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Predicting Nitrogen Fertilizer Needs of Iowa Soils: IV. Evaluation of Nitrate Production as a Criterion of Nitrogen Availability¹

ABSTRACT

Preceding papers in this series have traced the development of a suitable method for assessing the relative nitrogen-supplying power of soils and its correlation with crop response to nitrogen fertilization in the field. This method has been shown to be of value in predicting the need for nitrogen fertilizer in Iowa soils.

The present study, conducted under controlled greenhouse conditions, was designed to evaluate critically the utility of soil nitrate release as a basis for establishing the relative nitrogen status of soils. The 21 soils used in this investigation represent a wide range in nitrogen-supplying capacity. Four levels of nitrogen fertilizer were applied to pots and German millet was grown.

The relation between total nitrogen uptake by the crop and level of applied nitrogen was linear for all soils studied. Extrapolation of the linear regressions provided values which reflected the relative contents of available nitrogen in the soils. These values which have been termed "N-values" are expressed in units of applied nitrogen. Also, N-values correlated highly with the total nitrogen uptake of the check pots. Both N-values and nitrogen uptake by the plants were highly correlated with nitrate nitrogen released during a 2-week period of incubation. Considerably lower correlations were found in relating these criteria to total soil nitrogen or to values obtained by the alkaline permanganate method.

¹ Journal Paper No. J-2736 of the Iowa Agr. Exp. Sta., Ames, Iowa. Project 1190. Presented before Div. IV, Soil Science Society of America, Nov. 10, 1954.

² Graduate assistant and Professor of Soils, respectively.

Received for publication October 19, 1954.