The release of exchangeable and nonexchangeable potassium from different soils upon cropping

Ladino clover was grown in the greenhouse in 11 different soils for periods ranging from 271 to 391 days. From four to six crops were harvested from each of the different soils studied and the amount of potassium removed by the tops and roots of each crop was determined. The exchangeable potassium content of the soil was determined at the start and at the time of each crop harvest. The amount of nonexchangeable potassium released was calculated by subtracting the decrease in the exchangeable potassium content of the soil from the total amount of potassium removed by the crop. Upon continuous cropping the exchangeable potassium content of the different soils decreased very rapidly at first, then more gradually until a certain level was reached, when the potassium-supplying power of the soil was determined largely by the rate at which the nonexchangeable potassium was converted into the exchangeable form. The dry weights and the potassium content of the plants of the successive crops were closely associated with the amount of exchangeable potassium in the soil. The potassium content of the plants decreased upon continuous cropping. Symptoms of potassium deficiency appeared on the clover leaves when the potassium content of the plants fell below 0.8% on the oven-dry basis. Of the 11 different soils studied, Honeoye silt loam, Chenango gravelly silt loam, and Ontario loam released large amounts of nonexchangeable potassium; whereas Mardin silt loam, Gloucester loam, and Honeoye gravelly silt loam released only small amounts of nonexchangeable potassium upon continuous cropping. These findings are related to crop responses to potassium under field conditions. The relative significance of the exchangeable and the nonexchangeable forms of potassium in determining the potassium supplying power of soils is discussed in light of the results obtained in this study.