Nitrogen

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. The key to successfully managing N is to determine the relatively narrow range between too much and too little – this is not an easy task. Having an understanding of the forms of N in the soil and the factors that influence its behavior 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, as illustrated in figure 1, 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 cycle.

Figure 1. The Nitrogen Cycle

 

Nitrogen Inputs

As can be seen from the N cycle, there are two forms of the N used by plants: ammonium (NH4) and nitrate (NO3). In addition to commercial fertilizer sources, available N may be added to the soil through mineralization (the microbial conversion of organic N to ammonium and then nitrate) of soil organic matter, manure and other organic residuals.

Nitrogen in soil organic matter: Organic matter contains the largest pool of soil N, usually comprising more than 90 percent 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” or 7” 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, but can be large in some years relative to the amount of N needed for plant growth. Research has shown that for most soils 2% to 4% of the total organic matter N is converted (mineralized) annually to forms plants can use. This is roughly equivalent to 20 to 40 lb of available N per acre for each 1% of organic matter in the surface 6” or 7” of soil. A soil with 8% organic matter content, therefore, will mineralize, or make available to plants, 160 to 320 lb N/acre from the organic matter. This amount of N would be sufficient for most vegetable crops in a relatively dry year with little leaching or denitrification. This mineralization is not constant throughout the growing season. A flush of available N is mineralized in late spring with lower rates of mineralization occurring during the season. Moisture conditions will greatly influence the mineralization during the season, with high rates when the soil is near water holding capacity in a well aerated soil, and lower rates when the soil is dry. Small flushes of N will be mineralized when soils are re-wetted during the season. The rate of mineralization is dependent on microbial activity, especially bacterial activity. 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.

Nitrogen in manures and other waste products: The N content of manures is highly variable. Differences in N content 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. The values in Table 1 are based on numerous analyses of Connecticut manures as well as published data from other states. If specific manure analysis data for the farm are 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 ammonia (NH3) which is volatile. The following manure application methods are listed in order of being most effective to least effective for reducing ammonia volatility: Tine injection, disc injection, immediate incorporation after surface application, band spreading with trailing hoses or trailing shoe without incorporation, broadcast application with incorporation. Broadcast application of slurry manure without incorporation should be avoided at all times because this method increases air to ammonia 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 conserved if applied during cold temperatures, low wind speeds and especially to a growing cover. A growing cover also reduces manure run-off or leaching. NOTE: Manure often contains human pathogens. Serious illness has occurred from eating produce where fresh manure was applied without an adequate waiting period (see Food Safety).

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

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

Table 1: Nitrogen Credits from Manure Incorporated Before Planting

 

Time(s) of Application

 

April/May1

Fall Only2

Other Times3

Manure

  lbs N/ton  

Dairy (cow)

 

 

 

  Solid

5

2

3

  Liquid

16

18

12

Poultry, cage layer

     

  fresh (20-40% D.M.)4

16

5

8

  sticky-crumbly (41-60% D.M.)

22

7

11

  crumbly-dry (61-85% D.M.)

32

10

16

1 "April/ May" credits are for manure applied and incorporated in April and/or May for spring-planted crops and for manure applied and incorporated within four weeks of planting at times other than spring.

2 Use "fall only" values for manure applied in no-till or maintenance situations where the manure is not incorporated.

3 "Other times" means times other than April and/or May or fall only for manure applied for spring-planted crops. "Other times" also means any combination of times from fall through May other than April and May for spring-planted crops. Examples: March, February, March and April; November, April and May.

4 Dry matter.

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 appreciable 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 2). 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 2: Nitrogen Credits for Previous Crops

Previous Crop

Nitrogen Credit
Lb N per acre

"Fair" clover (20-60% stand)

40 to 60

"Good" clover (60-100% stand)

60 to 90

"Fair" alfalfa (20-60% stand)

60 to 90

"Good" alfalfa (60-100% stand)

100 to 150

"Good" hairy vetch winter cover crop

120 to 150

Grass sod

20 to 40

Sweet corn stalks

30

Synthetic  fertilizers: Fertilizers used to supply N include urea (46-0-0), diammonium phosphate (DAP: 18-46-0), monoammonium phosphate (MAP: 11-48-0), ammonium nitrate (not readily available), urea-ammonium nitrate solution (UAN: 32-0-0), calcium ammonium nitrate (this has generally replaced ammonium nitrate), calcium nitrate, potassium nitrate and various manufactured and blended fertilizers such as 15-8-12, 15-15-15 and 10-10-10. In bulk blended or custom blended mixes, N-containing fertilizers with almost any grade can be provided.

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 that 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 cover crops following cash 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 causing eutrophic conditions.  

Denitrification Losses: These losses occur when nitrate is converted to gases such as nitrous oxide (N2O) and nitrogen (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 tend to 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 the 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, it is a powerful greenhouse gas. The impact of 1 pound of nitrous oxide on atmospheric warming is over 300 times greater than 1 pound of carbon dioxide. Agricultural activities account for over 70% of nitrous oxide emissions in the US. 

Immobilization: Immobilization occurs when soil micro-organisms 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 soil via crop harvest. For example, good sweet corn crops may remove over 150 lb of 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/A. 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 preventing it from being lost via leaching or denitrification.