Site-specific potassium application based on the fertilizer potassium availability index of soil

Soil varies in its potassium (K⁺) content and ability to supply K⁺ to crops. Site-specific K⁺ management aims to optimize crop production and minimize K⁺ loss from the soil. The spatial variation of available K⁺ prior to fertilizer application, the K⁺ fixation capacity of soil and soil texture need to be taken into account for variable-rate K⁺ application. This study was done to measure the spatial variation of the fertilizer K⁺ availability index (A_I), which shows the potential for K⁺ fixation, and to develop a strategy that takes the spatial distribution of this index into account for site-specific K⁺ application. To determine the fixation capacity, the linear relation between the amount of K⁺ added to soil and the amount of K⁺ fixed was determined on 40 topsoil samples. Samples of soil were equilibrated in a moist condition for 3 weeks after the addition of 0, 25, 75, 225 and 675 mg K⁺ kg^?1. The increase in exchangeable K⁺ was described by a linear relationship. The fertilizer K⁺ availability index (slope) varied from 0.20 to 0.49, indicating 51–80% of added K⁺ was converted to the non-exchangeable form. Principal component analysis (PCA) showed that the first two components accounted for most of the variation, 48.7 and 26.3% of total variation, respectively. A non-hierarchical cluster analysis (k-means clustering) identified four groups and the amounts of fertilizer K⁺ required were calculated for each group. The results suggested that such classes could form a basis for variable-rate application to maintain an adequate K⁺ status for crop production and to reduce potential K⁺ loss from soil by leaching.