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Determination of Fixed Soil Potassium¹

ABSTRACT

After making preliminary tests of methods proposed by other investigators for determining the fixed potassium of soils and also of several new approaches, the approach found most promising — heating at 500° C for two hours — was investigated in considerable detail. It was found that on heating ammonium saturated soils, illite, and bentonite in this manner, substantial release of nonexchangeable or fixed potassium could be effected without breakdown of primary potassium minerals. Not all of the fixed potassium can be released in one heating, the amount thus released representing an equilibrium point related to the amount which still remains fixed.

The procedure finally adopted consists in heating ammonium saturated soil at 500° C for two hours, then extraction of the potassium made exchangeable with ammonium acetate solution, and finally determination of the potassium in the solution by means of the flame photometer. This heating, extraction, and determination are repeated once, and from the amount of fixed potassium released during the second heating, the total amount of fixed potassium present may be calculated by use of a summation formula for a geometric progression. The method is simple and is adaptable to rapid analysis of large numbers of samples. On the basis of the results obtained with soils and minerals, it is believed that the method is reasonably specific for fixed potassium, and offers the possibility of being a practicable and reliable method.

¹ Contribution from the Dept. of Soils, University of Wisconsin, Madison, Wis. Published with the permission of the Director of the Wisconsin Agr. Exp. Sta. This paper includes part of a thesis submitted by the senior author in partial fulfillment of the requirements for the degree of Doctor of Philosophy, Univ. of Wis. This work was supported in part by grants from the American Potash Institute and the University Research Committee through funds derived from the Wisconsin Alumni Research Foundation. Presented before Division II, Soil Science Society of America, Cincinnati, Ohio, Nov. 18, 1952.

² Graduate fellow (now agronomist, E. I. du Pont de Nemours & Co.) and professor of soils, respectively.

Received for publication February 13, 1953.