Nitrogen Needs Assessed More Accurately

Farmers routinely use soil-testing laboratories to help determine how much nitrogen, phosphorous, and potassium fertilizers their farms need.

Apply too little and crop yields are limited, reducing income. Apply too much and risk polluting the environment, particularly with nitrogen fertilizer.

But soil labs that farmers contract with do not always give accurate nitrogen fertilizer recommendations; they generally measure only the amount of inorganic nitrogen in the soil samples that farmers provide. There isn't a good mechanism for the labs to use to factor in the amount of nitrogen that is released naturally by microbial degradation—nitrogen mineralization—when soilborne bacteria chew up organic matter such as residues left from the previous crop.

But that may change, now that Agricultural Research Service scientists have found a way to more accurately assess microbial breakdown of straw and stalks. The new method cuts in half the error between measured and predicted nitrogen mineralization rates.

"Using more accurate nitrogen fertilization recommendations will help farmers nationwide, and it could reduce nitrogen fertilizer costs by $10 to $30 per acre," says Merle F. Vigil, an ARS soil scientist at the Central Great Plains Research Station in Akron, Colorado. "'And more accurate rates will help reduce the environmental threat of excess nitrogen seeping downward toward water supplies."

The new technique uses computers to predict the effect of temperature on mineralization rates in soils.

The scientists collected data from soils mixed with different crop residues. They took measurements for up to 160 days at four temperatures ranging from 40° F to 90° F.

They plugged that data into a computer model called MINIMO—for Mineralization and Immobilization—that was originally developed by scientists in Israel, The Netherlands, and the United States. It is a subroutine of CERES-Maize, developed by ARS, Texas A&M, and Michigan State University scientists.

"The technique will be of more benefit to farmers in the East and Midwest—where soils contain more native organic matter—than in the more arid areas of the western United States," says Vigil.

Vigil is now working on ways to help soil-testing labs give more accurate assessments of how much cattle and swine manure, plowed-under green crops like rye and alfalfa, or sewage sludge help meet crop fertilizer requirements and reduce the need for commercial sources. -- By Dennis Senft, ARS.

Merle F. Vigil is at the USDA-ARS Central Great Plains Research Station, 40335 County Road GG, Akron, CO 80720; phone (970) 345-2259 ext. 0517, fax (970) 345-2088.

"Nitrogen Needs Assessed More Accurately" was published in the October 1995 issue of Agricultural Research magazine.