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(1)), 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.