Corn is sensitive to a deficiency of potassium (K),² and yields of corn on K-deficient soils often are increased by K fertilizer applications. K deficiencies in corn have been observed or yield increases have been obtained from K fertilizer applications on many soils in the North Central Region of the United States. However, many other soils in the region have high levels of available K, and applications of K fertilizers on these soils have not increased corn yields. Therefore, it is important to have effective methods of estimating plant availability of K in different soils and to develop methods of predicting the yield response which can be expected from applications of K fertilizer for corn grown on different soils.

The increases in yields of corn and other crops obtained from applications of K fertilizer have been shown in some studies to be inversely related to the level of exchangeable K in air-dry samples of the surface soil (5, 7). Therefore, this determination is commonly used in soil testing laboratories to estimate K availability. Other studies, however, have shown that the amount of exchangeable K in some soils is markedly changed by drying the soil (1, 2, 9, 17, 18). In some of these studies, the level of exchangeable K in undried soil samples provided a better estimate of K availability to plants than did the exchangeable K in dry soil samples (2,9,17). Matthews and Sherrell (18), on the other hand, studied the relation between exchangeable K in the soil and the yield of potatoes grown on sandy soils in Ontario, Canada and found a higher correlation with exchangeable K values for oven-dry soils than for undried soils.