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Recovery of Fixed Ammonium by Corn in Greenhouse Studies¹

ABSTRACT

The availability of native fixed NH₄ and applied NH₄ fixed in the Ap and B1 horizons of a Dodge silt loam was investigated. In addition, the effect of freezing on the fixation of applied NH₄ was studied. By applying K and N to either the same or separate layers of soil in green-house pots it was also possible to evaluate the effect of K on the release of fixed NH₄. When NH₄ and K were applied in the same layer of soil, the Ap and Bl horizons fixed 5 and 15% of the added NH₄, respectively. An additional 5% of the added NH₄ was fixed in each horizon when the soil was frozen after applying NH₄. However, when NH₄ and K were applied to separate layers of soil, negligible amounts of NH₄ were fixed. Highly significant correlations between exchangeable K in the moist soil after cropping and the amount of applied NH₄ fixed as measured by differential N uptake showed that the level of exchangeable K in the soil controlled the release of fixed NH₄ to the crop. Correlations between two methods of measuring fixation of applied NH₄, namely, differential N uptake and analysis of the soil after cropping, were highly significant.

These studies also indicated that about 15% (0.12 me. per 100 g.) of the native fixed NH₄ in the Bl horizon was released when the level of exchangeable K in the soil was very low. However, very little native fixed NH₄ was released from the Ap horizon, presumably because sufficient K remained in the soil of this horizon to block release of fixed NH₄. These studies indicate that even under the most advantageous conditions very little of the native fixed NH₄ is available to crops.

¹ Contribution from the Soils Department, University of Wisconsin, Madison. Published with the permission of the Director of the Wisconsin Agr. Exp. Sta. Presented before Div. IV, Soil Science Society of America, Nov. 16, 1959, at Cincinnati, Ohio.

² Assistant Professor and Associate Professor, respectively, Soils Department, University of Wisconsin.

Received for publication May 7, 1962. Accepted for publication July 3, 1962.