The present work was conducted in a greenhouse at the Department of Soil Science of the Federal University of Lavras, Minas Gerais State, Brazil, in the period of February to June 2002, to investigate the response to potassium fertilization by rice plants (Oriza sativa L. cv. Jequitibá) cultivated in flooded soils, the contribution of the K form on their nutrition, and to estimate the critic levels of the nutrient in the soils and in the plants. The experimental design utilized was the totally randomized, in the 4 x 5 factorial scheme, with four replications. Four flooded soils (Mesic Organosoil - OY, Melanic Gleysoil - GM, Haplic Gleysoil - GX, and Fluvic Neosoil - RU) and five levels of potassium saturation of the potential CEC (natural content, 4, 6, 8 and 10% of the pH - 7.0 CEC saturated with K) were used. Natural soil K saturations (percentage of pH-7.0 CEC) were 1.55 for the OY soil, 1.30 for the GM soil, 1.65 for the GX soil, and 1.49 for the RU soil. Initially, sufficient samples of 4 dm³ for all soils received limestone to elevate the V to 50% and were incubated for 30 days. Following that, samples of 3 dm³ received the potassium treatments and a basic fertilization with macro and micronutrients and were incubated for more 30 days, in vases with capacity for 5 dm³. In the end of this period, the soils were flooded, to receive two rice plants per vase, on which were measured the leaf content of potassium at the flowering period and the yield at the end of the cycle. In sub-samples obtained from the soils of the vases before and after cultivation, the forms of potassium total K (Ktot), nonexchangeable K (Kne), exchangeable K (Ke), K in solution (Ksol) and available K by the Mehlich-1 extractant were measured. From the results, it was verified that the plants responded in grain yield to the potassium fertilization in all the studied soils. There was uptake of Kne by the crop, however, it was the Ke form that contributed most to the supply of the nutrient.The critic saturation values of K to obtain 90% of the maximum production were 4.0, 8.1, 7.8, and 4.4% for the RU, GX, GM and the OY soils, respectively. On the same way, the critic leaf levels of K were 9.6, 11.1, 10.2 and 11.5 g kg-1.
exchangeable K; nonexchangeable K; total K; lowland soil; Oryza sativa