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Residual Effects of ¹⁵N-Labeled Fertilizers in a Field Study¹

ABSTRACT

Residual effects of two ¹⁵N-labeled N fertilizers, urea and oxamide, were compared during the second cropping season after they had been applied at rates of 0, 56, 112, and 168 kg N/ha. ‘Sudax SX11’ sorghum-sudan grass hybrid (Sorghum sudanense) was grown and harvested three times during this second cropping season while the residual effects were being measured.

Fertilizer N removed in plant tops during this second year of cropping contained 13–18% of the residual fertilizer N in the soil at the end of the first season and was equal to 4–6% of that applied originally in the fertilizers. At the end of the second cropping season, 22 and 26% of the initial applications of N in urea and oxamide, respectively, remained in the soil. The effect of carriers on these amounts of residual fertilizer N was not significant. The removals in crops during the second season plus those amounts remaining in the soil at the end of the second season were essentially equal to the residual fertilizer N which had been in the soil at the end of the first season. Thus, no evidence was found for loss of residual N by leaching, denitrification, or other processes during the second cropping season. Since removals of N in crops were considerably greater than the additions in fertilizers, it is hypothesized that nearly all inorganic fertilizer N was taken out of the soil and most of the residual fertilizer N was in relatively stable organic forms. Conditions for these experiments had been selected to conserve N, and as a consequence the deficits of fertilizer N reported here may be expected to be smaller than might occur under other practices.

¹ Contribution from the Department of Agronomy, Univ. of Illinois, Urbana, and the Ill. Agr. Exp. Sta. This research was done in cooperation with the Division of Agricultural Development, TVA. This work reported here is part of the Ph.D. thesis by the senior author.

² Research Assistant and Professor, respectively, Univ. of Illinois. Senior author is now Assistant Professor, Dept. of Agr. Chemistry and Soils, Univ. of Arizona, Tucson 85271.

Received for publication May 24, 1971. Accepted for publication August 27, 1971.