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

The Nitrogen Cycle

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

 

Figure 1. The Nitrogen Cycle

 

Nitrogen Inputs

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

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

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

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

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

Table 1: Nitrogen Credits from Previous Crops

Previous Crop	Nitrogen Credit Lb N per acre
"Fair" clover (20%-60% stand)	40-60
"Good" clover (60%-100% stand)	60-90
"Fair" alfalfa (20%-60% stand)	60-90
"Good" alfalfa (60%-100% stand)	100-150
"Good" hairy vetch winter cover crop	120-150
Grass sod	20-40
Sweet corn stalks	30

 

Nitrogen Losses

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

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

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

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

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

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

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

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

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

Topdressing and Sidedressing Nitrogen

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

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

Figure 2. Generalized nitrogen accumulation curve for annual crops

Pre-sidedress Soil Nitrate Test (PSNT)

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

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

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

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

CROP	SOIL SAMPLING TIME FOR PSNT	SIDEDRESS lbs n per ACRE2
Beets	After thinning (2-4 leaves)	30
Cabbage, Brussels sprouts, broccoli	2 weeks after transplanting	60
Cauliflower	2 weeks after transplanting	30
Celery	2 weeks after transplanting, again 3-4 weeks later	40 twice, 3-4 weeks apart
Cucumber, muskmelon	Before vines are 6" long	40
Eggplant	3-4 weeks after planting, again 3-4 weeks later	30-50
Lettuce, endive, escarole	2 weeks after transplanting or after thinning (2-4 leaves)	30-50
Pepper	3-4 weeks after planting, again 3-4 weeks later	50, and 40 later at fruit set
Spinach	2-4 leaves, again after first cutting	30
Sweet corn	when plants are 6-10" tall	60-90
Tomato	3-4 weeks after planting, again 3-4 weeks later	30 twice, 3-4 weeks apart

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

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

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

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

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

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

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

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

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

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

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

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

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

Table 3: Relative Tolerance of Vegetables to Boron

Tolerant	Semitolerant	Sensitive
artichoke asparagus beet broad bean carrot parsley spinach tomato	bell pepper broccoli cabbage cauliflower celery corn lettuce muskmelon pea potato pumpkin radish sunflower sweet potato turnip	bean cucumber garlic Jerusalem artichoke lima bean pea

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