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Nature of Residual Nitrogen in Iowa Soils¹

ABSTRACT

Increased quantities of soil nitrogen resulting from nitrogen applied to corn 1 year previously were characterized as to the form and location in soil. Under the experimental conditions the residual soil nitrogen appeared to be chiefly in the form of nitrates in the 6- to 21-inch layer. No appreciable quantities of residual ammonium nitrogen were found. Nitrification rate tests showed that effects of residual nitrogen on nitrifiable forms of nitrogen were negligible.

Quantities of residual nitrates in soil layers 0 to 6, 6 to 12, and 12 to 21 inches deep were related to the nitrogen yield of oats. As much as 90% of the total variations in nitrogen yield of oats were accounted for by the multiple regression of nitrogen yield on nitrate nitrogen found in the 3 soil layers. Simple linear regressions showed a high correlation between effects of residual nitrogen on nitrogen yield of oats and measured residual soil nitrates at the beginning of the oat season. Results of isotopic nitrogen measurements obtained from a greenhouse experiment provided additional evidence of residual nitrogen being present mainly as nitrates below the plow layer.

¹ Journal Paper No. J-3405 of the Iowa Agr. and Home Econ. Exp. Sta., Ames, Iowa. Projects No. 1189 and 1148, Department of Agronomy. Acknowledgement is made of financial assistance from the Nitrogen Division, Allied Chemical Corp. Presented before Div. IV, Soil Science Society of America, Atlanta, Ga., Nov. 19, 1957.

² Formerly Graduate Assistant and Associate Professor of Soils, respectively. The senior author is now Soil Fertility Extension Specialist, North Carolina State College, Raleigh.

Received for publication May 12, 1958. Accepted for publication June 25, 1958.