Potassium management over 60 crops: A long‐term study on an Oxisol under no‐till

Potassium (K) is a typical yield‐limiting nutrient for soybeans [ Glycine max  (L.) Merr.] in humid tropical regions. The effect of fertilization on K balance over 32 yr of continuous cropping was investigated in a highly weathered Rhodic Hapludox under no‐till. The nutrient balance was calculated based on the amounts of K added via fertilization and on the amounts removed by the crops. Potassium rates interfered with yields over time and modified the final balance of K in the system. Depending on K rates, the balance tended to be positive or negative, and the range of K rates evaluated allowed us to establish conditions of either continuous K depletion, resulting from K removals by harvests, or the accumulation in soil under high rates. In this range, indicators of the efficiency of K fertilizer also varied and made it clear that K fertilizer is not the only source of K for plants.