The low capacity to predict potassium (K) availability for rice by different evaluation methods is due to the greater potassium diffusion in the flooded soil solution and to potassium supply through non-exchangeable forms in the soil. The purpose of this study was to evaluate the supply capacity of exchangeable and non-exchangeable potassium forms by flooded soils with different characteristics, in successive rice crops. Rice plants were grown in a greenhouse in 12 dm³ pots in three soils (Typic Hapludales, Aquic Udorthents and Lithic Udorthents) that represent soils from different rice producing regions in the state of Rio Grande do Sul (Brazil), where the yield response to potassium fertilization is low. Soils were fertilized with potassium in three successive rice crops. Soil texture and mineralogy were characterized and rice plants were grown until flowering. Potassium was determined in the plant tissues and in exchangeable form in the soil, before and after rice cultivation. It was found that the soils differed in terms of potassium supply capacity to flooded rice and the contribution of non-exchangeable forms was significant. The short-term supply capacity (first crop) was related to the exchangeable form, and in the long term (2nd and 3rd crop) mostly by the potassium saturation ratio in the cation exchange capacity (K/CECpH 7,0) and by non-exchangeable forms, particularly, potassium feldspars, smectites and smectite interlayered with Al-hydroxy , found in different amounts in the analyzed fractions.
Oryza sativa; potassium absorption and availability; non-exchangeable forms
