POTASSIUM CYCLING AND BALANCE AND SOYBEAN YIELD IN AN INTEGRATED CROP-LIVESTOCK SYSTEM UNDER NO-TILLAGE

Nutrient cycling and balance in integrated crop-livestock systems (ICL) are processes that involves soil, plant and animal components, beyond fertilization. Animal management, by modifying soil biotic and abiotic conditions, also influences this process as it affects the residue decomposition rate. The objective was to evaluate potassium (K) cycling and balance in an ICL system with grazing (black oat and ryegrass) intensities in the winter, followed by soybean cultivated in the summer. The experiment was conducted as of May of 2001 in an area in the state of Rio Grande do Sul - Brazil, on a Rhodic Hapludox (Oxisol), under no tillage since 1991. The treatments consisted of continuous bovine grazing intensities at a pasture height of 10, 20, 30 and 40 cm, and a control (no-grazing), in a randomized block design, with three replicates. The forage/soybean cycle of 2007108 was evaluated in this paper. To calculate K cycling, the contents accumulated in pasture, soybean and animals in one pasture-soybean cycle were considered, and the balance, "in the field" and "in the soil", of K inputs (fertilizer) and outputs (soybean grains and animal tissue) and the K remaining in plant and soil, respectively, were calculated. Potassium cycling increased with increasing grazing intensity and the balance, despite negative, with a higher deficit in areas with higher grazing intensity, did not influence soybean yield.