Abstract
Potassium (K) is one of the most abundant nutrient elements in soil and one that is required in large amounts by plants. Of the huge amount of K in many soils only a fraction is available for immediate uptake by plants. In the longer term, and in the absence of any added K, the vulnerability of crop production to K availability is mainly related to the amount and rate of release of K from soil minerals. Maximizing the efficient use of K in the soil-plant system requires, whether from soil minerals, organic manure or fertilizers, a knowledge of critical soil K thresholds to maintain soil fertility and crop production. In cropping systems with a reduced K input, soil fertility will be compromised if soil K values fall below the critical level for the soil type and farming system. Successful K management in low-K-input systems depends on whether the release of indigenous K, minimizing leaching losses on sandy soils and, if economically viable, the return of K from urban areas, can supply sufficient K to meet crop demand. The extent to which soil K can be exploited will depend on soil type and mineralogy, cropping system and time-scale. Thus different soil types and cropping systems require a site-specific approach to K management.