Predicting Nitrogen Fertilizer Needs of Iowa Soils: III. Use of Nitrate Production Together with Other Information as a Basis for Making Nitrogen Fertilizer Recommendation for Corn in Iowa

A large number of field fertilizer experiments have provided a general basis for predicting nitrogen fertilizer needs of crops in Iowa. Nitrate production now provides additional information making possible more accurate prediction of nitrogen needs. The relationship between nitrate production and the yield increases obtained from different rates of application of nitrogen fertilizer on corn (limited to corn which does not follow a leguminous meadow) is presented and discussed. This relationship is summarized by the equation: log Y = log [104 (1‐10 ‐0.003525X )] −0.0098b 1 where Y = bushels per acre yield increase to be expected from an application of x pounds of nitrogen per acre on a soil with a nitrate production of b 1 ppm. nitrate nitrogen. The most profitable rate of application of nitrogen fertilizer is given by the equation: x = −21 + 284 log K −2.8b 1 where x represents the most profitable rate of nitrogen to apply to a soil with a nitrate production of b 1 ppm., when K = the price of corn per bushel divided by the price of nitrogen per pound. The method used here appears to hold much promise for the calibration of other soil tests. Various factors which may influence the interpretation of the test results and the fertilizer recommendations because of their effect on the test results or on nitrogen availability in the field are discussed.