Iowa State University's new nitrogen fertilizer recommendations for corn (outlined in Pm-1714) do not involve the traditional practice of calculating credits for N supplied by legumes or animal manures. Instead, adjustments for N supplied by these sources are made by using different crop categories and by soil and cornstalk testing to make site-specific assessments of N availability. This change was made to help producers make better use of N in organic fertilizers.

Much of the nitrogen contained in animal manures and plant residues is classified as "organic" by scientists. The meaning of the word "organic" used in this context has no relationship to the meaning of the same word when used to denote gardening or farming without chemicals. All corn producers use organic materials (plant residues and animal manures) as fertilizers every year, so a basic understanding of the N-supplying power of these materials is important.

Unlike nitrogen in commercially prepared fertilizers, nitrogen in organic materials must be "mineralized" to nitrate or exchangeable ammonium ("inorganic" or "mineral" forms) before it is available to plants. This mineralization occurs as microorganisms decompose the organic materials in soil. This decomposition process involves microorganisms feeding on the organic molecules. The mineralization of N always is accompanied by the formation of new organic materials, either as new microbial cells or as by-products of microbial activities. When soil temperature and moisture are favorable, the availability of organic materials is the primary factor that determines how much microbial activity occurs. The nitrogen-richness of the residue, however, is a major factor that determines how decomposition of the organic materials influences N availability for the next crop.

Characterization of the nitrogen-richness of organic materials is best achieved by use of carbon-to-nitrogen (C/N) ratios. C/N ratios of organic materials important during corn production can range from greater than 200 (nitrogen-poor) to less than 10 (nitrogen-rich). The C/N ratio that best distinguishes nitrogen-rich from nitrogen-poor materials varies with many factors, but it is appropriate to consider a ratio of 25 to be the "critical" C/N ratio in general discussions. This ratio is considered "critical" because the decomposition of organic materials with higher ratios results in a net loss of plant-available N through the process of immobilization.

Immobilization refers to incorporation of nitrate and exchangeable ammonium into organic forms that are not available to plants. This occurs as microorganisms build new cells during growth supported by the decomposition of organic materials. Immobilization is the opposite of mineralization, and both occur simultaneously in soils. The term "net amount of mineralization" is used to denote the amount of N mineralized, minus the amount of N immobilized. It is the net amount of N mineralized (or immobilized) that determines the overall effect of the residue decomposition on availability of N to plants.

Decomposition of nitrogen-rich organic materials in soils results in an immediate net release of plant-available N. This release occurs because the organic material supplies more N than needed by micro-organisms during the growth of new cells. This initial release of N causes nitrogen-rich organic materials to behave like commercially prepared fertilizers. One major difference, however, is that only part of the N is released during the first growing season.

Decomposition of nitrogen-poor organic materials usually has the opposite effect on N availability. During the initial stages of decomposition, soil microorganisms must use nitrate or ammonium from the soil to build new cells. The decomposition of nitrogen-poor residues, therefore, results in a short-term depletion of the plant-available N supply. Soil microorganisms that decompose nitrogen-poor residues compete with plants for available N. These microorganisms tend to win in the competition with plants for available N.

Nitrogen immobilized during the early stages of residue decomposition remains in the soil as organic matter and becomes available to plants only after it is mineralized. Immobilization of N and other associated processes that occur during the decomposition of organic materials is essential for maintaining soil organic matter concentrations and soil quality. Although this immobilization increases the nitrogen-supplying power of the soil in years to come, it frequently results in a substantial decrease in N availability for the first crop.

It usually is considered more convenient to describe the nitrogen-richness of organic materials by percentage N content than by C/N ratio. A major reason is that percentage carbon content is seldom measured. This need not be a problem because organic materials of interest usually have a carbon content of about 40 percent on a dry-weight basis. Therefore, there tends to be a predictable relationship between C/N ratio and percentage N content expressed on a dry-weight basis. A C/N ratio of 25 corresponds to 1.6 percent N on a dry-weight basis.

A notable problem with describing nitrogen-richness by percentage N is that this method of expression makes it easier to overlook important differences between organic and inorganic fertilizers. Important differences are overlooked, for example, when percentage N is multiplied by the application rate of organic material to estimate amounts of plant-available N supplied by the organic material. The important difference overlooked is the effects of C on amounts of N released for plant growth. This error is important when estimating the effects of the added manure on losses of N to groundwater supplies and when estimating N fertilizer needs for the next crop.

Use of percentage N content on a wet-weight basis poses serious problems because the C/N ratio of the materials remains essentially unknown. A low percentage N occurs when organic materials with high or low C/N ratios are diluted by water. Even when diluted by water, however, C/N ratios determine whether N availability is increased or decreased for the next crop. For this reason, the wet-weight nitrogen concentration of an organic material tells little about the amounts of N the material will supply for growth of the next crop.

The late-spring test for soil nitrate provides useful information about the nitrogen-supplying power of organic materials because it assesses N availability after the initial stages of decomposition have occurred. Subsequent use of the end-of-season test for cornstalk nitrate provides information about the effects of the organic material on N availability after soils were sampled in late spring. Together, these tests give site-specific information that helps producers learn about the nitrogen-supplying power of the organic materials applied to their soils. This information is needed to use commercially prepared N fertilizers as a profitable and environmentally safe supplement to the organic fertilizers normally added to soils during crop production.