Concern about nitrate nitrogen (NO3-N) and its effect on water quality has greatly intensified in recent years. The public has become aware of the potential health consequences of consuming high-nitrate drinking water, and this concern has led the government to initiate a much stronger regulatory system for domestic water supplies.
The U.S. Environmental Protection Agency (EPA) has set a maximum contaminant level (MCL) on water used for human consumption of 10 parts per million (PPM) of NO3-N. Although nitrogen (N) fertilizers represent only one of several possible sources of NO3-N in water, the tendency has been to blame commercial fertilizer use for high NO3-N levels in water.
With this background of public concern and political pressure, agricultural scientists are evaluating their experiments and experience about the efficient use of fertilizer N for crop production and its effect on water quality.
To manage soil NO3-N effectively, it is important to understand how N is lost from soil. With this knowledge, it is possible to develop cropping systems and N-use patterns that minimize losses of soil N to groundwater. The following discussion provides the best information currently available about this situation in Kentucky.
There are very few situations where available N is sufficient in soil for N-requiring crops. Most Kentucky soils cannot supply enough available N to provide economic levels of production of high-N-requiring crops such as tobacco, corn, small grains, and grain sorghum. Because of this, farmers commonly use commercial N fertilizers, animal manures, or cover crops to meet the needs of these crops.
Use of commercial N fertilizer has increased in Kentucky in recent years from about 100,000 tons N in 1970 to about 183,000 tons N in 1989. It is this source of N that has created the most public concern about NO3-N in water.
When N-containing materials are applied onto or into soils, certain generalized reactions take place that influence the relative availability of the N applied for crop uptake. These reactions are:
These processes are dynamic in nature, continually taking place to varying degrees, depending on environmental factors affecting any particular field.
Mineralizationthe process of transforming N from organic forms to inorganic N in soiloccurs in a number of steps. Organic matter decomposes by breaking down complex organic molecules to smaller and more soluble inorganic ones such as ammonium (NH4) and NO3.
Soil animals (fauna) perform much of the initial mechanical breakdown of plant residues. Then, soil microorganisms (microflora) attack the remaining materials and carry out enzymatic decomposition. Since soil fauna and microflora are living organisms, they are greatly affected by environmental factors. They also have relatively short life-spans. At death, they, too, are decomposed by other microbes. Through this process, organic forms of nitrogen in soil are converted to NH4 and NO3-N.
Ammonification is a part of the mineralization process in which the amide form (the form of N in proteins and many other organic N compounds) is changed to NH4.
Most soil microbes carry out mineralization and ammonification under a variety of conditions. Both aerobic and anaerobic organisms carry out mineralization using energy derived from carbon (C) contained in the decomposing organic material. Because of this, adding organic materials such as plant residues to soil stimulates microbial activity and, thereby, mineralization. Otherwise, mineralization in soil proceeds at a slow, constant rate, dependent largely on temperature and moisture.
Whether the decomposing material is from organic matter already present in the soil or from added plant residues or animal manures, many factors affect the rate of decomposition. Among these are the chemical nature of the organic matter, age and species of plant material, particle size of plant residue, N content of the residue, the C:N ratio of the residue, and type and amount of clay minerals present.
Generally, the materials that decompose most rapidly are soluble organic materials with simple molecular structures, young leguminous plants, and residues of high N content and low C:N ratio. Large amounts of clay tend to lower the rate of decomposition.
The ideal conditions for mineralization include:
The soil NH4 produced in this manner can then be held by the soil or absorbed by plants, but most NH4-N is subsequently converted to NO3-N.
Nitrification is the reaction that creates NO3-N from NH4-N. In most soils, NH4-N is oxidized to NO3-N soon after NH4 is formed by ammonification. Ammonium added in fertilizer undergoes this same nitrification process.
The microorganisms carrying out these reactions are able to meet their energy needs by oxidizing NH4-N. They incorporate C into their cells by using only carbon dioxide from the air. They do not require organic C. They are rather specific for these reactions. Whereas the general soil population of microorganisms may carry out ammonification, only a few species carry out nitrification and only under aerobic conditions.
Ideal conditions for rapid nitrification usually include warm temperature (75º to 85ºF), well-aerated soils, pH between 6.0 and 8.0, and soils with good water and nutrient levels. Soils containing organic matter with good fertility, good water content, and warm temperatures will produce NO3-N under natural conditions.
Excessively wet, cold, or acid soils do not contain much NO3-N. During the process of nitrification, acidity is created as NH4 is converted to NO3-N. Consequently, when large amounts of organic materials or NH4 fertilizers are added to soil, the soil pH will decrease because of nitrification.
Immobilization occurs when soil microbes assimilate plant-available N into their bodies or cells. Most typically, this happens when a large amount of crop residue with a high C:N ratio (such as wheat straw) is mixed with the soil. The addition of high C residues stimulates the growth of such a large population of microorganisms that the soil supply of NO3 or NH4 is incorporated largely in their bodies.
Because of this, crops being grown at the same time that the high C, low N residue is decomposing will suffer from a lack of N, at least temporarily. This possible N deficiency can be averted by mixing such residue with soil well ahead of the next cropping season.
Soil N that is immobilized is not lost from the soil. It is retained in the body mass of the immobilizing bacteria and in humus. After being immobilized, it slowly reverts to plant-available N through mineralization.
Even without the use of commercial fertilizers or manures, soils contain some residual level of NO3-N, largely due to mineralization and nitrification of plant residues on or in the soil. If plant residues are high in organic N content, residual soil NO3-N will be higher than that in soils containing residues of lower organic N content. In Kentucky, the amount of residual NO3-N has been shown to be related to the parent geology of soils.
Streamflow content of NO3-N from several watersheds varying in geologic origin and land use is shown in Table 1, and it demonstrates this relationship. The Cave Creek streamflow is much higher in NO3-N than that of the other streams. The soils of this watershed, in the inner Bluegrass area of Kentucky, were developed in highly phosphatic limestone of the Ordovician Age. Apparently, the high phosphatic content of these soils results in better growth of N-fixing plant species (legumes). Residues from legumes result in higher residual soil NO3-N content.
|Table 1. Nitrate-Nitrogen in Kentucky Streams in 1972 and 1990.|
|Stream||County||Land Use||Avg. 1972||Avg. 1990|
|Cave Creek||Fayette||Pasture; 2% tobacco||4.48||4.12|
|Flat Creek||Franklin||Pasture; 54% wooded||0.49||0.17|
|Plum Creek||Spencer||Pasture, corn, tobacco||1.08||0.63|
|McGills Creek||Lincoln||75% wooded; 25% pasture, corn||0.48||0.54|
|W. Bays Fork||Allen||20% wooded; 80% pasture, corn, tobacco||0.64||0.82|
|Rose Creek||Hopkins||Corn, soybeans, hay; 5% wooded||2.74||1.20|
|Perry Creek||Graves||Corn, soybeans, hay||0.93||1.12|
Except for the relatively small amount that exists as NH4 and NO3 (1% to 2%), all of the N in soils of humid regions is contained in and is an essential part of the organic matter or humus. Thus, the N content of a soil is indicative of the humus content and vice versa. Nitrogen constitutes about 5% to 6% of the soil's organic matter by weight. The N in soil results from biological fixation and from accumulation of partially decomposed plant residues over a long period.
For mineral soils of the United States, the approximate range of total N content in the surface 6 inches is 0.02% to 0.50%; most cultivated Kentucky soils range from 0.05% to 0.10% N (1000 to 2000 lb/acre), only a small portion of which becomes available during a growing season. Since N is contained largely in humus, which forms from plant residues, its concentration decreases with soil depth.
Climatic factors influence the N content of soils through the effects of temperature and water supply on the growth of plants and activities of soil microorganisms. Production of plant material, and consequently plant residues and the resultant soil N content, generally is greatest in areas of highest rainfall; plant production decreases as amounts of annual rainfall decrease. However, soils in areas with higher temperatures have lower organic matter content than those in areas with lower temperatures, regardless of rainfall. Plant residues decompose more rapidly in warmer temperatures.
Over narrow geographical areas, the texture of the soil affects soil N content, which is generally higher in soils with finer texture. Therefore, clayey and silty soils would be expected to have more N than sandy soils.
These factors may be responsible for the variation in soil N content with texture: differences in water-holding characteristics or in drainage, aeration, or fertility and the tendency for lignin and certain clay minerals to form complexes with organic N materials.
Sources of fertilizer N used in Kentucky are listed in the following discussion with a brief explanation of how they react when applied into or onto soils. Of these sources, ammonium nitrate, urea, anhydrous ammonia, N solutions, and NH4 in ammoniated phosphates are the most widely used.
Although most of the mixed fertilizers used in Kentucky contain some N, the amount varies considerably, depending on the grade. Most of the N in mixed fertilizer is present as urea or NH4. Diammonium phosphate and monoammonium phosphate are also commonly available N-containing fertilizers, with all the N being in the NH4 form.
Fertilizers that contain only N are sometimes referred to as straight N fertilizers. In recent years, the tonnage of these straight N fertilizers has increased rapidly in Kentucky. They are marketed in solid and liquid form. Nitrogen materials commonly sold in Kentucky are discussed below.
Ammonium nitrate (NH4NO3), a solid N fertilizer, contains 33.5% to 34.5% N. Half the N is in the NH4 form, and half is in the NO3 form. Ammonium nitrate dissolves rapidly in the soil. A large proportion of the NH4 initially reacts with the negatively charged soil particles before being nitrified to NO3, while the NO3 remains in the soil solution and moves with the soil water.
Urea, CO (NH)2, contains 45% to 46% N in the solid form. When applied to the soil, the enzyme urease quickly converts urea N to NH4-N. Consequently, urea behavior in soil is essentially that of NH4, except for the potential for some ammonia volatilization loss (loss from the soil surface as a gas) unless urea is mixed into or is in contact with the soil.
Nitrogen solutionsAlthough the N content of commonly available N fertilizer solutions ranges from 28% to 32%, 28% N solution is used in Kentucky mostly because of its low salt-out potential. Half the N is from ammonium nitrate, and half is from urea in the N solutions most commonly used for direct soil application.
Anhydrous ammonia (NH3) is the highest analysis N fertilizer available. It contains 82% N. At ordinary temperatures and at atmospheric pressure, it is a gas. For this reason, it must be kept under pressure to stay in liquid form, which is necessary for transportation, handling, and application.
When anhydrous ammonia liquid is released from the pressure in the applicator tank, it immediately changes to gas. Therefore, anhydrous ammonia must be injected 6 or more inches deep into the soil and then covered immediately to prevent loss of ammonia gas to the atmosphere. When injected into the soil, the ammonia molecule (NH3) reacts with water and becomes ammonium (NH4). Soil particles then hold the positively charged NH4 until it is either converted to NO3-N by nitrification or is absorbed by plant roots or soil microorganisms.
Ammonium sulfate, (NH4)2SO4, contains about 20% N and 24% sulfur (S). All of the N is in the NH4 form, which is temporarily adsorbed by the clay and organic matter of the soil until it is nitrified to NO3-N or used by plants or microorganisms.
Nitrate of soda (NaNO3) contains 16% N, all of which is in the NO3 form and readily soluble in the soil solution.
Slowly available nitrogen compoundsSome organic N materials, such as cottonseed meal, fish scraps, tankage, S-coated urea, and urea formaldehydes, are used as slow-release N fertilizers because their N solubility is slow. Their use has not generally extended to field crops because the cost of the N in these products is much higher than in the more common N fertilizers.
One of the major ways that soils accumulate N is through conversion of N2 gas in the atmosphere into organic N by rhizobia bacteria. These bacteria are found in the nodules on roots of properly inoculated leguminous plants such as clover, alfalfa, lespedeza, soybeans, and peas. The air contains about 35,000 tons of N above each acre. Fixation of N by the rhizobia bacteria may amount to as much as 200 lb/acre/year.
Another group of soil microorganisms, called non-symbiotic N2 fixers, incorporate some atmospheric N into their cells. This can later become available for plant growth. Because it is usually less than 10 lb/acre/year, it is not considered a very important source of plant-available N in agricultural soils.
Very small amounts of NH4 and NO3-N from the atmosphere are returned to the earth in rainfall each year. Although this amount may be a little greater near industrial areas, it cannot be considered a major source of soil N since it is only about 5 to 10 lb/acre/year.
The amount of biologically fixed N is variable with climate, species of plants, and bacterial activity but ranges from 20 to 300 lb/acre/year.
Crop residues returned to the soil are important sources of N for succeeding crops if they are properly managed. Mineralization of N from legume or legume-grass residues can release as much as 100 lb N/acre when cover crops or sods are killed or plowed under. However, decomposition of some high C:N residues such as straw, cornstalks, and sawdust may reduce the amount of N available to the crop during the initial stages of decomposition.
Since these residues are high in carbonaceous material, the microbial population of the soil increases rapidly during the early stages of decomposition and will compete with the growing crop for N. More N fertilizer is needed when large quantities of these high-C crop residues are added to the soil just before planting a crop. This immobilization of plant-available N is not important if the crop residues contain much N (legumes or young plants) or when these more carbonaceous residues are applied well ahead of crop planting time.
The N content of manures varies considerably, depending on the kind of livestock, age of animals, ration fed, and amount of bedding. As a general rule, a ton of barnlot manure contains about 10 lb of N. About one-half of this will be available to plants during the first growing season after the manure is applied. Table 2 lists the N content as well as P2O5 and K2O of some manures and tobacco stalks and stems.
|Table 2. N-P2O5-K2O Content of Animal Manures and Tobacco Stalks and Stems.|
|Material Applied||Pounds Per Ton|
|Dairy Cattle (80% water)||11||5||12|
|Hogs (75% water)||10||6||9|
|Poultry (55% water)||31||18||8|
|Horses (60% water)||14||5||14|
|Sheep (65% water)||28||10||24|
As previously indicated, soil microbes may immobilize available N. Although immobilized N is not lost from the soil, it is not readily available for plant uptake. However, N can be lost from soil in several ways:
An understanding of conditions contributing to these losses is helpful in managing N fertilization of crops.
Most of the organic matter in upland soils is in the top 6 to 8 inches of soil. Therefore, a considerable amount of N may be lost each year by erosion. A high rate of erosion, such as 10 T/acre, can result in a loss of 10 to 20 lb N/acre. Good soil conservation practices will minimize N losses by reducing erosion of surface soil.
Leaching losses of soil N occur because much of the N added in fertilizers ends up as NO3-N, which largely remains in solution in the soil water. During periods of excess moisture, particularly in winter and early spring, the downward movement of soil water leaches NO3 from the soil.
During the late spring, summer, and fall months, transpiration of moisture by the growing crop and evaporation at the soil surface remove moisture rapidly. With such rapid removal of soil water during the growing season, precipitation is seldom great enough for water to move through the soil beyond the depth of the plant root except in sandy soils. For this reason, leaching of N during the growing season is usually negligible on silt loam and finer textured soils under Kentucky's climatic conditions. However, leaching during the growing season can be greater from no-tillage soil than from conventional-tillage soil.
Although much of the NH4-N in soil is held by soil particles and does not readily leach, it rapidly converts to NO3-N as the soil warms. Conversion of NH4 to NO3 is very slight at soil temperatures of 32ºF but is very rapid by the time soils warm to 50º to 60ºF.
Kentucky's soil temperature does not usually stay cold enough over an extended time to prevent conversion of NH4 to NO3 during the winter. University of Kentucky field research has shown that considerable N is lost during the winter and early spring when fertilizer N is applied on fallow land in the fall for crops that are to be planted the following spring.
Anaerobic bacteria in soil reduce NO3 to gaseous forms of N under conditions of poor aeration or a low supply of oxygen. The presence of crop residues and other decomposable organic matter increases the rate of denitrification. After NO3 is denitrified to nitrous oxide (N2O) or molecular N (N2), these gaseous forms of N escape to the atmosphere. Most NO3-N may be lost in this manner from low, wet areas of a field, especially during periods of warm weather and heavy rainfall when the soil stays saturated for periods of two to six days. Also, due to the presence of the surface mulch and more moisture, there is some potential for denitrification loss of surface-applied N on no-till crops. A sidedressing of N fertilizer may become necessary following such conditions due to the denitrification loss of previously applied N.
Applications of anhydrous ammonia, urea, and N solutions should be managed to minimize the loss of ammonia gas from the soil.
Since the urea is quickly converted to NH3 when applied to soil, some NH3 may escape to the atmosphere before it converts to NH4 and reacts with the soil. These losses are most likely to occur at temperatures above 50ºF and on soils having a pH greater than 7.
Another factor influencing the risk of volatilization loss from topdressing is the amount of vegetative cover. When urea is applied onto residue-covered, untilled land where much of it is held on the vegetative cover or plant residue, the risk of volatilization loss is greater than when the urea is in contact with bare soil. If rainfall occurs soon after urea is surface-applied, it will wash the urea into the soil and prevent volatilization.
Kentucky studies comparing ammonium nitrate and urea as N sources for topdressing forage grasses have shown urea to be less effective than ammonium nitrate, particularly when applied after May 1. Effectiveness of N solutions has been less than ammonium nitrate but greater than urea in topdressing trials comparing the three N sources.
As N rates are increased, soil acidity increases since most N fertilizers make soil more acid. This increases the need for an adequate liming program. Thus, maintaining soil pH at proper levels is a basic step in efficient use of soil N, since crop growth is improved, thereby channeling more soil N into the crop.
Nitrogen is subject to loss from the soil by leaching, denitrification, volatilization, and erosion and can be immobilized by soil microbes during decomposition of organic residues. Because of the possibility of loss, nitrogen should be applied as near as possible to the time the crop requires it, which will be shortly before rapid growth begins or after the crop is actively growing.
Two or three applications during the growing season will result in more uniform growth of forage grasses. Despite the crop, part of the N should be applied as a sidedressing on sandy soils because of the high risk of leaching losses from coarse-textured soils. Sidedressing with N may also be profitable on soils that remain wet for several days following rains, since much of the NO3-N may be lost by denitrification.
Sidedressing and delayed application of N have been consistently beneficial on no-tillage soils because of the high potential for early season N loss with no-tillage. Four to six weeks after planting is the optimum time to apply most of the fertilizer N to corn.
Except urea and anhydrous ammonia, fertilizer placement is not usually as important with N as with other plant nutrients, since NO3-N moves with the soil water and is carried to the plant roots during periods of adequate rainfall. Some research indicates that denitrification losses and immobilization are reduced by subsurface banding of N below the organic matter accumulation at the surface on no-tillage soils. By placing the N below the surface residues, there is much less likelihood that these microbially induced reactions will occur.
In recent years, some chemicals have been developed for use with N fertilizers to slow the rate at which they are converted to NO3-N. This is an attempt to keep fertilizer N in its more stable NH4 form for a longer period, thereby diminishing the potential for NO3 losses, particularly during the early growing season. These chemicals are often called N stabilizers.
As stated previously, NH4 forms of N adhere to the surface of soil colloids (clay, organic matter) and are not as readily lost as is NO3. Stabilized N generally leads to improved fertilizer N use efficiency and thereby to decreased potential for groundwater pollution.
Most stabilizers function as nitrification inhibitors, which inhibit for varying periods of time the activity of certain soil bacteria that normally convert NH4 to nitrite (NO2), the first step in nitrification. Numerous compounds have the capacity to inhibit nitrification but only a few are registered for use in the United States. Perhaps the most widely used is nitrapyrin1, developed by Dow Chemical Company and sold under the trade name N-serve Nitrogen Stabilizer®. A second is etridiazol, developed by Olin Corporation and sold under the trade names Dwell® or Terrazole® by Uniroyal Chemical Company. A third product is dicyandiamide (DCD), which is sold under the name Super N® by Allied Corporation. However, it is sold as a slow-release organic N source containing urea and DCD rather than as a nitrification inhibitor.
Urease inhibitors also stabilize N, although they are not classed as nitrification inhibitors. Rather, their action is through the prevention of NH3 volatilization from urea. Urea hydrolysis to NH3 is catalyzed by the enzyme urease, and urease inhibitors slow the rate of urea hydrolysis to NH3.
Nitrogen stabilizers have been widely tested for crop response in Kentucky and the United States. Generally, they have been shown to reduce the rate at which NH4 is converted to NO3. However, crop response to their use has been variable and inconsistent. In one summary of 173 experiments conducted in the Midwest, corn yields were increased about 60% of the time. Crop response to nitrification inhibitors is most likely when N is applied in the spring before planting on soils known to be conducive to N loss.
1Nitrapyrin has been labeled for use on both conventional and no-till corn in Kentucky. However, it has not been labeled for use on tobacco.
Where trade names are used, no endorsement is intended, nor criticism implied of similar products not named.