Potassium Quantity‐Intensity Relationships in Calcareous Vertisols and Inceptisols of Southwestern Spain

Quantity‐intensity (Q/I) curves and derived parameters provide general information about soil K availability. This investigation was conducted to study the relationship between the K Q/I parameters and the properties of 24 calcareous Inceptisols and Vertisols of southwestern Spain. Three Q/I parameters were considered: activity ratio at equilibrium (AR o ), buffer power at equilibrium (BP o ), and buffer power in the 20 × 10 −3 to 30 × 10 −3 activity‐ratio range (BP 20‐30 ). Close linear correlations were found between buffer powers (BP 20‐30 and BP o ) and cation‐exchange capacity (CEC) ( r 2 = 0.949 and 0.888, respectively), and between AR o ) and initial exchangeable K percentage (EPP o ) ( r 2 = 0.762). A good correlation ( r 2 = 0.873) was also found between EPP and AR for all the soils within the range of K saturation studied. Twelve of the 24 soil samples were intensively cropped (nine wheat [ Triticum aestivum L.] croppings); the K balance showed that 80% of the K extracted by the crops came from the nonexchangeable K pool. The Q/I curves of the soil samples after cropping were displaced upward with respect to the original ones, and the AR o values were, consequently, lower; the displacement of the Q/I curves was similar to the decrease in exchangeable K. Following cropping, the BP 20‐30 values increased slightly. In spite of these changes, the shape of the Q/I curves remained nearly unchanged; therefore, this can be considered a characteristic of the soils studied. The good correlations between AR o and EPP o and between buffer powers and CEC allow prediction of the Q/I parameters of these soils.