Fertilizer Management
Soil nitrogen behavior in soils
Constantly changing nitrogen
Nitrogen in soil is in a constant state of change in Mississippi's warm, humid climate. We know a lot about the process of nitrogen transformations, but the quantities are very hard to specify with certainty.
There is a great deal of changing nitrogen activity occurring in soils when the temperature is above 40º F. Natural nitrogen in soils comes from decomposing organic materials in a process called mineralization, or the changing of nitrogen forms from organic to inorganic. Immobilization can be loosely characterized as the reverse: converting inorganic form nitrogen into organic form. The rate of this transformation depends on the quantity and type of organic matter present in the soil
Losing nitrogen from soil
Nitrogen may be lost from soil by leaching, denitrification, or volatilization. Leaching losses occur when nitrate form nitrogen in the soil water moves downward. Nitrogen lost in this way is a contaminant if it reaches ground or surface waters. Leaching can be a major problem in some areas, but measurements in Mississippi indicate our soils may not be as prone to loss via this mechanism.
On the other hand, denitrification in waterlogged soils has been proven a major loss pathway in Mississippi. As water content of soils increases, some soil microbes use the oxygen in nitrate and release nitrogen gases to the atmosphere.
Waterlogged soils and relatively warm temperatures are perfect for denitrification to occur.
Volatilization is the loss of volatile ammonia gas to the atmosphere. This can occur from anhydrous, urea, or N solution fertilizer sources on the surface and are minimized through management of fertilizer applications. The potential loss is greater with temperatures above 50° F, high pH soils, and pastures or high residue tillage systems.
Testing for nitrogen in soils
The shifting of nitrogen forms and constant loss means testing soils for nitrogen is not straightforward as for the other plant nutrients. The interpretation can be very tricky, however testing is available.
The Mississippi State University Soil Testing Laboratory can test for nitrate nitrogen if the appropriate information sheet is filled out and accompanies the sample(s). The nitrate nitrogen sheet(s) and box(es) can be picked up at a local county Extension office or at the soil testing laboratory at Mississippi State. Cost of this service is $3.
While knowing the residual nitrate nitrogen of the soil may be useful, the test is only good for the nitrate nitrogen that is present when the sample is collected and has very little value unless the following items are considered.
- Since nitrate nitrogen is lost throughout the year in Mississippi, take samples before nitrogen fertilizer application.
- Samples must be collected deeper than 6 inches and to depths of 36 inches depending on the crop being grown. Samples need to be taken in 6-inch increment and need to be representatively collected in the field with the appropriate number subsample in order to make up one soil sample.
- Research in Mississippi showed that nitrate nitrogen samples need immediately air-drying after taken. This can be accomplished by spreading the samples out in metal or plastic trays.
