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Investigation of a Thermal Method for the Determination of Fixed Potassium in Soils¹

ABSTRACT

The release of K from soils according to the thermal method proposed by Kolterman and Truog was studied. This method involves heating an NH₄-saturated sample at 500°C. for 2 hours, followed by extraction of the K made exchangeable with ammonium acetate. The proponents of this method found that successive treatments released K from certain soils and clay minerals in a definite manner such that the slope of the lines formed by plotting the log K released against the treatment number was constant for all of the materials tested. The assumption that the slope would remain constant for all soils permitted the calculation of the total amount of fixed K from the results of only one or two heating and extraction treatments.

A group of soils with known K release characteristics was subjected to six successive heating and extraction treatments. The release of K from the different soils did not follow the expected pattern, however, since the slope of the log K plots did not remain constant for all soils. This discrepancy invalidates the method of calculating the total fixed K in the soil as proposed by Kolterman and Truog.

The data suggest the possibility of using the thermal method for determining the relative capacity of soil to supply nonexchangeable K to plants. A single heating and extraction treatment was as effective as a series of successive treatments in this regard.

Potassium release characteristics of a number of Maryland soil profiles were also studied by means of the thermal method. Potassium released by successive treatments varied greatly among soils and among horizons of each soil profile.

¹ Scientific article No. A643, Contribution No. 2829 of the Maryland Agr. Exp. Sta., Department of Agronomy. Part of a dissertation submitted by the senior author in partial fulfillment of the requirements for the Ph.D. degree. Presented before the Northeastern Branch, American Society of Agronomy, Amherst, Mass., June 19, 1957.

² Senior author is with the Agricultural Research Service, USDA, Beltsville, Md., and the junior author is Associate Professor, Univ. of Maryland. The authors express their appreciation to Dr. R. F. Reitemeier for his excellent cooperation in obtaining some soil samples from his previous studies for use with this experiment.

Received for publication December 9, 1957. Accepted for publication March 14, 1958.