Exchangeable Soil Potassium as Influenced by Seasonal Cropping and Potassium Added in Crop Residues

Detailed sampling of soils and crops from three long‐term field experiments permitted evaluation of the seasonal depletion of exchangeable soil K and the influence of K added as crop residue. Exchangeable soil K was markedly reduced during the growing season by a growing crop. For corn ( Zea mays L.), the regression of the decrease (spring to fall) in exchangeable K (pp2m) on the initial (spring) level of exchangeable K was characterized by a quadratic relationship, Ŷ = −23.5 + 0.62X − 0.00072X 2 (r 2 = 0.78**). Increases in the exchangeable K from fall to spring were directly related to K additions in crop residues. Within a single crop rotation, the exchangeable K increased as a result of K additions from corn residues and declined when meadow was grown. When maintained in a field moist condition before the determination of exchangeable K, reduced levels of exchangeable K resulting from cropping were not restored by release of non‐exchangeable K. When the exchangeable K was reduced to a very low level, however, nonexchangeable forms of K were released to the exchangeable form to maintain a “minimal” level of exchangeable K.