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Relationship of Ammonium Nitrogen Distribution to Mineralogy in a Hapludalf Soil¹

ABSTRACT

Distribution of fixed and exchangeable ammonium were examined in three soil profiles of the Shrouts series from the Knobs region of eastern Kentucky. Shrouts soils (fine, mixed, mesic, Typic Hapludalfs) have high base status; exhibit strong, prismatic structure; and are derived from calcareous, high magnesium, soft (weakly cemented) clay shale. Soil pH ranges from 6.3 in the Ap horizon to 7.9 in the C. Analyses, using the Silva and Bremner procedure, showed considerable fixed NH₄⁺-N in all horizons of each profile. Mean fixed NH₄⁺-N in the profiles was 365 ppm for the Ap, 463 ppm for the B21t, 433 ppm for the B22t, 469 ppm for the C, and 543 ppm for the Cr horizons. Exchangeable NH₄⁺-N also increased with depth, with values being about 1.2 to 1.5% of the fixed NH₄⁺-N values. X-ray diffraction analyses of the clays and silts revealed predominantly mica in the 2–0.2 and <0.2 µm fractions, with some vermiculite present, and large quantities of mica in the silt fractions. High correlation (r = 0.92) was found between fixed NH₄⁺-N and exchangeable NH₄⁺-N. A correlation exists between fixed NH₄⁺-N and mica + vermiculite content with an r value of 0.68. The fixed NH₄⁺-N in the lower horizons is presumed to be indigeous, since no NH₄⁺-N has been added recently to the soil.

¹ Contribution from the Department of Agronomy, University of Kentucky, Lexington, KY 40546. Published with approval of the Director of Kentucky Agric. Exp. Stn. as Journal Article no. 77-3-98.

² Graduate Research Assistant, Associate Professor, and Professors of Agronomy, respectively.

Received for publication July 1, 1978. Accepted for publication March 17, 1979.