Nitrogen (N) can be lost from the soil through three principal pathways, (1) DENITRIFICATION, (2) LEACHING, and (3) SURFACE VOLATILIZATION.
The form of N a farmer chooses should depend on how serious a problem he has with the above N losses. Cost of N is another consideration when choosing a fertilizer source.
DENITRIFICATION occurs when nitrate-N (N03-) is present in a soil and there is not enough oxygen (02) present to supply the needs of the bacteria and microorganisms in the soil.
If 02 levels are low, bacteria and microorganisms strip the oxygen from the nitrate and the end result is the production of N gas (N2) or nitrous oxide (N20), which volatilizes from the soil.
Three conditions that create an environment that promotes denitrification are wet soils, compaction and warm temperatures for high activity of soil microorganisms.
LEACHING losses of N occur when soils have more incoming water (rain) than the soil can hold. As water moves through the soil, nitrates (N03-) in the soil solution are picked up and moved with the water. Ammonia (NH4+) forms of N have a positive charge and are held by the negative sites on the clay in the soil; therefore, NH4+ forms of N leach very little. In sands where there is very little clay, ammonium forms of N can leach. Relatively coarse soils (sands and some mucks) are the only ones in which significant leaching of N is important.
One way to minimize N leaching and denitrification is to minimize the time the N is in the soil before plant uptake. This cuts down on the time when conditions are favorable for losses. Most of the N is needed by corn after the plant is three to four weeks old (June 1).
SURFACE VOLATILIZATION of N occurs when urea forms of N break down and form ammonia gases and where there is little soil water to absorb them. This condition occurs when urea forms of N are placed in the field but not in direct contact with the soil. This situation can occur when urea is spread on corn residues or 28% is sprayed on heavy residues of cornstalk or cover crop.
The rate of surface volatilization depends on moisture level, temperature and surface pH of the soil. If the soil surface is moist, the water evaporates into the air. Ammonia released from the urea is picked up in the water vapor and lost. On dry soil surfaces, less urea-N is lost. Temperatures greater than 50oF and pH's greater than 6.5 significantly increase the rate of urea conversion to ammonia gases. Applying urea-type fertilizers when weather is cooler slows down N loss. If the surface of the soil has been limed within the past three months with two tons or more of limestone per acre, DO NOT apply urea-based fertilizers unless they can be incorporated into the soil.
To stop ammonia volatilization from urea, the urea must be tied up by the soil. To get the urea in direct contact with the soil it must rain enough to wash the urea from the residue or the farmer must place urea-based fertilizer in direct contact with soil by tillage, banding or dribbling.
If the residue is light, 0.25 to 0.5 inch of rain is enough to dissolve the urea and wash it into the soil. If the residue is heavy, 0.5 inch or greater of rainfall is required.
The common N fertilizers are anhydrous ammonia (82% N), urea (45- 46% N), solutions (28-32% N), ammonium sulfate (21% N) and ammonium nitrate (34% N).
Anhydrous ammonia (82%) converts to nitrate N the slowest of any form of N fertilizer. Therefore, it would have the least chance of N loss due to leaching or denitrification. It must be injected into the soil; therefore, it would have no loss due to surface volatilization. The disadvantage of anhydrous ammonia is that it is hazardous to handle. It must be injected into the soil, and on steep slopes erosion can be a problem.
Urea (45-46%) converts to nitrate N fairly fast, usually in less than two weeks in the spring. Denitrification on wet or compacted soils can be serious. Leaching can be a problem in coarse soils. In no-till situations surface volatilization can be a problem if the urea is not placed in contact with the soil and it is dry for several days after spreading.
Twenty-eight to 32 percent (28-32%) N solution is usually made up of urea and ammonium nitrate. The nitrate in this product is subject to leaching and denitrification from the time it is placed in the field. The urea components are subject to the same loss mechanisms as urea.
N solutions can be banded on the soil surface easily by dribbling. This method of application minimizes the amount that sticks to the residue; therefore, surface volatilization is minimal.
Ammonium sulfate (21%) is a nitrogen source with little or no surface volatilization loss when applied to most soils. Ammonium sulfate is a good source of sulfur when it is needed. Its disadvantage is that it is the most acidifying form of N fertilizer - it requires approximately 2 to 3 times as much lime to neutralize the same amount of acidity as formed by other common N carriers.
Ammonium nitrate (34%) is 50% ammonium N and 50% nitrate N when added to the soil. The ammonium N quickly converts to nitrate N. For soil subject to leaching or denitrification, ammonium nitrate would not be preferred. Ammonium nitrate has no urea in it; therefore, surface application would be a good choice where volatilization of urea is expected.
Jay W. Johnson
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