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A Comparison of Methods for Determining Extractable Soil Potassium in Fertilizer Test Plots¹

ABSTRACT

A Bridgehampton silt loam which had received various amounts of potassium fertilizers for 44 years was analyzed for extractable or exchangeable K by 6 methods. Total K and that removed by boiling 1N HNO₃ were also determined. The methods measured significantly different quantities of K. The Peech-English method extracted more than did the North Carolina, Maine, or Morgan procedures. Neutral normal ammonium acetate extraction and a method employing IR-120 resin (Amberlite) were intermediate. The use of exchange resin, although too lengthy for rapid soil testing, appeared more effective in estimating crop response (mixed hay yields 1954–55) than did the buffered acetate solutions. The North Carolina acid extraction and the HNO₃ method for nonexchangeable K were least effective in this respect.

Nonexchangeable K, although related to the exchangeable fraction K, was not significantly associated with the total applied, indicating that the shifting of exchangeable to less available forms of K was not cumulative and is probably a reflection of the very low clay content of this soil. However, greater residues of K were found in topsoils than in subsoils indicating a relationship with soil organic matter which constitutes most of the exchange material. Average recovery of K from muriate and sulfate applications was essentially the same. There was no accumulation of extractable K in spring samples where less than 80 pounds of potash had been applied in the preceding fall.

¹ Contribution No. 898 of the Rhode Island Agr. Exp. Sta., Kingston, R. I. Presented before Div. IV, Soil Science Society of America, Cincinnati, Ohio, Nov. 12–16, 1956.

² Associate Professor of Agricultural Chemistry and Professor of Agricultural Chemistry, respectively. Since 1929, T. E. Odland, head, Department of Agronomy, has been in charge of this project.

Received for publication September 24, 1956. Accepted for publication November 27, 1956.