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The Influence of Soil-Fertilizer Geometry on Nitrification and Nitrite Accumulation¹

ABSTRACT

Nine granules (330 ppm N) of either urea, (NH₄)₂SO₄, or diammonium phosphate (DAP) were spaced in a square pattern at distances of 0, 0.5, 1.0, or 1.5 cm on 30 g of soil. Samples were watered, incubated, and extracted with I N KCl. The extracts were analyzed for NH₄⁺, NO₂^-, and NO₃^-. Generally NO₂^- accumulation for the three N carriers increased with granule spacing; the geometric effect was DAP > (NH₄)₂SO₄ > urea. Nitrite varied from 140 to 265 ppm N. Increased NO₂^- accumulation with increased granule spacing was accompanied by a corresponding increase in NH₄⁺ oxidation rate with little change in NO₃^-. Geometric effect with four granules (55 ppm N) spaced up to 2.5-cm apart showed that both NO₂^- and NH₄⁺ decreased with increased granule spacing accompanied by increased NO₃^-. Single-granule studies suggested that the greater geometric effects of DAP resulted from the lower solubility and relatively low mobility of phosphate ions. The subsequent periphery pH decrease minimized outward movement of NH₄⁺. These effects were less for (NH₄)₂SO₄ and for urea.

¹ Contribution from the Minnesota Agr. Exp. Sta. as Scientific Journal Series Article no. 7436. The results were used by the senior author in partial fulfillment of the Ph.D. degree.

² Formerly, Research Assistant, now Assistant Professor of Agronomy, University of Maryland, College Park 20742; Professor of Soil Science; and Head, Soil Science, University of Minnesota, St. Paul, respectively.

Received for publication December 21, 1970. Accepted for publication July 7, 1971.