Relation of Available Potassium to Soil Moisture

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Abstract
Preliminary studies revealed that exchangeable K levels in Iowa soils fluctuated in response to vapor pressure changes and air drying. This phenomenon was investigated in relation to (1) K nutrition of plants in the greenhouse and (2) the problem of predicting the relative K‐fertility status in a wide range of Iowa soils. The thirteen soils employed in the greenhouse study showed variable increases in exchangeable K content upon air drying. Accompanying increases occurred in K uptake by the corn plants. Effects on K absorption by plants associated with drying of the soils often were equivalent to applying 60 to 120 lbs. K per 2 million lbs. of soil.Relative ability of the soils to supply K to plants under green‐house conditions was better correlated with exchangeable K content of undried than of air dried soils. This was true whether or not soils were air dried for cropping.Under field conditions, fluctuations in soil moisture levels in the surface one‐inch layer were accompanied by marked changes in content of exchangeable K over an 11‐week period. Below this layer, exchangeable K remained nearly constant since moisture content did not drop so low as in the surface. Results obtained in both field and laboratory showed that not much increase in exchangeable K occurred until soil moisture level had dropped to near 5 per cent or below. On the basis of overall results obtained it is concluded that for Iowa soils exchangeable K values determined after air drying may not be as reliable as those determined on moist soils, in predicting K supplying abilities of soils. Additional field studies are needed, however, to evaluate effects of alternate drying and wetting of soil on K nutrition of the crop.
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