Potassium availability for plants depends on the contents of the K forms present in the soils (non exchangeable, exchangeable and soluble K), which vary according to the degree of pedogenetic development of soils. The objective of this study was to evaluate K availability in soils with different degrees of pedogenetic development, based on the extraction of K from the soils with different extractants and with corn plants. Samples of the surface layer (0-30 cm) were collected from 12 representative soils in the state of Paraiba, of which six were more and six less developed. To the three replications of all 12 soils, five K doses (0; 50; 100; 200, and 300 mg dm-3) were applied in the form of KCl in solution and incubated for 21 days. Following, a sub-sample of 0.2 dm-3 of each soil was taken to determine the content of available K by the extractors Mehlich-1, Mehlich-3 and ion exchange resin. Then, corn was grown on the soils in a greenhouse for 30 days and the dry matter production and K plant content evaluated. The extractants Mehlich-1, Mehlich-3 and ion exchange resin extracted similar K amounts in the more developed soils. In the group of less developed soils, the extractants Mehlich-1 and Mehlich-3 extracted not very different K amounts, but much greater amounts than by ion exchange resin, especially in the soils with higher clay content and a larger proportion of 2:1 minerals. Unlike the extractants Mehlich-1 and Mehlich-3, ion exchange resin was not well-related with the exchangeable K content in the less developed soils. In this soil group, Mehlich-1 correlated best with the plant K content. In most soils the dry matter production of the plant did not increase with increasing K doses, but in all soils the K plant content increased in response to K fertilization.
extractants; availability potassium; potassium forms; semi-arid
