The K supplying and fixing capacity of A and B horizons in seven Latosols and seven Andosols from Costa Rica was determined by successive cropping in the greenhouse, using tomato (Lycopersicum sculentum) as indicator plant, and by successive soil extractions and incubations.The K supplying capacity of A and B horizons in Latosols and Andosols, measured by plant dry weight, K uptake, and exchangeable K in the soil, was high for the first two successive crops but dropped drastically for the succeeding crops. While some exchangeable K was fixed during the first two crops, some nonexchangeable K was released during the third crop to replenish the low levels of exchangeable K. Potassium fertilization increased the exchangeable K fixed and decreased the nonexchangeable K released during successive cropping in both soils and horizons.The K supplying capacity, measured by successive extractions with normal, boiling HNO3, was similar to that indicated by plant growth and K uptake. While soil incubation under continuously wet conditions favored fixation of exchangeable K, incubation under wet-dry cycles resulted in release of nonexchangeable K.