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Potassium Availability in Nason Soil as Influenced by Ammonium and Lime¹

ABSTRACT

A greenhouse experiment was established to find if NH₄⁺ increases K⁺ availability when Nason silt loam is limed to pH 7.0 as suggested by previous laboratory studies.

Two oat crops were grown in soil at 3 lime levels (1, 5, and 15 g/1,000 g), 2 K levels (0 and 0.5 meq/100 g) and 2 N sources [Mg(NO₃)₂ and NH₄NO₃]. At high soil pH, potassium uptake was significantly greater where the N source was NH₄NO₃ as compared to Mg(NO₃)₂. The results support soil extraction data which show that at about pH 7.0, NH₄⁺ ions are considerably more efficient in displacing K⁺ from this soil than are Mg²⁺ ions. It is concluded that the smaller NH₄⁺ ions are able to penetrate to exchange positions not accessible to large hydrated Mg²⁺ ions.

The results do not support an explanation of the differential K uptake based on the antagonistic effect of Mg²⁺ ions on K⁺ nor due to an effect caused by lower soil pH with NH₄NO₃ fertilization.

Although K fixation occurs at high pH in this soil, this fixation does not appear to be associated with greater collapse of expansible vermiculite. In fact, it is shown that liming caused the vermiculite to be less subject to collapse on K saturation. The reason for the greater fixation at high pH may be that there is less competition for the specific sites by H₃O⁺ ions and thus greater opportunity for selection of K⁺ ions which are of similar size.

¹ Contribution from the Agronomy Department, Virginia Polytechnic Institute. This research was supported from Hatch funds for regional Project S-14. Presented before Div. S-4, Soil Science Society of America, Kansas City, Mo., Nov. 19, 1964.

² Graduate student in Agronomy and Professor of Agronomy, respectively. The authors gratefully acknowledge the assistance of the Department of Statistics, V.P.I., in the statistical analysis of the data.

Received for publication April 1, 1965. Accepted for publication July 2, 1965.