packages. These packages test the biological activity in soils (active carbon, soil respiration, extractable protein), and physical characteristics (soil texture, aggregate stability, compaction, available water capacity), in addition to the traditional soil chemistry results (soil pH, organic matter, P, K, micronutrients). If using these tests, keep in mind that soils vary considerably from site to site. These results should be used as a baseline against themselves year over year, as opposed to comparing your results any listed “average”, or neighboring farms. These test results can be used as an indicator to evaluate the long-term effects of various soil health practices that you may be implementing. 

The appropriate soil test methods for a given region are selected based on soil characteristics and climate. Different analytical procedures can result in vastly different results. This is especially true of the different extraction solutions used to estimate a soil's nutrient supply. The two extraction solutions used in New England are the weak acid modified Morgan extract and the strong acid Mehlich 3 extract. These are both universal extraction procedures, meaning they are used to determine all major nutrients and many of the micronutrients simultaneously. Most New England Land Grant University labs (ME, VT, MA, and CT) use the weak acid modified Morgan extract. University of New Hampshire uses the strong acid Mehlich 3 extraction solution. A saturated media test using water as an extractant is used to measure nutrient availability in greenhouse potting media and in some cases, high tunnel soils. See Soil Testing Labs in New England for contact information.

The quantity of extractable nutrients may be reported in different units. Some labs report the concentration of nutrients in ppm (parts per million) which is equivalent to mg/kg (weight basis) or mg/dm³ (volume basis). Other labs report nutrient values in lb/acre (weight per area basis). Nutrient concentration in ppm can be converted to lb/acre by multiplying by 2. This is based on the assumption that there are approximately 2,000,000 lb of soil in the surface 6 inches of one acre. Differences in reporting units are important to be aware of, but they are of little consequence to our interpretation of soil test results. Soil test results only provide an index of nutrient supply that must be interpreted based on correlation and calibration data developed by nutrient response research in local soils. It is important to understand what the nutrient interpretations mean.
 

Soil test results are of little value without an appropriate interpretation. To be useful, extractable nutrient values must be shown to relate to: 1) the soil's ability to supply that nutrient to crops (correlation); and 2) crop response to application of that element (calibration). Figure 3 illustrates the conceptual relationship between soil test level and yield. This relationship is determined by conducting nutrient response research, under local conditions with representative soils ranging from deficient to adequate for each nutrient of concern. These data form the foundation of our interpretation of soil test results. The exact relationship between soil test level and crop response to different nutrients may vary considerably, but the general shape of the response curve is relatively consistent. At low soil test levels, yield is limited by a lack of the nutrient. As the soil test level increases, yield increases until a point where the nutrient is no longer limiting and the curve levels out; this point is known as the critical soil test level. The critical soil test level is defined as the extractable nutrient concentration in soil above which an economic yield (or quality) response to added nutrient is unlikely. Nutrient levels are considered sufficient when the concentration is just above the critical soil test level. This is known as the Optimum soil test range. Soil test values are interpreted based on how they compare to the critical soil test level and optimum range.  

Figure 3. Conceptual relationship between soil test level and crop yield or relative yield. The critical level is defined as the soil test level for a given nutrient above which there is a low probability of a response with addition of that fertilizer.

A critical step in soil testing is sample collection. Be sure to read and follow sampling procedures from the specific lab you will be using. For all testing, it is important to obtain a representative sample; a poor sample may result in erroneous soil test results and poor recommendations. The first step is to determine the area that will be represented by the sample. Soil physical appearance, texture, color, slope, drainage, and past management should be similar throughout the area. It may be helpful to draw a map of the farm and identify areas where you will sample separately. Using a clean bucket and a spade, auger, or sampling tube collect at least 12-15 subsamples to a depth of 6-8 inches from random spots within the defined area and place them in the bucket. This may be accomplished by walking a zig-zag pattern across the sampling area and collecting a subsample periodically. Avoid sampling field edges, old fence rows, areas where manure or lime were stockpiled, and other non-representative areas. Next, break up any lumps or clods of soil, remove stones and plant debris, and thoroughly mix subsamples in the bucket. This step is very important, because only a few tablespoons of your sample will actually be used for testing. Once the sample is thoroughly mixed, scoop out approximately one pint of soil and, if specified by your lab, spread it on a clean piece of paper to air-dry (brown paper bags work great). Once the sample is dry, place it in a labeled container provided by the lab (or a zip lock bag), and complete a sample submission form. For each sample, indicate the crop to be grown, recent field history and any concerns.

Soil pH and extractable levels for certain nutrients (e.g., P and K) vary throughout the year. While the seasonal fluctuation is not typically large enough to significantly influence interpretation and recommendations, it can make it difficult to compare soil test values from the same field over time. For this reason, it is a good idea to be consistent about timing of sample collection from one year to the next. Although soil samples can be taken any time, many prefer to take samples in late summer or fall because this allows time to apply any needed lime, plan a fertility program and order materials well in advance of spring planting. Avoid sampling when the soil is very wet or within six to eight weeks after a lime or fertilizer application. Routine soil analysis should be conducted once every two to three years.

Nutrient recommendations are determined by soil test results; however, even when two labs use the same methods and generate equivalent results, their nutrient recommendations may differ. These disparities arise due to differences in soil test interpretation and recommendation philosophies. Over the years, three basic philosophies have emerged. These include the sufficiency approach, the build and maintain approach, and the base cation saturation ratio (BCSR) theory. Both the sufficiency approach and the build and maintain approach follow the general concept that there are definable critical levels of nutrients in soil, and that below this level crops are likely to respond to additional nutrients applied. When nutrient concentrations are in the optimum range, just above the critical level, there is a low probability of crop response to the addition of that nutrient. With the build and maintain approach, fertilizer recommendations are made with the goal of building the soil's nutrient levels into the optimum range, then maintaining these levels by applying nutrients at rates that approximate crop removal. The sufficiency approach is a more conservative philosophy where nutrient recommendations are intended to meet crop needs, not build soil fertility. No nutrients are recommended above the critical soil test level. The sufficiency approach is designed to “feed the crop” while the build and maintain approach is designed more to “feed the soil.” In theory, the sufficiency approach is a more profitable system since fertilizer is only applied when there is likely to be an economic return. However, in practice the sufficiency approach is also more risky due to the inherent uncertainty associated with soil testing.

The third philosophy, the BCSR theory, promotes the idea that maximum yields can only be achieved by creating a balanced ratio of calcium (Ca), magnesium (Mg), and potassium (K) in the soil. At one time, many private labs and a few public labs used the BCSR concept to interpret soil test results and make nutrient recommendations. Over time, as the body of research evidence illustrating the flaws of the BCSR concept grew, leading most private labs and essentially all of the public labs to abandon the system. Most of the guidelines developed by Land Grant Universities and used by both public and private soil test labs follow a combination of the sufficiency and build and maintain approaches, the goal being to provide adequate, but not excessive, levels of essential nutrients to promote healthy plant growth. Nutrient recommendations provided in this Guide are a reflection of this compromise. The nutrient guidelines are intended to help growers optimize crop yield and quality, maximize return on fertilizer investment, and minimize nutrient losses to the environment.

The nutrient recommendation tables in this Guide are applicable to the New England soil test results given as very low, low, optimum (medium or high), and above optimum (very high or excessive). Table 5 provides a brief interpretation of each of these categories. Generally, nutrients should be in the optimum range for good yield and quality. When levels are below the optimum range (very low or low), the addition of more of the nutrient will usually improve production and provide a return on fertilizer investment. Nutrient recommendations are intended to meet crop needs and provide enough to slowly (over several years) build soil test levels to the optimum range. When soil test levels are in the optimum range, crop response to application of that nutrient is unlikely, but some amount may be recommended to maintain soil tests levels by replacing a portion of crop removal. In the nutrient recommendation tables for each crop listed in the guide, these build and maintain application amounts are indicated by a range such as 0-50 lb per acre. Crops and even cultivars of the same species vary in their uptake and removal of nutrients and this is accounted for in the nutrient recommendations. If a nutrient is in the above optimum range, crop response is very unlikely and application of that nutrient is generally unjustified. It is important to keep in mind that factors other than nutrients may limit crop growth, and simply adding more nutrients will not improve yield. To optimize yield and maximize response to fertilizer, sound agronomic practices must be used (e.g., crop rotation, timely planting and harvest, pest control, soil health, and water management).  

Table 5: Interpretation of Soil Test Level Categories

An additional tool that can be used for long season crops such as eggplant, pepper, potato, tomato, squashes, sweet corn, and pumpkin is plant tissue analysis.  While leaf tissue laboratory analysis can be used to verify symptomatic deficiencies in any nutrients, it can also be used to detect sufficiency levels of critical nutrients such as N, P, K, Ca and Mg.  If performed early enough in the season, corrections can be made by topdressing, side dressing, or fertigation.  Prior to making nutrient corrections, other potential issues should be addressed first, such as incorrect pH, inadequate soil moisture, root disease or insect infestation. Leaf tissue analysis requires collecting an adequately sized sample (from all over a planting) of whole leaves from designated locations in the plant canopy. These locations vary among crop species and specific sampling and handling instructions are available from university and private laboratories.  For the purpose of comparison, laboratory reports will also present results in tabular form alongside reference sufficiency ranges of nutrient concentrations for the crop tested. Nutrient status of some crops can also be determined using laboratory testing of whole leaf petioles, and for nitrate-N and K, it is possible to test petiole sap in-field using a portable meter.

Connecticut:
Soil Nutrient Analysis Lab
6 Sherman Place, Unit 5102
Storrs, CT 06269-5102
Telephone: 860-486-4274
Website: http://www.soiltest.uconn.edu/
Email: soiltest@uconn.edu

Gregory Bugbee, State Laboratory
The Connecticut Agricultural Experiment Station
123 Huntington St., P.O. Box 1106
New Haven, CT 06504
Telephone 203-974-8521
Website: http://www.ct.gov/caes/cwp/view.asp?a=2836&q=378206
Email: Gregory.Bugbee@po.state.us

Maine:
The Analytical Laboratory and Maine Soil Testing Services
5722 Deering Hall, Room 407
Dept. Plant & Soil & Environmental Sciences
Orono, Maine 04469-5722
Telephone: 207-581-3591
Website: http://anlab.umesci.maine.edu/
Email: hoskins@maine.edu

Fertilizer grades refer to the guaranteed percentages of plant nutrients. The ratio refers to the proportion of nitrogen (N), phosphate (P₂O₅) and potash (K₂O) in the fertilizer. For example, the grade is 5-10-5, and the ratio is 1-2-1. High analysis fertilizers are those with grades such as 20-20-20, 35-0-0 or 0-46-0, and are more economical to use on the basis of price per pound of nutrient.

Liquid starter fertilizers are materials that are completely water soluble and are high in phosphorus content such as 16-32-16, 10-52-17 or 9-45-15. These materials are used at the time of transplanting. Dry starter fertilizer can be banded at the time of seeding. The band is normally placed 2" below and 2" to the side of the seed. When using either dry or liquid starter fertilizer, follow label directions because an excess of starter could burn seeds and young seedlings. Starter fertilizer promotes early and rapid growth that leads to greater yields with certain crops such as tomato, pepper, and melon. See individual crops for rates.

Table 6, "Plant Nutrient Content of Various Fertilizer Sources" lists many fertilizers commonly used for vegetable cropping systems. Similar information for fertilizers and amendments commonly used in certified organic cropping systems are provided in the section entitled Guidelines for Organic Fertility Management (Table 10).

Table 6: Plant Nutrient Content of Various Fertilizer Sources (% by weight)

Table 6a: Conversion Factors for Boron Nutrient Sources

¹ Best for fertilizer blends.

Animal manure is an excellent source of nutrients and organic matter. Many of the nutrients in fresh livestock manure, especially nitrogen, are readily available.  Nutrient content varies by animal species, their diets and the form of their manure. There are times when readily available nitrogen is needed, but many people prefer to compost manure before field application (see Compost section below). Manure application rates are now regulated in many New England States (see Nutrient Management Regulations).

Nitrogen in manures and other waste products: The N content of manures is highly variable. Differences are due to the species of animal, the animal's age and diet, the moisture content of the manure, handling and storage, and the amount of bedding in the manure. The N fertilizer equivalent of a manure varies not only with the total N content of the manure, but also with the timing and method of manure application. Manure samples can be analyzed by the Universities of Maine and Vermont Laboratories. The values in Table 7 are based on analyses of Vermont manures as well as published data from other states. If specific manure analysis data is not available, growers should estimate N credits using these or other book values. The time elapsed between spreading and incorporation of manure is also important. About half of the N in dairy manure and three quarters of the N in poultry manure is in the form of ammonium (NH₄), which easily turns to ammonia gas (NH₃) and is volatilized (lost to the air). The longer that manure is left on the soil surface, and not incorporated, the greater NH₃ volatilization losses become (Table 7a). Broadcast application of slurry manure without incorporation should always be avoided because this method increases air contact and allows time for all ammonia to be lost. Research has shown that in reduced or no-till fields where manure must be surface applied without incorporation, ammonia can be best conserved if applied during cold temperatures, low wind speeds and especially to a growing cover. A growing cover also reduces manure run-off and leaching losses.  NOTE: Manure often contains human pathogens. Serious illness has occurred from eating produce where fresh manure was applied without an adequate waiting period (see Produce Safety).

Previous manure applications: Up to 50% of the total N in cow manure is available to crops in the year of application. Between 5% and 10% of the total N applied is released the year after the manure is added. Smaller amounts are furnished in subsequent years. The quantity of N released the year after a single application of 20 tons per acre of cow manure is small (about 15 lb N per acre). However, in cases where manure has been applied at high rates (30-40 tons per acre) for several years, the N furnished from previous manure increases substantially. The buildup of a soil's capacity to supply N resulting from previous applications of manure has important consequences for efficient N management, including: 1) The amount of fertilizer N needed for the crop decreases annually; and 2) If all the crop's N needs are being supplied by manure, the amount of manure needed decreases yearly.

With poultry manure (as compared with manure from cattle) a higher percentage of the total N in the manure is converted to plant-available forms in the year of application. Consequently, there is relatively less carry-over of N to crops in succeeding years. This does not mean, however, that there is never any carry-over of N from poultry manure applications. If excessive rates of poultry manure (or commercial N fertilizers) are used, high levels of residual inorganic N, including nitrate (NO₃), may accumulate in soil. High levels of soil nitrate in the fall, winter and spring have the potential to pollute groundwater and coastal seawater.

Table 7: Nitrogen Credits from Manure Applied Before Planting

Adapted from Nutrient Recommendations for Field Crops in Vermont (2018). Dairy manure values are from Vermont samples analyzed by University of Maine, 2012-2016, others are adapted from University of Nebraska-Lincoln NebGuide G 1335 and Penn State Agronomy Guide (2016). Values do not include bedded pack. Manures vary greatly, so obtaining a manure analysis is always best practice. n/a = data not available.

Table 7a: Availability of ammonium nitrogen from spring or summer applied manure (% fertilizer N equivalent)

Composting livestock manure and other organic matter stabilizes the nutrients by partially decomposing the materials.  Nutrients are releaed more slowly from finished compost than from fresh livestock manure.  Compost is considered mature (i.e., finished) when most of the easily decomposed components of the material have been broken down and biological activity has slowed. At this time, the pile returns to ambient temperature, and it does not reheat when mixed or turned. The composting process results in a dark-brown material in which the initial constituents are no longer recognizable and further degradation is not noticeable. The length of time needed to achieve finished compost will vary with many factors and can range from a couple of weeks to over a year.

Application of unfinished compost could affect plant growth adversely because the compost-making microbes may compete with the crop for nitrogen. Applying compost at least one week before transplanting or seeding a crop will allow a margin of safety in case the compost is immature. Immature composts made from nitrogen-rich feedstock are also often high in ammonium, which can change to ammonia gas and be toxic to plant growth. High ammonium concentrations are not typically a problem if the compost is field applied, but if compost will be used in a greenhouse mix, it is important that it be low in ammonium. 

Vegetable growers can make compost on the farm although most don’t have enough raw materials to satisfy their needs. Some bring in additional materials such as manure or municipal yard wastes to compost on-site. Others purchase compost from commercial composters. 

Compost as a nutrient source. Finished compost is a dilute fertilizer, typically having an analysis of about (1-1-1 N-P₂O₅-K₂O), but the analysis can vary greatly depending on the types of materials used to make the compost and how they were composted. Composts should be analyzed for their available N, total N, P₂O₅, and K₂O content before application to agriculture fields.

Carbon to Nitrogen Ratio. The recommended C:N ratio for finished compost is 15-18:1. The C:N ratio plays a crucial role in the availability of nitrogen in any organic material added to the soil. If the C:N ratio is much above 30:1 microorganisms will immobilize (i.e., consume and make unavailable for plant uptake) soil nitrogen. This soil nitrogen will remain unavailable until the carbonaceous material is consumed by the bacteria.

Table 8: Typical Carbon-to-Nitrogen Ratios

Nitrogen. The majority (usually over 90%) of the nitrogen in finished compost has been incorporated into organic compounds that are resistant to decomposition. Rough estimates are that only 10%-30% of the nitrogen in these organic compounds will become available in the first season following application. Some of the remaining nitrogen will become available in subsequent years and at much slower rates than in the first year. Repeated annual applications of compost at high rates above 400 pounds of nitrogen per acre can result in excessive amounts of nitrate in the soil.

Phosphorus. There is not much research information published about the availability of phosphorus from compost. The few papers published show that composts made primarily from manures supply phosphorus over the growing season at 70%-100% of the availability of triple superphosphate fertilizer. The amount of organic amendments that can be added without building up excessive phosphorus depends primarily on: 1) the existing soil test P level of the field; and 2) the P₂O₅ content of the amendment. Table 9 shows the effect of both soil test P categories and the P₂O₅ concentration of an organic amendment on the suggested maximum amount of material to apply. If these rates of amendments are applied every year, analyze the soil for extractable P annually to ensure that soil test P has not risen to excessive levels. Additional compost applications to soil that tests optimum for P could increase P to above optimum levels. If a soil test shows an above optimum P level, avoid compost applications until P returns to the optimum range. 

Table 9. Maximum Compost or Organic Amendment Application and total P₂O₅ per Soil Test Category and P₂O₅ Concentration¹

¹ Assumes moisture content of the compost or organic amendment of 45%.

² Average rates used to calculate amounts of P₂O₅ applied for various rates of compost applications.

Potassium. Potassium in finished compost is much more available for plant uptake than nitrogen because potassium is not incorporated into organic matter. However, some of the potassium can be leached from the compost because it is water soluble. In one study, potassium levels were reduced by 25% when finished compost was left uncovered in the open over a winter.

Soluble Salts. In general, soluble salts are not a concern from additions of composts to field soil. However, soluble salts can be a serious problem when using compost in greenhouse mixes. Incorporation of 40 tons per acre of compost in the top 6 inches of field soil would be a ratio of 50 parts soil to one part of compost. Compost used in the preparation of greenhouse media will make up a much greater percentage of the whole mix and therefore will have a greater influence on all aspects of fertility, including soluble salts. It is important to have composts tested for salt levels. Electrical conductivity (EC) is a measure of salt level, and compost used in greenhouse mixes should have EC < 1 mmhos/cm.

Compost and pH. The pH of finished compost is usually slightly alkaline. In general, composts will not raise soil pH to undesirably alkaline levels because of the low total alkalinity of composts. However, caution should be taken if the compost has been “stabilized” with the addition of lime (thus increasing the total alkalinity) or with heavy applications to certain crops such as potatoes, for which the soil pH should be about 5.2. Heavy applications can cause increases in soil pH that might last for a growing season.

Heavy Metals and Trace Elements. The danger of heavy metals in some composts has received much attention. At one time, some heavy metals in some composts were high enough to be toxic to plants (copper, nickel, zinc) or of concern to human health (cadmium). There have been documented cases where elements such as boron have been raised to toxic levels with repeated applications of compost. These composts with high metals or boron were made from materials with high concentrations of these elements. Governmental regulations control the materials that may be used in composts for applications to farmland. None of these toxicity problems are likely to occur with compost that has been made from farm manures or crop residues or with the commercially available composts of today.

Herbicide Residues in Compost. There are broadleaf herbicides registered for use on turfgrass, pastures, and hay crops that retain activity in the manure of animals that have fed upon them, as well as through the composting process of crop residues from areas treated with these herbicides. There have been many cases where vegetable growers have unknowingly purchased organic amendments such as manure and composts that are contaminated with herbicides and have damaged vegetable crops. If you purchase organic amendments, you should be aware of this possibility and get assurance that herbicides are not present in the manures and composts that you purchase. 

Have Compost Analyzed. No compost should be applied to field soil or used in greenhouse mixes without testing for nutrient content. If the compost will be used in greenhouse mixes, it should also be tested for maturity. Some soil test labs will test compost. Check to be sure the lab analyzes compost before submitting samples, and make sure to have it tested as a compost sample, not as field soil.

Take Soil Test After Applying Compost. A good way to evaluate the effect of compost on the fertility of a soil is to obtain a soil test after applying compost. It is best to wait six to eight weeks after application before testing the soil to allow the compost and soil to equilibrate. 

There are many different types of commercially available soil amendments and plant nutrient sources on the market today.   While some products contain detectable quantities of nutrients that become available to plants in the near term, other products may instead increase availability of existing plant nutrients in the soil. Many have not been well tested in controlled studies.  There are many  categories of such products available for purchase. Some, but not all, have been approved for organic production.

Organic Residuals

Organic by-products of industrial processes fall into this category. Note that the use of the word “organic” in this case refers to the nature of the material itself, e.g. derived from biological sources. It does not necessarily indicate acceptability for certified organic production. Materials included in this category include processed slaughterhouse wastes, leather processing waste, biosolids, papermill sludge, and composts. In general, as these products decompose, plant nutrients are released. Many are sold with the nutrient analysis content listed, which is generally very low on a “percent-by-weight” basis. The greater benefits are usually for soil conditioning, and in some cases, liming activity. Not all of these products are acceptable for certified organic production, and acceptability for use in food production should be verified.

Foliar Amendments

Foliar feeding has become a more common practice among some vegetable farmers. Many products are now available on the market, for use in both conventional and certified organic production. Foliar feeding is not recommended as a major source of nutrients for a growing crop, but it can be used for supplemental feeding under certain circumstances. Such circumstances include: 1) when soils are cold and N and P mineralization rates are low; 2) at the onset of nutrient deficiency symptoms in rapidly growing plants (verified by properly conducted leaf tissue testing); and 3) during periods of high nutrient demand, especially fruiting. Even so, nutrient deficiencies often result from indirect causes, such as water issues, soil compaction, pH, root diseases or even macronutrient (N, P or K) deficiencies that can be limiting micronutrient uptake or availability. Addressing these issues is likely a more long-term, as well as time- and cost-effective way to ensure crop micronutrient needs are met.

New England soils are glacial in origin and are considered “young.” For this reason, our soils are not typically lacking in micronutrients. In soils with pH greater than 7, metal cations become less available to plant roots, and plants may show signs of deficiency. Most soils in New England are acidic, requiring periodic lime applications. Where soils are alkaline, the best way to correct deficiencies of Zn, Mn, Fe and Cu may be to apply foliar sprays of these nutrients in chelated form. Certified organic growers should ensure that they are using forms allowed under organic certification. In some cases, it may be necessary to lower soil pH using products such as elemental sulfur, aluminum sulfate or ammonium sulfate.

Plant Biostimulants, Biofertilizers, Microbial Biostimulants, Microbe-containing Bio-products

A biostimulant is a substance or microorganism (or mixture of one or more of these) applied with the intent of enhancing a crop’s nutrient efficiency, abiotic stress tolerance and/or quality traits, regardless of the material’s nutrient content. There are now hundreds of commercially available products that fall into these categories. This does not include products labeled for pest control purposes, however, which fall under strict EPA guidelines mandating EPA registration.

One category of these products is familiar to most: various strains of species of Rhizobium inoculants for legumes. Research has consistently shown the benefit of legume inoculation to realize full nitrogen fixation potential of legumes, provided that the plant and bacterial species are properly matched.

There has been a proliferation of mycorrhizal fungus inoculant products. These fungi are symbiotic with many crop plants (excluding brassicas and a few others) and extensive research has shown their beneficial effects on plant nutrition, growth, and stress reduction in field, nursery pot and greenhouse conditions. The fungi live inside plant roots, where they obtain a carbohydrate energy source from plants. In turn, the fungal mycelia transfer water and mineral nutrients to plants, which they can extract from the soil volume more efficiently. In this way, the fungi “extend” the rhizosphere that surrounds plant roots. Unfortunately, real-world test results of these products are not readily available. It is unknown at this time whether inoculation has short or long-term economic impact in annual vegetable production.

There are numerous other soil microbial inoculant mixtures available from commercial suppliers. Peer-reviewed research with many of these organisms has shown some positive potential. They are intended to influence crop plants’ rhizosphere, promoting potential availability of mineral nutrients already present in the soil, sometimes by stimulating plant responses to stresses or diseases. They do not, however, directly supply nutrient elements to plants. There may well be a promising future for microbial inoculants, particularly if it means reduced fertilizer input, but product effectiveness has not been well documented at this stage.

Compost Tea

The legal definition of compost tea from the National Organic Program is “A water extract of compost produced to transfer microbial biomass, fine particulate organic matter, and soluble chemical components into an aqueous phase, intending to maintain or increase the living, beneficial microorganisms extracted from the compost.” Microbial species content is highly variable, depending on the source of compost and the “brewing” conditions. Compost teas have a very low analysis of plant nutrients. Although they are widely produced and used on farms of various scales, research evidence of their efficacy is inconsistent at best. Benefits of using commercial microbial inoculants are variable, and using compost tea is even less dependable despite its widespread popularity. If you do plan to use compost tea, care must be taken to avoid cultivating bacteria harmful to human health (e.g., start with finished compost, use potable water, and avoid using additives like molasses).

Humates, Humic Acids, Humic Substances

These materials are made of very large and complex molecules. Most commercial products are extracted from peat or soft brown coal deposits of lignite. Extraction processes and treatments vary widely, so it is difficult to make comparisons between various products on the market. Humic materials contain only small amounts of plant nutrients, thus are not considered fertilizers. Their usage has been promoted by some to provide physical, chemical, and biological benefits to soils. These materials have been studied for over 50 years, mainly in controlled settings, with mixed results from laboratory and greenhouse studies; some resoundingly positive reports, many neutral, and a few detrimental. Under field conditions there are few documented positive effects from their usage. Naturally occurring compounds in soil organic matter effectively perform the same functions, such as chelation of micronutrient metals, and possibly producing plant hormonal effects. Nevertheless, there are many commercial products available, and little consistency among them.

Seaweed Extract Products

Seaweeds have been applied to agricultural land for at least a few thousand years and until recently, their primary benefit was considered to be similar to that of other organic amendments, releasing nutrients through decomposition. It was discovered over 50 years ago that seaweed nutrient content was too low to directly boost soil test nutrient levels and that other growth stimulating mechanisms must be involved. Seaweed has been proposed to have several different effects on the root zone environment and on plants themselves.

Though seaweed extracts are used in crop production in large quantities world-wide, there is surprisingly little published research on their use and effectiveness in field settings. One of the more common claims is alleviation of the effects of environmental stresses, such as temperature and moisture extremes. Subtle effects are difficult to measure in the field alongside many other possible factors. Therefore, when used during typical conditions, their effects are hard to detect.

Soil Moisture

A soil is saturated when all of its pore spaces are filled with water. This is likely to be the case after a heavy rain or irrigation. After one or two days, when excess water has drained due to gravitational pull, the soil is at field capacity. At this point, the remaining water is attracted strongly enough to soil particles to prevent further drainage, yet it is readily available for uptake by plant roots. Soil water is further depleted by evaporation from the soil surface and transpiration through the plant leaves. The combination of these processes is called evapotranspiration (ET). As soil water is depleted, what remains is held more tightly by soil particles. As this happens it is increasingly difficult for plant roots to extract moisture from the soil. Eventually a point is reached where the remaining water is so tightly held by soil particles that it is unavailable to plants. This is the permanent wilting point or wilting coefficient. Soil moisture in the range between field capacity and the wilting coefficient is usable by plants and is called available water, although moisture stress will occur as the lower end of this range is reached. It is advisable to begin irrigating vegetables before half the available water has been used. If soil moisture is depleted below this point, plants will be under increasing stress, even though they may not show visible wilt symptoms at first. Moisture stress can greatly reduce yield and cause numerous disorders such as tip burn of leafy crops and blossom-end-rot in tomatoes and other fruits. It may be advisable to begin irrigation when as little as 30% of the available moisture has been used.

To achieve the most benefit from an irrigation system, it is necessary to apply the correct amount of water at the right time. This means replacing the water lost through ET and doing so before plants are under stress. The rate of ET is affected by a number of environmental factors including solar radiation, temperature, wind speed and relative humidity. When it is hot, sunny and windy with low relative humidity, up to 1/3" of water per day can be lost through ET. That is about 2" per week. When it is cool, cloudy and damp with little wind, losses are quite low. As canopy area increases, evaporation from the soil decreases due to shading, but transpiration from the leaves increases and generally ET increases. If weeds are present, their leaf canopy increases ET losses to the detriment of the crop. Evaporation from the soil surface is reduced by the use of organic mulch and nearly eliminated under areas covered with plastic mulch.

Evaporation pans can be used to estimate ET loss. Pans should be filled with a measured amount of water, such as 1", and placed in or next to the field in a sunny spot. The loss of water from an evaporation pan will approximate the amount lost through ET. Although this is not exact, it provides a good indication of the rate of loss when sprinkler irrigation is used. When irrigation occurs, the evaporation pans should be filled with the same amount of water as was applied to the field.

It is important to know the amount of available water that a particular soil can hold. This varies considerably with soil type. For example, a slit or clay loam can hold several times as much available water as a sandy soil (Table 12). Soil organic matter can substantially increase a soil's ability to store available water. It has been estimated that for each percent of soil organic matter, water holding capacity is increased by about 1/2" per foot of soil depth. This depends on the state of decomposition, but it is clear that organic matter has a profound influence on moisture holding capacity. Crops growing on soils with a high available water holding capacity require as much water as those on a soil of low available water holding capacity, because ET is about the same on both types, but the required frequency of irrigation is different. Soils with a high available water holding capacity need less frequent irrigation than those with a low capacity. However, when irrigated less frequently, a greater amount of water should be applied per application. This results in less labor for moving and setting up pipes and sprinklers.

Table 12: Available Water Holding Capacity Based on Soil Texture

When to Apply Water

In general, if you wait for crop symptoms (wilting) to decide when to irrigate, the crop will already be damaged. While experienced growers learn their soils and how they interact with water over seasons of experience, utilizing a soil moisture measuring device or sensor of some sort enables a grower to attach a number to their observations and track trends over time. Readings provided by sensors can also be utilized to fine tune irrigation management strategies and better manage the growing environment for specific plants.  Proper installations of sensors is critical for accurate readings. Sensors can quickly and easily be moved from one location to another in order to better understand the dynamics of soil moisture in relation to soil types, irrigation cycles, topographical changes, etc., so long as the installation instructions are followed along with each move.

Soil tensiometers for measuring soil moisture are available at a cost of $75 to $100. They can be purchased through several field equipment suppliers. To use a tensiometer, place the porous tube of the tensiometer at the depth you desire moisture measurement. You can calibrate your tensiometer to a particular soil so irrigation is done when the tension on the gauge reads a certain value (a number specific to your soil and the crop you are growing). A maximum value (usually 30-35) would be used for a sandy loam soil and this value may vary with the particular tensiometer purchased. In utilizing tensiometers, be certain you are aware of soil variability within a given field. Three to four tensiometers per field may be needed to adequately account for this variability and, in addition, two depths (commonly 6" and 12") may be necessary to adequately reflect the most critically stressed areas. The 6" unit indicates when soil moisture near the surface is being depleted (begin irrigating) and the 12" one shows when the moisture has moved to the bottom of the root zone (stop irrigation). In a drip irrigation system, a rule of thumb is to place the tensiometer about 6" from the tape at a depth of one-third the entire root zone. During irrigation, the tensiometer indicates when field capacity has been attained at the depth of the porous tube.

Another type of soil moisture sensor gaining popularity in the northeast is the granular matrix sensor. This category of sensors provides a reading based on the electrical resistance between two electrodes embedded in the granular matrix within the sensor. The more soil moisture available in the soil, the lower the resistance and the number on the reader. This resistance reading is reported in kilopascals (kPa) or centibars. This measure of resistance can be used to better understand the force a plant root must overcome to extract water from a given soil. 

Established guidelines for maintaining soil moisture in specific crops and soil types are available. Using these recommendations, paired with visually observing the crop and soil, provides growers with additional information on which to base their irrigation decisions. In most soils, other than heavy clay, the decision to irrigate would generally happen in the range of 30 to 60 kPa. Differences in soil type should be considered when determining the appropriate range in which to irrigate. This is because different soil types have varying levels of plant available water at various soil moisture readings. To clarify, a soil moisture tension reading of 40 kPa in a sandy loam would mean that approximately 50 percent of the water in the soil is plant available. Comparatively, a loamy sand soil might have only 35 percent plant available water at the same 40 kPa reading. 

This reinforces the importance of knowing your soil type, along with monitoring soil moisture and visually observing crops and soil to make an informed irrigation decision. Additionally, the irrigation method makes a difference as to when a grower might decide to irrigate. For example, overhead irrigation is recommended to begin when the available soil moisture is no less than 50 percent, whereas drip irrigation, taking comparatively longer to distribute substantial volumes of water, should be started before the plant available water drops below 80 percent. 

When using drip irrigation on plastic-covered raised beds, during rain events where less than one inch of rainfall has fallen, run the drip irrigation system as normal.  When greater than one inch of rainfall has occurred, delay the application of water through the drip irrigation system.  

Critical Periods for Moisture Needs

Vegetable crops should not be under stress at any time. Each crop has its particular periods of critical moisture needs. In many crops (such as sweet corn, beans, and peas), the most critical period is during or just after flowering. These crops have flower development in a much more concentrated period of time. Other crops (tomato, peppers, eggplant, and potato) also have a critical moisture need during fruit or tuber development. Check individual crops for details (Table 13). Some crops, such as onions, potatoes, pumpkins and winter squash, benefit from dry conditions at the end of the growing season when the crop is curing.

Table 13: Critical Periods of Water Need of Vegetable Crops

Note: These are stages of critical water demand, but vegetable crops should not be subjected to stress at any time during growth

Water Application Rate

The application rate of irrigation should not exceed the infiltration and percolation rates of the soil. Infiltration is the entry of water through the soil surface and percolation is the downward movement of water through the soil. If the application rate exceeds either the infiltration or the percolation rate, water will accumulate on the surface and is subject to run-off and erosion. Soil compaction inhibits both infiltration and percolation and should be minimized. Soil crusting also interferes with infiltration. Soil organic matter reduces soil compaction and crusting. Compaction and crusting can also be minimized by using appropriate tillage practices and restricting traffic over the soil.

Sprinklers

Sprinkler size should be chosen based on the crop, distance between laterals, pumping pressure and volume, and the infiltration and percolation capacities of the soil. Sprinkler placement should be staggered with those on adjacent laterals. This provides a triangular pattern in the field. Sprinklers are designed to operate with patterns that overlap according to the manufacturer's specifications. A triangular arrangement with overlapping patterns provides the most uniform coverage.

Wet conditions are favorable to most diseases. Time the use of sprinklers so that foliage can dry rapidly when irrigation is complete. This is usually in the morning. Irrigating thoroughly to achieve field capacity to a depth of the majority of the root system, and never permitting plant stress, can reduce diseases. Irrigating more often than necessary may encourage disease development by maintaining wet foliage for long periods of time. This can also leach nutrients.

Frost Control

Early- or late-planted vegetables can be subjected to freezing temperatures in spring or fall. Using an overhead sprinkler system that applies about 1/10" of water per hour, during periods when the air temperatures in the crop canopy are below freezing, can reduce or prevent crop losses. Greater or lower applications may be necessary depending on minimum temperature, length of freeze period, and wind speed. Be certain to start irrigating before the temperature reaches freezing and continue irrigating until ice melts. The degree of protection depends on wind speed and other factors. However, protection below 20°F has not been attained.

A well designed trickle or drip irrigation system benefits the environment by conserving water and fertilizer and requires little labor to use once it is set up. Water is applied either on the surface, next to the plant, or subsurface, near the root zone. In dry years, fewer weed seeds germinate between rows because there is less water available beyond the plant root zone, although weed roots may grow toward the wet zone, especially when plastic mulch is used. With drip irrigation there is less evaporation than with sprinklers. Evaporation losses from the soil surface are further reduced when drip irrigation is placed under plastic mulch.

It requires some expertise to install and operate a trickle system. Consultation with a knowledgeable professional is wise. A poorly designed system can result in yield variability in the field due to areas of over- or under-watering and clogged lines. Trying to save money by cutting costs on initial equipment purchases will likely be more expensive in the long run. Any or all of these problems can completely offset the potential cost savings from using drip irrigation, but they can be avoided by using good equipment and proper design and installation.

Water source. Organic materials such as plant materials, algae, small living organisms, and inorganic sand, silt, and clay are likely to be found in surface water such as a pond or stream. Well water is likely to have some sand, silt or clay particles, although not as much as most surface supplies. These particles can clog the small diameter emitters in the tape. Filters are used to remove particulate matter as discussed later. Surface water might have contaminants from run-off, which can include plant pathogens such as Phytophthora.

Slope. A slope of 2% or less is the ideal for drip irrigation. Many fields in New England have slopes greater than 2%. A difference of 2.3' in elevation will change water pressure by 1 psi, decreasing as elevation increases and vice versa. The length of lateral lines, the pump size, and pressure regulators are chosen based on the slope. It is best to run the rows horizontally across a slope, but if rows must run up and down, it is best to have the water flow downward through the drip tape. Water should also flow downward in headers. This probably requires that water be pumped to the top of the field so it can flow downward from there. With slopes greater than 5%, pressure-compensating drip tape is needed.

Soil. The soil type determines the soil wetting patterns. Soil wetting patterns in turn influence the depth of the drip tape and the distance between emitters. The duration and frequency of irrigation are also determined by the soil type. Over-watering can move fertilizer away from the root zone. On sandy soils, water goes primarily downward rather than horizontally so emitters should be at relatively close spacing. Spacing between emitters can be greater in heavier soils where there is considerable movement laterally. In sandy soils, irrigate more frequently, but run the water for a lesser amount of time. In heavier soils, irrigate less often, but run the water for a longer duration. In both cases, this should lessen the chance of leaching fertilizers away from the root zone.

Drip tape should apply water uniformly throughout the crop root zone. The emitters should be close enough so that there is uniform wetting of the soil. As discussed above, the spacing of the emitters is affected by soil type. Drip tape should have a coefficient of manufacturing variation (CV) number that reveals how much variation in uniformity there is from one emitter to the next. A CV of 0.05 is considered excellent and a CV between 0.05 and 0.1 is acceptable. The rate of water delivery is a function of the size and spacing of the emitters and typically ranges from about 2.5-5.0 gallons per minute per 1,000' of tape (0.25-0.5 gpm per 100').

The length of the drip lines is another important consideration. The length is determined by the pump size, the field size(s), and the slope of the land. Any one of these factors will influence wetting uniformity because the emitters will discharge water at different rates if there are changes in pressure along the line. Because of variation in water pressure, tape is rarely laid out longer than a length of 400' on fairly level land and less on slopes. Tape should run across the slope, but if this is not practical, it should run downhill so the pressure loss due to friction is counteracted by the gain as the elevation decreases. If the tape runs up hill, there is a double loss due to friction and increasing elevation.

The choice of tape thickness, measured in mils, is based on how long you want the tape to last and the expected highest water pressure in the lines. The longer the tape is expected to be in the ground, or the higher the pressure in the lines, the thicker the tape should be. Tape thickness is usually between 4 and 10 mil, though thickness of up to 25 mil can be purchased. Tape can be reused for two to three years, but the labor costs of retrieving and cleaning the tape usually make this uneconomical for annual vegetable crops.

Placement of drip tape. The tape should be placed as close to the plant as is practical for the specific crop. This is critical on porous soils, but if necessary, tape can be placed between 6" and 12" from the plants on soils with good lateral water movement. Tape is often laid between double rows of crops such as peppers. The soil will be wetter on the side of the crop where the tape is and most of the roots will be concentrated there. The tape should be placed so that the emitters are pointed upward so that any particulate matter will settle away from the emitters after the water stops flowing.

Tape can be placed on top of the soil or a few inches below the surface. If tape is on the surface, it is easy to observe wetting patterns and to make repairs if needed. The disadvantages are that there is greater evaporation in the initial stages of the crop's growth and the tape is more likely to be damaged by production practices, wind, and animals. As tape is heated by the sun during the day, it expands and takes on a snake-like pattern in the row. This is particularly a problem with higher temperatures under plastic. This can be avoided by burying the tape.

Pumps. For a given area, trickle irrigation uses less water and at a slower rate than a sprinkler system. Therefore smaller pumps can usually be used. The required pump capacity is determined by the flow rate of the tape, total length of tape to be used at a given time and pressure loss due to friction and increases in elevation. Pump size should be determined when designing a system.

Filters. The choice of filter is based on the quality of the water passing through the system and drip application requirements.  Typical screen mesh size is 150 mesh (100 micron). The filter should be sized for the longest application flow rate needed.  Water particulates change through the growing season and filter flow rates are affected with dirtier water and more suspended particulates.  It is better to design a system larger than is required, compared to smaller.

Screen filter. Screen or mesh filters are inexpensive and easy to install. Mesh filters work well if there are moderate to low contaminants in the water, such as those coming from a well or public water supply. Screen filters have a limited ability to store contaminants. If the water comes from a river or pond, the screens will probably have to be flushed often. This could result in considerable down time and labor.

Mesh screen sizes are between 20 and 200 mesh. The larger the number, the smaller the particle the screen will filter out. The screens are made from stainless steel, nylon, or polyester. Follow the recommendation of the tape manufacturer as to mesh.

Disc filter. A disc filter consists of a series of discs that are stacked on top of each other. The discs have microscopic grooves that radiate out from the center and filter out particles. Equivalent mesh sizes are between 40 and 600 mesh. They require less water for cleaning than do sand filters.

Sand filter. Sand filters are preferred over screen filters if the contaminant load is moderate to heavy. A sand filter can run longer than a screen filter before it needs to be cleaned by back flushing. This results in less down time and labor. The filters can be set up in pairs so that clean water from one filter is used to back flush the other filter. A screen filter can also be used to provide clean water for back flushing a sand filter.

The correct filter size is important. Under-sizing will increase pressure loss and there is considerable down time for cleaning. It is better to be too big than too small. The sand used for trickle irrigation should be of the correct type, made up of crushed, sharp edged silica or granite. 

Pipe/Mainlines. Mainlines can be metal pipe, PVC pipe or lay-flat hose. They deliver water from the pump to the submains and laterals. Proper pipe diameter is a function of the pumping rate and distance the water must travel. Larger diameters can move greater volumes and have lower friction losses, but are more expensive than smaller pipes. The longer the pipe and the more elbows or junctions, the more pressure loss due to friction. A qualified designer can help determine the appropriate pipe size for the system.

Care should be taken when laying the pipe to prevent soil or debris from getting into the system and clogging the lines. The lines should be flushed before the tape is connected to remove any dirt that got into the lines during installation.

Back flow preventers or check valves allow water to flow in one direction only. They are used to prevent water from flowing backward into a water source after the system is shut off. This is especially important to prevent injected fertilizer or pesticides from contaminating water supplies. They are required by law for systems with injectors. In some areas they may be required even if no injector is used. Vacuum-relief valves are installed to prevent soil from being sucked into the emitters when a vacuum is created after the system is shut-off.

Pressure regulators maintain the desired pressure as the water flows through the system. They are required to supply the appropriate pressure to tape and may be needed to protect filters and other components. They should be sized according to the rate of water flow. Drip tape typically operates at 12 PSI. 

Fertigation

Fertigation is the injection of soluble fertilizer into irrigation water. Nitrogen and potassium are available in liquid or soluble solid form and can be applied through a drip system. Phosphorus, if needed, is usually broadcast at the beginning of the season.

By using a fertilizer injector, trickle irrigation can be used effectively to apply N and sometimes K during the growing season. The need for supplemental N can be determined using the PSNT as it is with other application methods. Samples for the PSNT should be taken from under the plastic, if used. Use a soil sampler to punch a small hole in the plastic and remove a core of soil. Be sure to avoid cutting the irrigation tape when sampling under plastic.

With conventional topdressing or sidedressing, it is common to apply all the N in one or two applications. With trickle irrigation, it is convenient to apply small amounts of N weekly or even daily, which is desirable from a N management standpoint. For example, if you want to apply about 50 lb N per acre, you can inject a little over seven lb N per acre per week for seven weeks, or about one lb per day if you prefer. Small weekly applications provide for more efficient crop use of N than one or two larger applications. Daily application offers little advantage over weekly application, but may be necessary if the injector cannot inject a week's worth of N during the appropriate irrigation run time. To prevent leaching, the irrigation system should not run longer than necessary to effectively wet the root zone of the crop. If there is not enough time to inject all the fertilizer needed for the week in one injection, then smaller, daily injections are preferable. Before injecting fertilizer, the entire system should be filled with water at full operating pressure. When all the fertilizer has been injected, the system should run long enough to flush all fertilizer from the lines. If fertilizer is left in the lines, clogging may occur due to chemical precipitates or growth of bacterial slimes.

Water Problems

Certain fertilizer materials may react with chemicals in irrigation water. If the water pH is below 7.0, there is little potential for problems, but at pH 8.0 and above, the risk is high. At levels above 40-50 ppm, calcium and magnesium are likely to react with phosphorus, if present in the fertilizer, causing precipitation of phosphates. If fertilizer containing calcium is added to water with concentrations of bicarbonates above 2 meq/liter, calcium carbonate may precipitate. Sulfates in fertilizers can react with calcium in the water resulting in the precipitation of gypsum. These precipitates can clog emitters.

Phosphorus- and sulfate-containing fertilizers, if needed, should be applied before planting because we are not concerned about these leaching. Nitrogen is the element that is most appropriate for injection into trickle irrigation water. Calcium nitrate has the potential to cause clogging if the water pH and bicarbonate levels are high, as noted above. If calcium nitrate causes clogging, potassium nitrate or urea can be used as an alternative N source.

Water testing labs can analyze water for pH, calcium, magnesium and bicarbonates. You can also perform a simple test: Mix fertilizer into a container of irrigation water at the same concentration it will be after injection into the trickle system. Cover the mixture to exclude dust and let it sit for at least the length of time it will be in the system before it reaches the soil. If the water becomes cloudy or a precipitate collects on the bottom of the container, you can expect this to happen in the irrigation system with the likelihood of clogging. If it is necessary to lower the water pH, acid can be injected into the irrigation water. This requires special handling precautions and special injection equipment. Be sure to carefully follow directions to avoid personal injury or damage to crops or equipment.

Plastic mulch is generally 0.75-1.25 mils thick, 4-6 feet wide, in rolls 1,000-4,000 feet long. It is available in a multitude of colors ranging from clear (transparent) to opaque (black or brown). Recently, colored mulches have been investigated for their influences on insect control and plant yields. For example, reflective or silver mulches have been shown to reduce the incidence of onion thrips and aphids. Check with your local extension office for the most recent research findings proven to work in your area.

Plastic mulch functions to warm the soil, conserve moisture, and prevent nutrient leaching. It also protects ground-level fruit from soil pathogens. However, plastic mulch restricts rainwater from reaching to the roots. Therefore, drip irrigation should generally be used with plastic mulch. Clear plastic has the highest soil warming capability (8-14ºF over bare soil), but weed growth underneath can be extreme. An herbicide is necessary to keep weeds under control with clear mulch. Black mulch will prevent weed growth by prohibiting light transmittance to the soil and will warm the soil 3-5ºF over bare ground. On the other hand, white-on-black (white on the top) mulch is used to cool the soil.

Wavelength selective or near-infrared transmitting mulch (formerly referred to as IRT mulch, but now "IRT" is part of a trade name) is a "hybrid" of black and clear mulch characteristics. They are more expensive than conventional plastics. Specific pigments incorporated into the film during manufacture selectively block out blue and red wavelengths of light (which cause weeds to grow). This inhibits weed growth similar to black mulch. At the same time, infrared light is transmitted through the mulch warming the soil (similar to clear mulch). The wavelength selective mulches are generally brown or green in color. However, don't purchase them on color alone. The pigments embedded in the plastic impart these specific properties. Commercial recommendations are to lay wavelength selective mulches 7 days prior to transplanting. Within reason, the additional cost for this mulch film is compensated for by increased yields due to early soil warming. On small farms or in small fields, black, brown, or wavelength selective mulches are often the preferred way to eliminate the use of herbicides. This is a viable option for weed control on many organic farms. Crops that respond best to mulching are those that require higher soil temperatures (e.g. muskmelon, watermelon, cucumber, squash, tomato, pepper, okra, and sweet corn).

Apply plastic mulch after fields have been leveled and smoothed and fertilizer has been applied, and when there is good soil moisture (at or near field capacity, which is the amount of moisture left after a rain or irrigation event after surplus water has moved out of the root zone by gravity). In the case of black mulch, good uniform soil contact is essential as the soil is warmed by heat conduction. Commercially, the simplest way to apply mulch film is with a mechanical mulch layer. Plastic mulch can be laid flat against the ground or on raised beds. Raised beds offer additional soil drainage and early warming. Hand application is an option, but applying more than a half-acre can be difficult and time consuming.

Generally, plastic mulch is laid in the spring as soon as the land can be prepared. However, some spring seasons are wet and can delay normal land preparation and planting activities. An alternative is to lay plastic in the fall. Fall mulch application will require similar land preparation as in the spring, but use of a cover crop between the rows is recommended to prevent soil erosion. Oats will winter kill, but winter rye will need to be terminated by using an herbicide (such as Roundup or Gramoxone), or by mowing and cultivation.

After harvest, plastic mulches should be removed from the field and disposed of properly according to local ordinances on incineration and landfills. Alternatives to minimize disposal challenges of used PE are biodegradable mulch films and recycling programs to alleviate landfill accumulations. Recycling is very difficult to implement because mulches are dirty after field use, recycling facilities are limited, and it can be challenging to transport used plastic to recycling facilities. Soil and plant debris adhere to the mulch, adding up to 70% by weight and the presence of soil can abrade the recycling equipment. Research is ongoing to assess the potential for recycling the plastic into higher value products through pyrolysis and other chemical recycling methods that can accept some level of soil and debris in the used plastics.

Biodegradable Plastic Mulch

Degradable plastic mulch has been in development for decades. Some of the first commercialized products were photodegradable, and would break down when exposed to light. Many growers who used these products reported uneven and incomplete breakdown, particularly after tillage buried the plastic fragments at the end of the season. However, degradable mulches prepared from biodegradable polymers now exist. They are designed to be tilled into the soil after their service life, after which they will undergo aerobic biodegradation by soil microorganisms, producing CO₂, water, and microbial biomass.

The most widely available and studied biodegradable polymer is Mater-Bi, made in Italy by Novamont. Some mulches that use this polymer are Bio360 and BioTelo (Dubois Agrinovations) and BioAgri (BioBag Americas). Mater-Bi is made primarily from starches, cellulose, vegetable oils plus proprietary biodegradable complexing agents derived from renewable, synthetic, or mixed sources. While Bio360 mulch is approved for use on European organic farms, at this time no biodegradable plastic mulch is approved for use on USDA-certified organic farms. This is because currently available biodegradable plastic mulches have a maximum 25% biobased content while one of the requirements of National Organic Program is that the mulch must be completely biobased. Further, most commercially available biodegradable plastic mulches are produced through fermentation using genetically modified yeast and bacteria for increased productivity, and that is not allowed in US organic agriculture. US organic regulations do allow the use of synthetic (polyethene) mulches, but they must be removed from the soil at the end of the growing season.

Research has shown that the biodegradable plastic mulches performed comparably to polyethylene mulch in controlling weeds, raising soil temperatures and increasing crop yields despite some breakdown of biodegradable mulch during the growing season. Biodegradable mulch does not have a significant impact on soil quality. Research at Washington State University modeled five years of mulch degradation data from a field study and predicted the timeframe of 21 to 58 months for 90% degradation of biodegradable plastic mulch after tillage. As biodegradable mulch starts to degrade during the growing season, mulch adhesion to fruit surface can be an issue for heavy-fruited crops like pumpkin and watermelon, where fruits rest on the mulch for extended period. Up-to-date information can be accessed at the Washington State University Small Fruit Horticulture Research & Extension Program's Plastic Mulches page, https://smallfruits.wsu.edu/plastic-mulches/.

Biodegradable mulches can range from 2-3 times the cost of standard black plastic, but end-of-season labor and disposal costs are avoided. The mulch is thinner (it comes in 0.5-0.8 mil thicknesses) than typical black polyethylene (1.25 mil), and when starting to lay the plastic, extra care is required to prevent tears. When laying mulch, do not stretch as tightly as you normally would with black plastic. Applying in early morning when temperatures are cooler can help. The mulch starts to break down more quickly when stretched. Apply right before planting because the mulch will start to break down as soon as it makes soil contact. Buy what you need each year – do not try to store biodegradable mulch. The mulch can start to break down in storage, particularly if storage conditions are moist and/or warm. Store the mulch upright, on ends of rolls. The mulch can start to degrade or stick together under pressure of its own weight. Biodegradable plastic mulches undergo degradation even under ideal storage conditions and may perform best if deployed within 2 years of their receipt date.

WeedGuardPlus (Sunshine Paper Co.) is a brown paper mulch with soil-cooling properties. It is OMRI listed and is effective under low rainfall and low wind conditions. WeedGuardPlus is also effective in controlling nutsedge unlike polyethylene and biodegradable plastic mulches. However, it is more expensive than biodegradable plastic mulch. 

Row covers function to enhance growth and yield by modifying the temperatures around plants in the spring and fall, or in combination with low tunnels, during the winter. They are also used for frost, hail, and wind protection, and to exclude certain pests. There are two general types: slitted or perforated plastic, and spun-bonded fabric. Heavier weight row covers can provide several degrees of frost protection, while lightweight "non-heating" or summer weight covers offer less heat enhancement and can be used in summer for insect protection. These materials can be used with or without hoops ("floating row cover") depending on its weight and the fragility of the crop underneath. Newer types of knitted or woven lightweight row cover (for example, 'ProtekNet' by Agrinovations) are available; they can be used with or without hoops, are quite durable, and will exclude insects. 

Row covers are installed right after planting and are left covering the crop for several weeks, depending on crop type and season. For fruiting crops and cucurbits, covers can be left in place for approximately 3-5 weeks until pollination is needed or the crop outgrows the space under the cover. Other crops that are low-growing and do not require pollination can remain under cover as long as the temperature benefit is useful. Sweet corn may be left covered with spun-bonded row cover until pretassel stage. If the crop is pressing against the cover, either loosen or remove it. Row cover removal timing is more critical in some crops, such as tomato and pepper, as they cannot tolerate extremely high temperatures that might develop under the covers (especially polyethylene). Covers must be removed for crops requiring insect or wind pollination.

Slitted or perforated row covers are clear polyethylene films with slits cut or holes drilled to provide ventilation when the plastic loosens under hot conditions. Under cool conditions, the plastic is taut and the slits remain closed. Very little water condensation occurs under perforated plastic covers. There is generally less frost protection under slitted or perforated row covers than under a solid cover. 

Plastic row covers will require support with wire hoops. A piece of No. 9 wire cut about 65" long makes a hoop that is about 3' wide at the base and 14" tall in the center of the row after inserting each leg of the hoop in the soil. Secure the edges of the cover with soil.

If you have a diversified vegetable and/or berry operation, row covers can be a cost effective and convenient tool for producing early, high quality crops. Edges are usually held down with soil, soil-filled bags, boards, smooth saplings or tree limbs or rocks. Row covers provide sufficient growth enhancement by raising air temperatures during the day and moderating cold temperatures at night. They also allow light and water to penetrate to the crop. The result is earlier harvests, and in some cases, higher total yields. While lightweight row covers do not provide reliable frost protection, they may be helpful when temperatures drop 2-3ºF below freezing. Heavyweight covers can provide more frost protection, but they block much of the sunlight, resulting in slower growth. A key benefit of row covers is that, if they are sealed along the edges, they exclude a wide range of insect pests that can damage crops.

Which types to use? There are several different weights, measured in ounces per square yard, ounces per square feet, or grams per square meter. Materials that are 0.5-0.6 oz/yd² provide growth enhancement and insect control, have high light transmission (85-90%), and are less expensive than heavier materials, but are more likely to rip from wind and sharp objects (fingernails, boots, deer hooves, stakes, etc.). One can expect 2 seasons with careful handling. A row cover that is 0.9-1.25 oz/yd² is heavy enough to be more tear resistant and last several seasons, has somewhat lower light transmission (70%), and provides growth enhancement and some frost protection in spring and fall. The heaviest covers are 1.25-2 oz/yd², have lower light transmission (30-40%), are used mainly for frost protection or for overwintering, and are durable enough to last for several seasons when handled with care. Non-heating row covers are useful when an insect barrier is needed during the hot part of the season.

Support and fastening. Many crops can handle floating covers without any support, including lettuce, greens, crucifers, onions, potatoes, strawberries, sweet corn. Those with tender, exposed growing points (tomatoes, peppers, and vine crops) should have some support to prevent damage from wind abrasion. Wire hoops or short stakes with a smooth top to prevent tearing placed at 3- to 6-foot intervals provide good support. Secure the edges of the cover with soil, with soil-filled plastic bags, or with metal or plastic pins or staples. For holding the cover in place, soil is the most secure in high winds, but the edges are difficult to unearth after repeated wetting and drying, while soil bags make it easier to lift or move covers and prolong the life of the material.

Widths. Row covers can be purchased in widths ranging 3-60 feet and in lengths 20-2,550 feet. Wider covers are more labor efficient because they have less edge to bury per covered area - but don't try to lay them in a strong wind!

Weed control. Watch for weed growth under the cover because they provide a good environment for weeds too. Covers can be rolled to the edge of the bed for cultivation or herbicide application, and then replaced.

Storage. Row covers should be stored away from direct sunlight as soon as they are removed from the field. While many have been treated to reduce UV degradation, they will last longer if unnecessary UV exposure is prevented. Fold or roll covers in a systematic way so they can be carefully unfolded for next year's use. 

Insect control. Some insects overwinter in the soil where the crop was grown, and emerge in the next spring. In such cases, only use row covers on rotated fields. Also, seal the edges of the cover immediately after installation. If the cover is removed for cultivation, it should be done when insects are less active, such as on a cloudy day or in the morning.

Cabbage root maggot fly. This pest is a concern in spring or fall crucifer crops. Pupae overwinter in the soil wherever they fed on fall brassicas. First generation adults fly from April to May and lay eggs at the base of the crop stems. Maggots feed on roots and kill early cole crop seedlings. Immediately after planting, place spunbonded row covers in the field and seal the edges to keep cabbage maggots out. It is important to rotate crops as pupae can overwinter in the soil and flies may emerge under the row covers and damage the crop. 

Flea beetles. There are many different species of flea beetles, each with a specific host crop. Because they typically spend the winter as adults around field edges, they can be effectively excluded by row covers if covers are in place soon after planting. Crucifer and striped flea beetles are tiny, black or striped beetles which cause shot-hole feeding patterns on any of the cabbage family crops. Covers can be used with spring or fall transplants, or all summer on direct-seeded crops, but are too hot for transplants in midsummer. Potato flea beetle causes similar damage to eggplant, tomato, and potato. Corn flea beetles cause feeding damage but are primarily a concern because they vector Steward's wilt. Excluding beetles with row covers prevents infection of young corn plants.

Spinach leafminer and Beet leafminer. These are pests of spring spinach, beets, and chard. The adult black fly emerges from overwintering sites in the soil and lays small eggs in the underside of leaves. Maggots tunnel inside the leaf, making unsightly pathways that render greens unmarketable. Row covers prevent flies from laying eggs on the leaves.

Striped cucumber beetle. This is a pest of cucumber, melons, summer squash, winter squash, and pumpkins. Row covers prevent feeding damage and transmission of bacterial wilt vectored by the beetle. Remove when flowers appear to allow for pollination by bees.

European corn borer. Adults emerge in late May or early June and lay eggs on corn. If row cover is left on into mid- to late June, after flight is peaked, it provides excellent protection to corn. If removed just as flight starts (e.g., first week in June) the larger, healthy corn that was covered may be just as infested as corn that was never covered. Row covers can be left on until tassel if enough slack is left for 3 to 4 feet of stalk growth.

Colorado potato beetle. This insect moves into potatoes and eggplant in late May and early June. Row covers should be removed before tuber initiation, which usually coincides with flowering, to prevent excessive heat.

Potato leafhoppers (PLH). Adults migrate from southern states where they overwinter. Adults usually arrive, reproduce and damage beans, potatoes and sometimes eggplants in June and July, but may last until September. Feeding causes a symptom known as hopper burn, where tips and edges of leaves begin to yellow, curl and die back. Adults and nymphs hide on the underside of leaves. Row covers can be used on beans from emergence until bud stage or the start of bloom. If removed for bloom, damage can be avoided and yields maintained. Lightweight row covers can also be used on potatoes to exclude PLH, flea beetles and Colorado potato beetles.

Avoid sites with inconvenient access, excessive water, poor quality soil, high winds, or low light levels. Ideally tunnels have year-round access, even when crops are not being grown, to allow for snow removal and other maintenance. Existing or potential access to irrigation water is essential, and access to electricity is desirable for inflation fans and mechanical air movement. Some growers have made use of micro-solar power systems to support these loads. It’s desirable to have good access roads and be close to wash/pack facilities. When siting your first tunnel(s) keep in mind future tunnel locations, so that your “build out” over the years allows for efficient access, materials handling and potential for multi-tunnel heating systems, etc.

The site’s topography should allow for drainage of “worst case” storm water and snow melt away from tunnels. A relatively level site is important to minimize structural stress on the tunnel due to uneven snow load. Moderately breezy sites can be helpful for passive ventilation, but high-wind sites create risk of damage to structure and/or plastic covering. Trees can provide a windbreak but consider their future height when locating tunnels to avoid shading. Also note that dense hedge rows or locations too close to wooded areas can reduce passive ventilation.   

Tunnels should be slightly elevated compared to the surrounding soil in order to allow water running off the cover and drain away from the interior, and to allow snow melt to move away from the tunnel when the ground is frozen. On some sites it is advisable to create a raised pad for tunnels. Some growers install tile drainage, French drains, or curtain drains along the inside or outside of tunnels to carry excess water away from growing areas. Water running through/under a tunnel takes away soil heat, prevents good root growth, and can create muddy working conditions. Orienting tunnels along an east-west axis provides optimal light for winter production, and a north-south axis is best to avoid shading inside the tunnel in other seasons, though most crops will have more light than they can use in the summer.  If using primarily passive ventilation in a low wind site, it may also be worth considering the direction of the prevailing wind when orienting the tunnel.

Construction. Do not skimp on the structural integrity of tunnels, as this can lead to collapse in bad weather. Plan for extreme snow and wind. Gothic style tunnels will shed snow better than Quonset hut style structures. Well-set ground posts, cross-ties, and other features that anchor the tunnel and keep it rigid are essential. Doors and vents should close securely to prevent winds from opening them in storms and seal well to help retain heat. It is advisable to have a plan to lower and secure roll up sides for the winter or during high winds. When building a tunnel, avoid driving equipment over future growing areas, as this can create compaction. Installing large doors in end walls or having removable / roll-up end covers to allow for tractor access can make tillage and addition of bulk soil amendments easier than with small equipment. Head houses or other structures make sense for storing tools and equipment, seed, and potting soil, rather than taking up valuable growing space in the tunnel.

Zoning and codes. Before you build, contact your state and local agencies to find out about regulations and tax policies for high tunnels. Some states and towns may require building permits; setback requirements and building codes vary among municipalities. Some consider tunnels to be real property (subject to tax) and others do not. It may be helpful to be very clear with local officials that the structure is not permanent and is used for producing agricultural crops.

As in the field, tunnel crop production will benefit from deep, well-drained, fertile soil that is not compacted. On most sites, soil amendments such as compost, peat moss or coir will be desirable to increase the organic matter level to optimize tunnel production. Lime and nutrients should be added based on soil tests prior to production. On sites with poor native soil, compaction and/or drainage problems, soil can be imported either into the entire tunnel, raised beds, containers or by using ‘grow-bags’ of pre-fabricated media.

Since tunnel soils are not exposed to regular leaching from rainfall, soluble salt levels can build up over time negatively affecting plant growth. Salts dissolve into ions in soil solution and come from the application of fertilizers and composts or manures. Crops remove some of these salts in their tissues, but the excess remains in tunnel soils, unlike in the field. Strawberry, green beans, and certain herbs are very sensitive to salts, but even tolerant crops such as tomato and spinach can show reduced vigor with very high levels. Salts tend to accumulate especially in the top few inches of soil, as they move upwards with evaporation. This can affect germination of winter crops while transplanted crops such as tomato may be more tolerant to high salt levels.  Deep tilling will remix those salts into the soil profile. Salt injury can be exacerbated if soils are allowed to dry out. Excessive salts can be reduced by diluting with the addition of peat moss, coir or topsoil. Irrigating with a large amount of water can move salts down in the soil profile, but is often impractical. Removing the plastic cover over winter is perhaps the easiest way to leach salts out of the root zone. Soil tests can be used monitor the buildup of salts over time.

Because high tunnels specifically and protected culture more generally increase heat in the soil and air, the season is extended and yields are increased. This leads to obvious questions about whether soil fertility information from field crop research can be used to effectively guide high tunnel crop fertilizer application. Tunnel tomato fertility recommendations in this guide have been updated based on yield goals. While similar updates for other tunnel crops are not available, soil tests, tissue tests and observations of nutrient deficiencies can help fine tune nutrient applications in tunnels.

Air exchange in tunnels is essential to avoid high temperature, high humidity, and low CO₂ in tunnels, leading to plant stress or disease. Passive ventilation using roll-up sides is common in tunnels, though some tunnels use mechanical ventilation with fans to pull air through the tunnel. Generally, you must pick one or the other or use them at different times. Fans pull from the point of least resistance, so running an end-wall fan with the sides rolled up simply pulls air from around the corner, not from the other end of the tunnel. When sizing fans for ventilation the basic rules of thumb are 8 CFM/ft² (of growing space) for summer cooling and 2 CFM/ft² to remove humidity during cooler months. Note that this guidance is for peak ventilation needs.  Staged fans (e.g., one small, one large) or variable speed fan controls can help moderate the ventilation for various times of the year.
Passive ventilation is less than ideal in locations where tunnels are crowded together, there are lots with trees or other significant wind breaks, or in calm sites with little wind. A dense crop canopy later in the growing season also reduces passive ventilation.  

Installing a ridge-vent (along the top peak of the tunnel roof) will greatly increase the effectiveness of passive ventilation, though these can be costly and they make installation of plastic cover more complicated. Some growers who have ridge vents have installed “cat walks” to ease maintenance.  These can make plastic replacement and repairs easier. Gable vents high up on end walls can also improve ventilation by acting as outlets for warm humid air in warmer seasons and by allowing for low volume ventilation in colder weather. A 24″x24″ gable vent on each end wall is recommended for a 30′x96′ tunnel. These can be made of plywood and manually operated with hinges, ropes or cables and tie-downs. If using a louvered vent, be sure it has a flanged seal to close against. Thermostatic wax cylinder actuators may also be used which require no electricity, are relatively inexpensive and are passively controlled by the wax cylinder based on temperature.

HAF Fans. Horizontal air flow (HAF) fans are hung from the inside horizontal structural tubing to mix the air inside a tunnel to create consistent growing conditions, they don’t improve ventilation.  They are for circulating and mixing the air inside the tunnel. When installed and used properly, they ensure that plants and any control sensors are seeing the “average” conditions of the space. The first fan should be placed about 10′-15′ from one end wall to pick up the air that is coming around the corner from the other side. Subsequent fans should be located 40′-50′ apart to keep the air mass moving. In a 30′x100′ greenhouse, four fans are required, and the total fan capacity should be 6,000 CFM (2 CFM/ft²). The “empty” corner, where there is no fan can sometimes become a spot without air flow. Check to make sure you can feel air flow in all locations in the house.  You may have to add fans or reorient the ones on the end to promote adequate mixing flow. If a tall crop such as tomatoes is grown or if there are hanging baskets, a slightly greater capacity is needed to overcome the additional air flow resistance. Small, 1/10-1/15 horsepower circulating fans work well in providing the air movement needed. A permanent split capacitor motor can save as much as one-third the electricity of the more common shaded pole motor. Some growers have used inexpensive, simple box fans and just plan for frequent replacement. High efficiency vane-axial fans can increase the “throw” of each HAF fan meaning you need fewer fans to provide the same mixing flow.
 

Typical high tunnel covering is greenhouse grade 6 mil polyethylene rated for 4-6 years. Using two layers, separated by air blown between the layers, reduces heat loss during cold season production and provides stability under windy conditions, reducing damage to the plastic. Solid plastic “spacers” are available to separate two layers of plastic in locations without electricity.  Some greenhouse plastics have additives to enhance durability and performance. UV stabilizers are essential to slow degradation of plastic. Anti-fog and anti-drip surfactants make water condense and run down to the sides of the structure, rather than bead and drop on the plants below. IR radiation-blocking additives reduce heat loss at night. UV-absorbing films have the potential to suppress certain insects and diseases. In summer, plastic may be covered with shade cloth to reduce light intensity and temperature. Shade cloth is rated by the percent of light blocked. Whitewash is also used by some growers to help keep tunnels cool in summer. 

When it’s cold outside, growers may want to heat the air and/or soil inside a tunnel, and this can be done using permanent heating systems, or emergency systems for coping with unusual conditions. Whatever system is used, a temperature warning system is important to provide notification when heating (or cooling) is urgently needed. To determine what size heater is needed, one must calculate the heat loss when a certain minimum temperature is desired inside the tunnel when there is a certain outside temperature. More information on heating, ventilation, and other engineering issues can be found at the University of Massachusetts greenhouse and floriculture web site and on the University of Vermont Extension agricultural engineering blog.

Water must be provided to replace that lost by evapotranspiration (ET), the combination of soil surface evaporation and water loss from plant leaves. Drip irrigation is an efficient way to deliver water and nutrients in a tunnel, while keeping the foliage dry, which reduces disease pressure. Drip tape is usually 8-10 mil thickness and is laid on the surface or buried an inch or two. Flow rates of drip tapes vary, a medium–flow tape provide 0.5 gpm per 100 feet. High-flow tape with 1.0 gpm flow is useful to prevent clogging and reduce irrigation time. Drip lines should be spaced on or under the soil surface to assure that the entire bed or row is wetted. Light-textured soils have less capillary movement of water than heavier soils, so in these soils, more drip lines may be needed to prevent dry areas in the bed. Many systems are available to add soluble fertilizer to irrigation water. Fertigation is a good way to provide plants with the nutrients they need over the season, or to supplement fertilizers applied at planting. Irrigation water quality should be tested. Water with high pH and high alkalinity may lead to increased soil pH over time.

A mature crop of tomatoes may require 2.5 quarts of water per plant per day, whereas winter greens may grow well only on existing soil moisture, and if irrigated, may develop disease. Determining the optimal amount and timing of irrigation is complicated since it depends on the crop, its stage of growth, soil texture, sunlight, temperature and humidity. Use of soil moisture sensors placed at several locations and depths in each tunnel, combined with irrigation and crop performance records, can help determine best practices on your farm. 

The tunnel environment differs from the field, so the type and timing of insects and diseases also differs. For example, spider mites and aphids are more frequent pests in tunnels than outdoors, and foliar diseases in tunnels are typically due to humidity levels. The use of biological insect controls is more practical in tunnels than outdoors due to the (at least partly) enclosed space, high crop value, and extensive information developed for many crops. See the University of Vermont’s high tunnel pest management web site for more information. Maintaining the area around the perimeter of the tunnel with mowing and trimming is a passive way of minimizing mammalian pests by reducing cover.

Pesticides. Outdoors, pesticide residues break down after application by exposure to ultraviolet radiation and rainfall. Inside tunnels, plastic coverings reduce UV light and rain, and as a result, pesticides break down differently. Each state’s pesticide regulatory agencies may have different interpretations of whether high tunnels are considered open fields or greenhouses, however it is safest to consider a high tunnel a greenhouse from the perspective of pesticide labels. The label may 1) specifically state that the product may be used in greenhouses and may provide different guidelines for greenhouse and outdoor use, 2) specifically state that the product may not be used in greenhouses, or 3) may not mention greenhouse use at all. The Environmental Protection Agency’s current position is that a label does not have to specify greenhouse as a site, provided the crop is on the label, in order to use the product in a greenhouse. If the label has multiple sections, and one of those sections is for greenhouse application, then the label must be followed explicitly for greenhouses with no exceptions. The rate for outdoor applications on those crops is for outdoor use ONLY and CANNOT be used for those crops in the greenhouse, since those crops were not included in the greenhouse section of the label. Using a pesticide inside a greenhouse where the label does not mention greenhouse use can increase risks to workers or plants. Also, when using a fumigant or smoke generator for an entire greenhouse, every crop in the greenhouse must be listed on the product label. We advise against applying a product in high tunnels unless the label specifically allows its use in greenhouses.

Adapted from Knott's Vegetable Grower Handbook, The New England Vegetable Growers Yield Guide Table, United Fresh Fruit Market Containers and Weights, The Boston Fresh Vegetable Wholesale Market and Arrivals Report, and the USDA-National Agricultural  Statisitics Service New England Vegetable and Strawberry Report (2014-2018).

Abbreviations for Vegetable Yield Table

• bun. = bunch
• ct. = count
• qt. = quart
• bu. = bushel
• doz. = dozen
• lb. = pound
• pt. = pint
• wt. = weight

NOTE: To convert yield per acre to yield per 100 feet of row:  multiply yield per acre by the number of feet between rows and divide by 4356.

* Yields vary depending on soil quality, weather conditions, farm management, location, etc. Low, Good and Excellent yields are based on national data, while 5-year averages (2017-2021) are from the USDA National Agricultural Statistics Service New England Vegetable Report and the USDA NASS New England Berries, Tree Fruit & Grapes Report. 

Information on optimum temperatures, relative humidity and storage life was adapted from USDA Handbook 66 and modified by experience under northeastern conditions.