Know What They’re Saying About Nitrogen Fertilizers

Our warm and humid Mississippi climate makes nitrogen (N) fertilizer management challenging. Let’s review the basics about N terminology as various product claims fly about with some puzzling promotional material being used. Be sure that you know what is being discussed, and that the person/material discussing it is accurate.

Nitrogen transformations depend on soil moisture conditions, soil acidity, temperature, and microbial activity. The ammonium form (NH₄⁺) can be adsorbed in soil particles or taken up by plants without transformation, but in most cases it is converted to nitrate (NO₃^–) soon after its formation or addition as fertilizer.

This process, called nitrification, involves two groups of soil bacteria. First, Nitrosomas bacteria produce nitrite (NO₂^–), then Nitrobacter species convert NO₂^– to NO₃^– soon after its formation. Two things to note: 1) NH₄+ exists in soils only a short time, and 2) H+ ions are produced, which increases soil acidity or lowers the soil pH.

Mineralization is the process of converting organic N to plant-available inorganic forms. It is a gradual breaking down of large organic molecules to smaller molecules by soil microorganisms. After these microbes complete their relatively brief life cycles, they are decomposed by other microbes. Energy for this process comes from carbon (C) in the material being used, so introduction of fresh plant materials rich in decomposable C stimulates this process.

Immobilization occurs when plants and soil microorganisms incorporate inorganic N into an organic N form. Because this process depends largely on microbes, the availability of carbon and other nutrients determines the rate of immobilization. When residues with high C:N ratios are being decomposed, all readily available N within the soil system may be used by the microbes and unavailable for plant uptake. It depends on the particular organic matter added to the soil as the fundamental issue is the carbon to nitrogen ratio: the critical value is 25. Hardwood sawdust is 400, wheat straw is 80, rye cover crop is 26, and poultry litter is 10. Above 25, soil nitrogen is immobilized, i.e. used up by soil microbes and unavailable to plants. Below 25, there is nitrogen in the soil solution for growing plants. Below 20, there is a flush of plant available nitrogen.

This effect is temporary. Eventually some of the microbial population dies and decomposes, releasing N that is available to plants. The risk of immobilization is avoided by mixing plant residue into the soil well before the next cropping cycle.

Loss of Nitrogen from Soils

Even though soil N may be unavailable to plants through immobilization, it is still present in the soil. Nitrogen can be permanently removed from soil by erosion, leaching, denitrification, or volatilization.

Organic matter, which contains the majority of native N, is concentrated in the plow layer of soils, making it susceptible to loss by erosion. Soil conservation practices will lessen loss through erosion.

Leaching loss occurs when NO₃– remains in the soil water and moves away from root uptake areas with downward water movement. This N loss is a contaminant if it reaches ground or surface waters. Nitrogen leaching is more likely when there is more rain than a crop can use.

A major mechanism of N loss in Mississippi is denitrification in waterlogged soils. As the water content of soils increases, the amount of air in soils decreases. Some soil microorganisms can use the oxygen in soil NO₃– and NO₂– instead of gaseous O₂. Ultimately nitrous oxide gases (N₂, NO, and N₂O) are produced and released to the atmosphere, resulting in a loss of N from the soil. Conditions necessary for denitrification are waterlogged soils, carbon sources (from organic matter or plant residues) for use by the anaerobic microbes, and N as either NO₃– or NO₂-. The rate is greatly accelerated by higher temperatures.  Note that nitrate is not a gas, and cannot be lost directly from soils via this process.

The loss of volatile ammonia gas (NH₃) to the atmosphere can also occur from anhydrous, urea, or N solution fertilizer sources. Losses by volatilization are minimized through proper management of fertilizer applications. Producers should apply anhydrous ammonia when soil conditions allow good sealing of the applicator slit after fertilization.

The first step in urea N conversion to NH₄+ is the production of NH₃. Volatile losses from urea are more likely with warm temperatures (>50°), high pH soils (>7), pastures, and conservation tillage with high residues.

Ideally, incorporate urea into the soil or apply it before a rainfall. Since urea volatilization is increased when broadcast applied to vegetative cover or plant residue, you must be careful when using urea as a topdressing N source. You should never place urea in contact with seed because of toxic effects of NH₃ on seedlings. The same precautions apply to N solutions, since they contain urea.

Future Mississippi Crop Situation postings by Dr. Tim Walker of the MSU Delta Research and Extension Center will look at various products used in corn and rice fertility programs.