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The Replacement of Nonexchangeable Potassium by Various Acids and Salts¹

ABSTRACT

A large proportion of the potassium absorbed by plants growing on Ramona loam is obtained from the nonexchangeable form. The nonexchangeable potassium may possibly be released to the solution phase before being absorbed by plant roots. This investigation concerns the release of fixed potassium from Ramona loam through the action of various acids and salts. The soil contains 20,000 ppm of potassium of which 78 ppm is exchangeable to neutral, normal ammonium acetate. Rye plants grown by the Neubauer technique removed in addition 112 ppm of nonexchangeable K from the soil. Water saturated with CO₂ at pH 4.0 leached through the soil for 50 days removed only 39 ppm of nonexchangeable K. It was noted that equal volumes of CO₂ saturated water which passed through the soil removed the same amount of K regardless of the rate of leaching or thickness of the soil column. The final concentration of potassium in the CO₂ saturated water leachates was 0.1 ppm K.

Extraction and leaching studies with dilute acid solutions varying in pH from 1 to 7 revealed a correlation between pH and the amount of K replaced from the soil. Potassium replaced with 0.1 N acid solutions with pH ranges between 3 and 7 was mostly from the exchangeable form, whereas that replaced at pH values below 3 included a large proportion of the non-exchangeable form.

A comparison of leaching and extraction studies demonstrates that for equal volumes and pH values the extraction method is more efficient in releasing nonexchangeable K than the leaching method. This holds even for water saturated with CO₂.

In extractions with various chloride salts, ammonium acetate, and hydrochloric acid, it was found that all ions except ammonium replaced nonexchangeable K from the soil. The ions in descending order of effectiveness of replacement of K from the soil are H⁺ > Na⁺ > Li⁺ > Ca⁺⁺ > Mg⁺⁺ > NH₄⁺.

¹ Contribution from the Division of Soils, University of California, Berkeley, Calif. Presented before Section II, Soil Science Society of America, State College, Pa., August 30, 1951.

² Assistant Soil Chemist and Professor, respectively.