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.

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 top 6" of soil, there are 1000 lb of N per acre. Thus, a soil with 3% organic matter contains about 3000 lb 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 top 6" of soil. This mineralization is not constant throughout the growing season, however. A flush of N is normally mineralized in late spring with lower rates of mineralization occurring during the growing season. Moisture conditions greatly influence mineralization, with the highest rates when the soil is well-aerated and near water holding capacity. Small flushes of N are 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 can be conservatively estimated as 10 lb of available N per acre for each 1% of organic matter in the top 6" 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

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 2 or 3 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. Nitrate leaching accounts for the vast majority of N losses from cropland. 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 the times when leaching potential is greatest. This is why nitrate remaining in the soil after the harvest of annual crops in early fall is particularly susceptible to leaching. Fall cover crops can take up this residual N and prevent it from leaching. The N can 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 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, nitrate leaching 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, as 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 deficiencies in the plants may develop. Immobilization takes place when highly carbonaceous materials such as straw, sawdust, or woodchips are incorporated into the soil, which stimulates microbial growth. The larger microbial population then absorbs a larger amount of available N. 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 uptake. For example, good sweet corn crops may uptake over 150 lb N per acre annually. Anticipated crop uptake of N is one of the factors used in making N fertilizer recommendations. Depending on the crop, variable amounts of the N taken up by the crop are returned to the soil after harvest in non-harvested plant parts. With sweet corn this can be as much as 100 lb N per acre with 50 lb N per acre harvested with the crop. 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' refers to applying fertilizer to a crop any time after planting. In popular usage, topdressing often refers to a broadcast application of fertilizer made after planting, compared to 'sidedressing', which refers to fertilizer applied as a band along the side of the row of a growing crop. Sidedressing is commonly done immediately before or during cultivation, which helps reduce N volatization losses when a urea-containing fertilizer is used.

Sidedressing with relatively soluble N fertilizer is an important component of efficient N management. The N accumulation pattern for annual crops is very similar to the crop growth pattern (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 sidedressing also allows growers to use the pre-sidedress soil nitrate test (PSNT) to help determine N needs.

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 1980s, 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 lb 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, for sweet corn, when the soil nitrate-N level is above 20–25 ppm there is rarely an economic response to the application of sidedress fertilizer N. Based on research and experience in New England, New Jersey, and New York, a threshold of 25–30 ppm is 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

Samples for the PSNT should consist of a well-mixed composite of 10–20 cores or slices of soil to a depth of 12". 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.

Phosphorus

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 lb 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. In soils with pH above 7.3 or below 5.5, P will bind with Ca and Al, respectively, making it 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

Potassium (K) is measured as potash (K₂O), similar to the way P is measured as P₂O₅. The K requirements for different crops vary 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

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 6.

Magnesium

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 (22% S, 11% Mg, 22% K) can be used. Blended fertilizer containing Mg can also be ordered.

Sulfur

Sulfur (S) is most often added to soil using calcium sulfate (gypsum), which contains about 17% S, Sul-Po-Mag (22% S), or potassium sulfate (17% S). Elemental sulfur (90% S) is mainly used to lower the soil pH for acid-loving plants but supplies substantial amounts of sulfur as well.

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. However, 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 micronutrients 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 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

Soil health (or soil quality) has been defined as the capacity of a soil to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation over a human time scale (thousands of years). Similar but distinct from soil health, soil fertility can be viewed as a function of the biological, physical, and chemical characteristics of soil that supply plant nutrient requirements. 

Holistically, a healthy and fertile soil must have: good structure and drainage, sufficient depth for root growth, sufficient (but not excessive) nutrient availability, small weed, insect pest, and plant pathogen populations, large populations of beneficial organisms, no toxins, and resilience to adverse conditions. 

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 contributes to soil structure and increases water holding capacity. Finally, SOM supports soil biology as a substrate for the growth of soil microbes. 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. The other component is active SOM, which is relatively undecomposed and is accessible to soil microbes for mineralization. It is important to maintain both types of SOM (see Building Soil Organic Matter).

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 (i.e. compounds that contain carbon) into carbon dioxide and their mineral constituents (including inorganic, plant-available forms of nutrients). Soil microbes are most active in warm soils (over 70°F) that are moist, well-aerated, and have a pH between 6 and 7, which are 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 abundant soil microbes. On average, there is a release of about 10-20 lb N per acre for each 1% soil organic matter over a full season. This is capped at about 4% or 40 lb per acre. Nitrogen contributions from SOM can be subtracted from the total amount of fertilizer-N recommended on soil tests, though because these releases of N vary with drainage and other soil conditions, they may not be well timed to crop needs, especially for early, short season crops. 

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. As microbes decompose SOM, they produce sticky compounds such as gums, carbohydrates, and resins, which bind soil particles together into secondary aggregates. These aggregates bolster 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 occurring simultaneously. 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 (i.e., soil oxygen levels), 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. SOM can be increased by including sod-forming crops in the rotation. A more rapid and direct method of increasing SOM is by adding carbon-containing amendments, such as mulches and compost. While the application of these 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_c/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.

'Base saturation' is the percentage of negatively charged sites on soil colloids that are filled with basic cations. Of the cations that account for the vast majority of those adsorbed on the soil colloids in New England soils—calcium (Ca++), magnesium (Mg++), potassium (K+), hydrogen (H+), and aluminum (Al+++)—Ca++, Mg++, and K+ are considered basic cations and have little to no influence on soil pH, while H+ and Al+++ are considered acidic cations because they tend to lower soil pH. If all the cations in a soil were basic and none were acidic, there would be a 100% base saturation and the soil pH would be close to 7 or neutral. In acid soils there is some amount of acidic cations adsorbed on the soil colloids—called the 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%.

At one time, many labs provided fertilizer recommendations to achieve very specific ideal Ca++, Mg++, and K+ saturation ratios. This approach was never well supported by data. Research conducted over the last several decades indicates that an ideal basic cation ratio does not exist and fertilizing to achieve a prescribed level of these nutrients is unjustified. Still, base saturation can provide useful information. When base saturation values reported on soil tests are well outside of ranges typically observed in New England soils, it is usually associated with deficient or excessive potassium or very acidic or alkaline soil conditions. Following the fertilizer and lime recommendations provided with your report will generally result in base saturation values within normal ranges. 

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 between 6.5 and 6.8, which is the ideal range for most vegetable crops. Scab-susceptible potato varieties require a lower pH of 5.0–5.2 to prevent scab development, but some lime may still be needed to maintain the recommended pH range. 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 in acidic soil. 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 below 6.0, bacterial activity is reduced and fungal activity increases. Acidic soil conditions also reduce the effectiveness of some pesticides and limit the ability of legume cover crops to fix N. 

The most effective way to manage soil acidity is to apply agricultural limestone, which is composed primarily of calcium carbonate. 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. Pure calcium carbonate is the standard and is assigned a rating of 100%; all other liming materials are rated in relationship to pure calcium carbonate. Suppliers can tell you the calcium carbonate equivalence of the lime you are purchasing. 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 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 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, 1–2 years or more. Agricultural ground limestone contains both coarse and fine particles. About half of 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 to adjust soil pH quickly 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 incorporation. 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 (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 and expected removal of nutrients from the soil by crops. See the following section, Removal of Nutrients from the Soil, and the section on Soil Testing to better understand nutrient recommendations.

Table 4 lists approximate 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 values can be used as a basis for adjusting your own application rates up or down on a trial basis. 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

Soil test methods used for vegetable production are designed to provide a measurement of soil pH and nutrient availability. Different 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, and manure, biosolids, compost, greenhouse media, and irrigation water testing.

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. Different extractants can extract different amounts of the same nutrient from the same soil. This means that soil test values depend on the extractant used and labs give fertilizer recommendations based on the type of extractant used. The two extraction solutions used for routine soil analysis 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. New England labs also offer a saturated media test, which uses water as an extractant to measure immediate 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, depending on the lab. 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.

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 Quality, or Soil Health test packages as well; check with your local lab. 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 to 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. 
 

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 nutrient (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.

A critical step in soil testing is sample collection. 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 field 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. 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 6 to 8 weeks after a lime or fertilizer application. Routine soil analysis should be conducted once every 2 to 3 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: 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. In general, the goal should be to maintain nutrient elements within the optimum range as reported on a soil test. When nutrient levels are within this range, the needs of most crops will be met, and crop response to application of a given nutrient is unlikely. Some amount of the nutrient may still be recommended to replace that which is removed by the crop and maintain soil test levels. If levels are below optimum (very low or low), increasing levels to optimum will usually improve production and provide a return on fertilizer investment. If levels are above optimum, application of that nutrient is very unlikely to result in a crop response and may attract pests and/or cause environmental harm.

Nutrient recommendations in this Guide are intended to meet crop needs and provide enough to build soil test levels to the optimum range over several years. 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. 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, squash, 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, sidedressing, or fertigation. Prior to making nutrient corrections, other potential issues, such as incorrect pH, inadequate soil moisture, root disease, or insect infestation should be addressed first. Leaf tissue analysis requires collecting a representative, adequately sized sample 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. Labs will present the results from the sampled tissue alongside reference nutrient sufficiency ranges 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

George Leigh Minor Plant and Soil Health Center
Soil Nutrient Analysis Laboratory
27A Manter Rd., Unit 4102
Storrs, CT 06269-4102
Telephone: 860-486-4274
Website: https://plantsoilcenter.cahnr.uconn.edu/ 
Email: soiltest@uconn.edu

State Laboratory - The Connecticut Agricultural Experiment Station
123 Huntington St., P.O. Box 1106
New Haven, CT 06511
Telephone: 203-974-8521
Website: https://portal.ct.gov/CAES/Soil-Office/Soil-Office/Soil-Testing-Offices-Instructions 
Email: Gregory.Bugbee@ct.gov 

Maine

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

Massachusetts

UMass Soil & Plant Tissue Testing Laboratory
203 Paige Lab
161 Holdsworth Way
Amherst, MA 01003   
Telephone: 413-545-2311
Website: http://soiltest.umass.edu 
Email: soiltest@umass.edu

New Hampshire

UNH Cooperative Extension Soil Testing Service
34 Sage Way Durham, NH 03824
Telephone: 603-862-0509
Website: https://extension.unh.edu/agriculture-gardens/pest-disease-growing-tools/soil-testing-services 
Email: soil.testing@unh.edu

Vermont

UVM Agricultural & Environmental Testing Laboratory
262 Jeffords Hall, 63 Carrigan Drive
Burlington, VT 05405-1737
Telephone: 802-656-3030
Website: https://www.uvm.edu/extension/agricultural-and-environmental-testing-lab
Email: AgTesting@uvm.edu

Fertilizer grades refer to the guaranteed percentages of plant nutrients in that fertilizer. The ratio refers to the proportion of nitrogen (N), phosphate (P₂O₅) and potash (K₂O) in the fertilizer. For example, if the grade is 5-10-5, 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.

Starter fertilizer promotes early and rapid growth that leads to greater yields with certain crops such as tomato, pepper, and melon. Liquid starter fertilizers are materials that are completely water soluble and are high in phosphorus, 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 can burn seeds and young seedlings. 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 Table 10 in the Guidelines for Organic Fertility Management section. 

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

Table 6a: Conversion Factors for Boron Nutrient Sources

Animal manure is an excellent source of nutrients and organic matter. Many of the nutrients in fresh livestock manure, especially nitrogen (N), are readily available to crops. Manure can be used fresh to make use of available N, or composted to stabilize nutrients and reduce risks from human or plant pathogens or weed seeds (see Compost section below). Nutrient content varies by animal species, their diets, and the form of their manure. 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 nutrient analysis for a specific batch of manure 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 manure is left on the soil surface and not incorporated, the more N will be lost as ammonia (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 to a growing cover crop when temperatures are cold and wind speeds are low. A growing cover crop also reduces manure run-off and leaching losses.  NOTE: Manure may contain human pathogens. Serious illness may occur 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, if cow manure is applied at high rates (30–40 tons per acre) for several years, the N furnished from previous manure applications increases substantially. The amount of fertilizer-N and subsequent manure applications needed for crops will decrease annually.

Comparatively, a higher percentage of the total N in poultry 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 leach from the soil and pollute groundwater and coastal seawater.

Table 7: Nitrogen Credits from Manure Applied Before Planting

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 released 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 can affect plant growth adversely because the compost-making microbes may compete with the crop for nitrogen. Applying compost at least 1 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 lb of nitrogen per acre can result in excessive amounts of nitrate in the soil.

Phosphorus. There is not much research published on the availability of phosphorus (P) from compost. The few papers published show that composts made primarily from manures supply P 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 P 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

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 1 part 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 that will be used as part of greenhouse media 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 scab-susceptible 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 compost 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 and soluble salts. 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 6–8 weeks after application before testing the soil to allow the compost and soil to equilibrate. 

In addition to fertilizers, manures, and composts, there are several other types of commercially available soil amendments and plant nutrient sources. 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. 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, i.e., derived from biological sources. It does not necessarily indicate approval for use in certified organic production. Materials included in this category include processed slaughterhouse wastes, leather processing waste, biosolids, papermill sludge, as well as 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 nutrient 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. Products labeled for pest control purposes do not fall into this category.

Rhizobium inoculants for legumes are one well-known type of these products. 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). While extensive research has shown their beneficial effects on plant nutrition, growth, and stress reduction in field, nursery pot, and greenhouse conditions, it is still unknown whether inoculation has short- or long-term economic impact in annual vegetable production. The fungi live inside plant roots, where they obtain a carbohydrate energy source from plants. The fungal mycelia grow out from the plant roots into the soil and extract water and mineral nutrients, which are then transferred, in part, to the plant.

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. 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. 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. There is surprisingly little published research on their use and effectiveness in field settings. The nutrient content of seaweed is too low to directly increase soil test nutrient levels but it has been proposed that seaweed has several different effects on the rhizosphere; one common claim is that seaweed extracts alleviate the effects of environmental stresses, such as temperature and moisture extremes. The effect of seaweed applications are hard to detect when used during typical conditions and published research has not yet proven these effects.

There is no simple method to accurately schedule irrigation events since all the above factors interact to determine the actual rate of evapotranspiration and crop water needs. If you wait until crops are wilted to irrigate, the crop will already be damaged. The following items should be kept in mind when deciding when and how much to irrigate, even if you are using the tools and techniques discussed immediately following. 

1. Soils vary greatly in water-holding capacity and infiltration rate. Silt and clay soils and soils high in organic matter can hold much more water than sandy soils low in organic matter (Tables 13 and 14). Soils with a high available water holding capacity need the same amount of water as those with a lower-water holding capacity, but less frequent irrigation. However, when irrigated less frequently, a greater amount of water should be applied per application since fully wetting the root zone is important for root development. 
2. Water loss from plants and the soil surface is much greater on clear, hot, windy days than on cool, overcast, humid days. During periods of hot, dry weather, when the crop is at full canopy, the rate of evapotranspiration or plant water use may reach 0.3 inch/day or about 2” per week. 
3. Research shows that irrigating to maintain soil moisture levels in a narrow range, just below field capacity (60%–80% available soil moisture), results in better crop performance than if the range is broader. Soil moisture monitoring is therefore a more accurate way to determine irrigation needs.
4. Plastic mulches reduce evaporation from the soil but also reduce the amount of water that can reach the root zone from rain. Thus, a large percentage of the natural precipitation should be discounted when scheduling irrigations for crops grown using plastic mulch.
5. On moderate moisture-holding capacity soils, apply 0.25–0.75 inches of water per irrigation event. This will ensure that water reaches active areas of the root zone. The exception is during early crop growth and establishment when lower rates may be appropriate. With sandy soils, daily irrigation applying only what can be used that day is best. Splitting or pulsing the daily application with a 2+ hour break between applications has shown benefits, particularly in very coarse soil.
6. If irrigation water has a high salt content (for example, wells in coastal aquifers or tidal streams), excess water should be applied during each irrigation event to leach any salts before they are concentrated by evaporation. It is necessary to regularly measure the salinity of tidal surface water to prevent crop damage.
7. Total weekly water needs for vegetable crops will increase to full canopy and decrease thereafter. Irrigation rates should be adjusted accordingly. Critical crop stages such as fruiting, or tuber bulking should also be considered in determining weekly irrigation rates (see Table 13). 

Scheduling Irrigation Based on Soil Moisture Levels 

Hand-Feel Method: This is the easiest and cheapest method to determine soil moisture levels for irrigation scheduling. Soil samples are collected using a soil probe or shovel, and the moisture level is estimated by “feeling” the soil and comparing it to known conditions. This method can allow for multiple depths of samples and is not susceptible to equipment failures; however, it does require an experienced operator to get consistent results. Charts are available to describe how different soils should look and feel at different moisture contents. See USDA- NRCS guide for Estimating Soil Moisture. 

Water Budget Method: The water budget method is an accounting process of daily water use and rainfall inputs. Plant water use is estimated daily based on crop development and climate conditions and is compared to the soil’s water-holding capacity. Often, pan evaporation is used to keep track of water losses. An evaporation pan is literally a pan with a gauge in it to determine how much water has evaporated, which is correlated with plant evapotranspiration.

Assume that the rooting depth of a crop is 8 inches on a sandy loam, high-organic-matter soil. After a soaking rain or after irrigation, the soil should hold about 1.2 inches of water (8 x 0.15 inches) (calculated using data from Table 14). Irrigation should begin when available water is depleted by 50%, or 0.6 inches in this example. If evapotranspiration removes about 0.22 inches of water per day from the soil, then after 3 days 50% of the available water is depleted, and water is required. With the water budget method, moisture loss is monitored and continuously calculated, and water is replaced based on the calculations. 

Tensiometers: Tensiometers are inexpensive tools for determining irrigation frequency because they indirectly measure water available in the crop root zone. Tensiometers are glass or plastic tubes filled with water, hermetically sealed with a porous ceramic tip at one end that gets submerged in the soil and a vacuum gauge at the other end. As the soil dries, its capillary action will try to suck the tensiometer water through the ceramic tip creating a vacuum. This vacuum is a measure of soil tension, “soil suction” or “matric potential.” Soil tension is a measure of how tightly water is held in the soil and is measured in pressure units of centibars (cb) or kilopascals (kPa). These are different units of measurement of the same condition: soil vacuum. The soil tension measured with tensiometers is an indirect indication of soil moisture content and can be used as an indicator of irrigation requirements.

Table 16 contains guidelines for using soil tension data to schedule irrigation events. Field capacity, the moisture content at which a soil is holding the maximum amount of water it can against the force of gravity, corresponds to soil tension levels ranging from 5 to 10 cb in coarse-textured soils and as high as 40 cb in fine-textured soils. The soil tension range corresponding to the time when irrigation should begin is also influenced by soil texture. In coarse-textured soils, irrigation should begin at soil tensions of 20 to 40 cb. In extremely coarse-textured soils, irrigation may be necessary at even lower tensions. Conversely, medium- and fine-textured soils do not need to be irrigated until soil tensions reach higher values. For all soil types, irrigate when a maximum of 50% of available water has been depleted. Lower depletion allowances may be used depending upon specific crop and management needs.

Table 16: Irrigation Guidelines for Tensiometers 

The utility of tensiometers in fine-textured soils is limited due to the range of detection. When soil dries beyond the 80 cb tension level, the column of water in the tensiometer "breaks," allowing air to enter the device. After breaking tension, the device ceases to operate correctly until it is serviced. Thus, tensiometers are the most practical in sandy or coarse-textured soils where normal soil tension levels are well below the point of breaking tension. In sandy soils it is often desirable to use ½-bar gauges that read in the 0–50 cb range rather than the standard 0–100 cb.

When locating tensiometers in the field, 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 irrigating). 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.

Resistance Meters - Granular Matrix Sensor: Another type of soil moisture sensor gaining popularity in the Northeast is the granular matrix sensor, which is categorized as a resistance meter. 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 (cb). This measure of resistance can be used to better understand the force a plant root must overcome to extract water from a given soil. 

Resistance sensors are calibrated for each soil type. As noted previously, this is because different soil types have varying levels of plant available water at various soil moisture readings. The irrigation chart (Table 15) can be used as a guide for when to water. In most soils, other than heavy clay, the decision to irrigate would generally happen in the range of 30–60 kPa. Prepare and install sensors according to manufacturer’s recommendations. Locate resistance sensors and meters in a similar manner to tensiometers to give accurate information on soil water depletion.

Volumetric Soil Moisture Sensors: Volumetric soil water sensors such as TDR (Time Domain Reflectometry) and FDR (Frequency Domain Reflectometry) sensors can also measure soil water accurately. They require power sources to operate (battery, solar, wired) and are typically much more expensive than tensiometers and resistance blocks. For irrigation scheduling, sensors at various depths and locations in the field are installed and monitored. Soil moisture is recorded as volume of water per volume of soil. This then can be related to the available soil water percent based on a specific soil type by calibration to produce a soil water curve.

Water Supply Considerations 

While irrigation is extremely important to successful production, the cost of irrigation is largely dependent on an adequate and dependable supply of good-quality water. The water source should be as close as possible to the area to be irrigated to minimize the pumping and supply line cost. Ponds, lakes, streams, springs, groundwater, and municipal water are all potential water sources.

The legal right to withdraw water for irrigation must be verified. Water rights in the eastern United States are called riparian, or landowner, rights because anyone is entitled to use any water associated with land ownership. However, a riparian owner’s rights are not absolute and are subject to reasonable use interpretations. Some states have water use registration and reporting requirements when a certain number of gallons of water are used, and a permit and/or an annual pumping report may be required. Contact your state environmental services department for information on water use regulations, especially if you plan to pull a significant quantity and/or the source is shared with neighbors or other farms.

Water sources must be able to provide water as often as it is needed, though this is becoming increasingly difficult for many farms, as many previously reliable water sources are becoming more precarious. In extreme cases of drought, some commercial growers have even needed to buy in or transport water from other locations. Streams and springs may not provide enough water during the summer when irrigation is needed most.

Sometimes municipal water systems allow connections for irrigation purposes. This option may be expensive if water-use prices are high. If the municipality enforces water rationing, it may not permit irrigation during droughts.

Seasonal Water Demand

Seasonal water demand is the total amount required by the planting for the entire growing season. Natural rainfall can contribute to supplying the seasonal water demand, but because it is unpredictable, supplementation from artificial sources is necessary. In the Northeast, field vegetable production on average requires 1–2 inches of water per week, or 25–30 inches per season. The warmer the annual temperatures, the higher the demand. Calculating a water budget, or the required amount of water needed for your specific crop production mix, allows you to determine whether your water supply can support your crop mix and helps you to make production decisions relating to crop mix, placement and potential to ration water, in a low rainfall year. 

Water Quality

Water quality should be assessed when choosing a water source. Water quality includes its physical, chemical, and biological constituents. 

Physical constituents refer to sand, silt, or other suspended materials in water. While the physical constituents are usually not damaging to vegetable crops, they can damage or clog the irrigation system. A high sand content, for example, damages pumps and sprinklers, and suspended materials can clog trickle systems. Surface water sources, such as ponds and rivers, usually contain more particles and should be evaluated carefully, especially if drip irrigation is being considered. 

Chemical constituents of water refer to pH, dissolved material, proportions of dissolved ions, and any organic compounds such as oil. Although generally not a problem, organic solvents or lubricants in the water can damage plants. If water pH is high (7.0 or above), consider lowering it to prevent nutritional problems in the field. Furthermore, certain pesticides can be deactivated by high-pH water, so it is important to be aware of the water pH. See Fertigation Section below. 

Biological constituents such as bacteria and algae are often present in surface water and in some wells. These bacteria are usually not harmful to crops, but they can affect irrigation system performance. See the section on Drip Irrigation Maintenance and Challenges. When planning water usage for vegetable production, it is also important to consider that human pathogens (e.g., Escherichia coli (E. coli), Salmonella, Campylobacter, and Listeria) can be spread through irrigation water, causing foodborne illness. See Agriculture Water in the Produce Safety section. 

A well-designed irrigation system will match water supply and distribution capacity with crop water requirements throughout the season. It is recommended to consult with an irrigation system design specialist before investing money into a new system or equipment upgrades. The information below gives a broad overview of the irrigation equipment used by New England farms. Sprinkler and drip irrigation systems have common components, such as the pumping unit, control head items (e.g., filters, pressure gauges, check-valves), and piping. Differences between systems are just the physical characteristics and the equipment that ultimately applies the water, either sprinklers or emitters.

Pumping Units: The pumping unit, which consists of a pump coupled to a power source, draws water from the supply source and pressurizes it for delivery through the irrigation system. A centrifugal pump or a submersible or deep well turbine pump may be used. Normally, a centrifugal pump cannot be placed more than 20 feet above the water line. Pumps are available in a wide range of flow capacities and delivery pressures. Selecting the correct style of pump and the proper size will depend on site characteristics and the final irrigation system layout. 

Since sprinklers usually have higher discharge and pressure requirements, pumps for sprinkler systems are generally larger than those for drip systems. This is one reason why drip systems are more energy-efficient than sprinkler systems. When growers tie into municipal or household systems, where water is already pressurized, a pumping unit is typically unnecessary. The primary concern is that the supply (volume and flow rate) is large enough. Pumping and pressurizing both surface and ground water can be expensive and should be incorporated into the pricing of the system design. 

Control Head: The control head is the combination of items that control, measure, or treat the water. Control heads can range from simple, manually operated heads with a single valve to quite sophisticated heads with automatic controllers and sensors, water meters, pressure regulators, filters, and nutrient or chemical injection equipment. Control heads for drip systems are often more complex because of the necessary water filtration equipment. Automated equipment can be adapted to some systems to measure soil moisture, start the irrigation system, and send water to the appropriate areas. The more complex and automated the control head and irrigation system, the greater the initial capital investment.

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

Backflow preventers or check valves: These devices 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. 

Filters: The choice of filter is based on the quality of the water passing through the system and application requirements. For drip irrigation, 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, disc, and sand filters are appropriate for various irrigation systems.

• Screen or mesh filters are inexpensive and easy to install. Mesh filters work well if there are moderate to low levels of contaminants in the water, such as with 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. For drip tape, follow the mesh size recommendation of the tape manufacturer.
• 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 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 drip irrigation should be of the correct type, made up of crushed, sharp edged silica or granite. 

Piping: Mainline pipes deliver water from the pump to submains (which are not always needed) and laterals. Mainlines can be metal pipe, PVC pipe, or lay-flat hose. 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. Lateral pipes deliver water from the mainline or submain lines to the sprinkler or emitters. They are of the same 3 general types as the mainline pipes but are usually smaller. Lateral pipe sizes are designed to minimize pressure losses so that sprinkler or emitter discharge at the far end of the lateral stays within 10% of the sprinkler or emitter discharge near the mainline to provide uniform water application. 

Overhead/Sprinkler Systems

The basic types of sprinklers include rotating sprinklers, stationary spray-type nozzles, and perforated pipe. Rotating sprinklers, such as the slowly rotating impact-driven sprinkler, are most common. Fixed spray nozzles are becoming more popular, although they are most widely used in landscaping. Perforated pipe is the simplest sprinkler, consisting of a pipe with numerous holes through which the water sprays. 

Rotating impact sprinklers come in many sizes and variations to meet various design conditions. Some micro-sprinklers operate at pressures as low as 10 psi, while the large gun types require pressures exceeding 80 psi. Sprinkler discharge ranges from a few gallons of water per minute to 1,000 gallons per minute for a big gun. Wetted diameters can range from only a few feet to several hundred feet. When irrigating new plantings, avoid big guns with high pressures. 

When selecting an irrigation system solely to provide water to the plants, use a high discharge rate with large droplets to minimize evaporative loss and the time that foliage is wetted, but do not use such a high rate as to cause puddling in the field. 

Pressure, discharge rate, and wetting diameter are the most significant characteristics of a sprinkler; but nozzle size, jet angle, wind speed and direction, sprinkler overlap, and sprinkler rotation speed are also important. These characteristics determine water application rates, sprinkler spacings, and water droplet sizes. For uniform water application, sprinklers are generally spaced so that 50%–60% of their wetted areas overlap. Annual operating costs of irrigation increase with increasing pressure and discharge requirements. Large gun-type sprinklers will require up to twice as much fuel or electrical energy as smaller sprinklers to apply equal amounts of water.

Drip Irrigation

Drip (or trickle) irrigation is used on a wide range of vegetable crops. Drip irrigation is a method of slowly applying small amounts of water directly to the plant root zone. Water is applied frequently, often daily or several times a week, to maintain favorable soil moisture conditions. The primary advantage of drip irrigation systems is that water use is more efficient than with overhead sprinkler irrigation systems. In many cases, one-half or less of the water applied with sprinkler or surface systems is required with drip systems because there is no evaporation loss from the soil surface. Most of the water conservation from drip irrigation occurs in the early season. The difference in the amount of water used between drip and sprinkler decreases once a full canopy is achieved. 

Drip irrigation systems have several other advantages over sprinkler and surface irrigation systems. Low flow rates and operating pressures are typical for drip systems. These characteristics lead to lower energy and equipment costs. Once in place, drip systems require little labor to operate, can be automatically controlled, and can be managed to precisely apply the amount of water needed by the crop, which also reduces operating costs. With most drip systems, disease and insect damage is reduced because leaves are not moistened by irrigation water. In addition, the areas between rows remain dry, which reduces weed growth and water use, as well as pests and pathogens in these areas of the field. Another advantage is that field management operations can continue during irrigation.

While there are several advantages to drip irrigation in vegetable production, various items also should be considered during design, installation and use. 

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. 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.

Drip tape selection: 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. 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 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 2–3 years, but the labor costs of retrieving and cleaning the tape usually make this uneconomical for annual vegetable crops.

Drip tape placement: 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. 

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 6”–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. When possible, 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.

Drip Irrigation Maintenance and Challenges: The equipment used in drip systems can present potential problems. Insects, rodents, and people can damage drip irrigation equipment. Pressure regulation is critical, and filtration is required. The drip system, including the pump, headers, filters, and connections, must be checked and ready to operate before planting. Failure to have the system operational could result in costly delays, poor plant survival or irregular stands, and reduced yield. Drip systems cannot be used for frost control. 

Emitter clogging remains the most serious problem with drip irrigation systems. As noted earlier, clogging can be attributed to physical, chemical, or biological contaminants. Filtration and occasional water treatment may be necessary to keep drip systems from clogging. Bacteria can grow inside drip irrigation tubes and form a slime, known as a biofilm, that can clog emitters and can serve as a source of plant and human disease-causing pathogens. Algae present in surface waters and in high iron wells can also clog emitters. Biofilms and algae can be effectively controlled by treating the lines with antimicrobial pesticides. Periodic treatment before clogging develops can keep the system functioning efficiently. The frequency of treatment required depends on the quality of the water source. 

Irrigation water containing high concentrations of iron (greater than 1 ppm) can also result in clogging problems due to the types of bacteria that "feed" on dissolved (ferrous) iron. The bacteria secrete a slime called ochre that may combine with other solid particles in the drip tape and plug emitters. The precipitated (ferric) form of iron, known commonly as rust, can also physically clog emitters. Treating water containing iron with an antimicrobial product (such as chlorine) can oxidize the dissolved iron, causing the element to precipitate so that it can be filtered and removed from the system.

Fertigation 

Fertigation is the injection of soluble fertilizer into irrigation water. Nitrogen (N) and potassium (K) 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. The need for supplemental N can be determined using the Pre-sidedress Soil Nitrate Test (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, being sure to avoid cutting the irrigation tape.

With conventional topdressing or sidedressing, it is common to apply all the N needed by a crop in one or two applications. With drip 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 7 lb N per acre per week for 7 weeks, or about 1 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. 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.

Fertigation and Water Quality Parameters

Various water quality parameters can present challenges for effectively fertigating through drip lines. 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. If the water contains calcium and magnesium at levels above 40–50 ppm, they are likely to react with phosphorus, if it is 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 also react with calcium in the water resulting in the precipitation of gypsum. These precipitates can all clog emitters.

Phosphorus- and sulfate-containing fertilizers, if needed, should be applied before planting because leaching is not a concern. 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 and will likely cause 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. Selecting the right mulch and installing it properly is essential for maximizing its benefits. Plastic mulches are available in several colors, but black is the most widely used, especially in temperate climates. The color of plastic mulch affects soil temperature and weed suppression.

Table 17: Plastic Mulch Types and Characteristics

Red, blue, green, or yellow mulches are used as they produce distinct radiation that reflects into the crop canopy. However, research has shown inconsistent results from colored mulches, and they are not widely used in commercial production except for green mulches in California strawberry production.

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). When using 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, traditional polyethylene (PE) mulches should be removed from the field and disposed of properly according to local ordinances and available waste-management options. Recycling of used PE mulch remains difficult because the mulch is often heavily contaminated with soil and plant debris, which can make up a large fraction of the recovered material by weight. As a result, relatively little agricultural mulch film is recycled, and much of it is landfilled or stockpiled. Incomplete retrieval can also leave persistent plastic residues in soil.

Biodegradable Plastic Mulch

Some early products marketed as "biodegradable" were photodegradable or oxo-degradable materials that fragmented under sunlight but did not reliably achieve complete biodegradation in agricultural soil. Current soil-biodegradable plastic mulches (BDMs) are different; they are designed to be tilled into the soil after their service life, and under aerobic conditions, soil microorganisms convert the material into carbon dioxide, water, and microbial biomass.

The term “biodegradable” is often misunderstood. A mulch may biodegrade in one environment, such as industrial compost, but not adequately biodegrade in agricultural soil. For that reason, biodegradability claims should identify the disposal environment and the standard used to support the claim. Commercial BDMs are typically engineered blends of biodegradable polymers, often including starch-based materials and biodegradable polyesters. Their field performance and degradation rate depend on product formulation, thickness, soil conditions, temperature, moisture, and management after incorporation.

The most recent research shows that BDMs generally provide weed suppression, soil warming, moisture conservation, and crop yield benefits comparable to PE mulch in many specialty crop systems, although performance varies by crop, season length, product, and local climate. Multi-year studies have generally reported limited or transient effects on soil physical, chemical, and biological indicators under tested conditions. However, degradation is strongly site- and product-dependent, and measurable residues can persist for multiple seasons after incorporation. Repeated-use studies suggest that reaching 90% degradation may take roughly 21–58 months, depending on product and field conditions, with slower degradation expected in cooler or drier soils.

As BDMs deteriorate, visible fragments may disappear before complete mineralization occurs. In some cases, macrofragments break down into smaller fragments before full biodegradation. For this reason, the disappearance of visible mulch should not be interpreted as proof of complete biodegradation.

Although the USDA National Organic Program added “biodegradable biobased mulch film” to the National List in 2014, no commercially available soil-biodegradable or compostable plastic mulch is currently allowed for use in U.S. certified organic production. Current products do not meet some of the National Organic Program requirements. Growers should confirm acceptability with their certifier before using any mulch product marketed as “organic.” 

Biodegradable mulches initially cost about two times more than standard black PE mulch, but end-of-season labor and disposal costs are avoided. Cost-effectiveness depends on seasonal labor, disposal fees, and mulch price. 

Biodegradable mulch is thinner (0.5–0.8 mil) than typical black PE mulch (1–1.25 mil), and extra care is required to prevent tears when laying it. When laying biodegradable mulch, do not stretch as tightly as you normally would with polyethylene mulch, as it breaks down more quickly when stretched. Applying in early morning when temperatures are cooler can help. 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. 

BDMs should be incorporated into the soil soon after harvest, and incorporation should promote good soil-mulch contact in the biologically active tilled zone. Degradation is generally faster in warm, moist, well-aerated soil and slower when fragments remain on the surface, are buried too deeply, or soils stay cold or dry.

Paper mulch is another option, and is most commonly used in organic systems. It can suppress weeds and biodegrade in the field, but it generally has a shorter service life than plastic mulches, can tear during installation, and may lose effectiveness quickly in wet or windy conditions or when mechanically disturbed.

Before purchasing, verify that a product meets soil-biodegradability standards or certification requirements appropriate for agricultural soil. EN 17033 and ISO 23517 are the most relevant current product-level frameworks for soil-biodegradable mulch films. Laboratory methods commonly used to assess soil biodegradation include ISO 17556 and ASTM D5988. ASTM D6400 and ASTM D5338 are composting standards and should not be used alone to support soil-biodegradability claims for field incorporation.

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 polyethylene, and spun-bonded fabric. In addition, fine-mesh insect netting is now commonly used in vegetable production for pest exclusion; unlike spun-bonded row cover, insect netting provides little or no frost protection and much less heat retention. 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 their weight and the fragility of the crop underneath. Newer types of knitted or woven lightweight row cover or insect netting (e.g., '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 or seeding 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.

A unique system using clear plastic has been developed for sweet corn, allowing earlier planting, improved germination, and maturity 5–12 days earlier. The corn is planted in two rows 16" apart, with seeds in shallow trenches and soil mounded between rows. Plastic is removed when plants reach 6–12 inches tall or sooner if temperatures under the plastic exceed 100°F, typically at 80–85°F ambient temperature. Floating fabric row covers can also be used successfully for longer periods, and are particularly effective for "super sweet" or "shrunken-2" corn varieties.

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. Compared with spun-bonded row covers, clear polyethylene low tunnels provide greater daytime heating.

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. Edges of the cover should be secured with soil.

In diversified vegetable and/or berry operations, 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, sandbags, or rocks. Row covers provide sufficient growth enhancement by raising air temperatures during the day and moderating cold temperatures at night. Spun-bonded materials also allow rain and overhead irrigation to pass through. 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.

A frequent mistake made when using row cover is allowing excessive heat buildup. Typically, if outside air temperatures reach around 80°F, it's advisable to consider providing additional ventilation. During unseasonably warm years, covers may end up causing more harm than good and should be removed shortly after application. When excess heat is a concern but insect exclusion is still needed, insect netting is often a better choice than spun-bonded row cover.

Which Types to Use? There are several different weights, measured in ounces per square yard, ounces per square foot, or grams per square meter.

• Lightweight: 0.45 oz/yd², allow 90% light transmission, mainly for insect control. Also used in heat-sensitive crops as it provides only a minimal heat increase during the day. Less expensive than heavier materials, but more likely to rip from wind and sharp objects (fingernails, boots, deer hooves, stakes, etc.). They last two seasons with careful handling.
• Medium weight: 0.5–1.0 oz/yd², allow 70%–85% light transmission and give 4–6°F for frost protection, for spring, fall, and overwintering crops. 
• Heavyweight: 1.5–2.2 oz/yd², allow 30%–50% light transmission and give up to 8°F of frost protection for overwintering crops. They are durable enough to last for several seasons when handled with care. 

Support and Fastening. Many crops can handle floating covers without any support, including lettuce, greens, crucifers, onions, potatoes, strawberries, and sweet corn. Those with tender, exposed growing points (tomatoes, peppers, and cucurbit 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, soil-filled plastic bags, or 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.

Sizes. Row covers can be purchased in 3–60' widths and 20–2,250' lengths. 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. Row covers provide a good environment for weeds too; watch for weed growth under row covers. 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 the following 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. Scout under row covers regularly for trapped pests, weeds, disease, and excess heat.

Cabbage Root Maggot. This pest is a concern in spring and fall brassica 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 crop stems. Maggots feed on roots and kill young seedlings. Immediately after planting, place spun-bonded row covers in the field and seal the edges to keep cabbage maggot flies out. It is important to rotate crops as pupae 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 brassica crops. Covers can be used with spring or fall transplants, or all summer on direct-seeded crops, but are too hot for transplants in mid-summer. Potato flea beetle causes similar damage to eggplant, tomato, and potato. Corn flea beetle causes feeding damage but are primarily a concern because they vector Steward's wilt. Excluding beetles with row covers prevents infection of young corn plants.

Allium Leafminer. This relatively new invasive pest attacks many allium crops and is of greatest concern in leeks and scallions. In the Northeast, allium leafminer typically has two flights per year, with one in mid- to late April through May and a second in September into early October. Adults leave a distinct line of white oviposition scars on leaves, and larvae mine the foliage and later move toward the base of the plant. Row covers or insect exclusion netting can effectively exclude flies if applied before flight begins and sealed securely along the edges. Because pupae may be present in crop residue or soil, covers should only be used if allium crops have been rotated to a non-infested field, or if residues have been removed from previously infested ground. 

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 on 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.

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 the flight has 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 3–4' 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 Leafhopper. Adults migrate from southern states where they overwinter. Adults usually arrive, reproduce, and damage beans, potatoes, and eggplants in June and July, but may last until September. Feeding causes a symptom known as hopperburn, where tips and edges of leaves turn 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.