Sorption and Desorption Quantity‐Intensity Parameters Related to Plant‐Available Soil Phosphorus

Phosphorus sorption quantity‐intensity ( Q/I ) relationships have been used more frequently than desorption to assess the capacity of soils to supply P to plants, although desorption properties would seem more appropriate when studying the soil‐plant system since P release is a key feature. The objective of this study was to compare sorption and desorption Q/I parameters as soil P availability indexes. Five soils with diverse chemical and physical properties were enriched with 0, 25, 50, 75, and 100 mg P kg −1 . Phosphorus sorption and desorption data were related to corn ( Zea mays L.) growth and P uptake results from a greenhouse experiment. The solution P concentration after a given P addition and the corresponding slope of the Q/I curve or P buffering power were used as the sorption intensity and capacity parameters, respectively. The solution P concentration when no P is desorbed by the sample ( I o ), the magnitude of slope of the Q/I curve at I o or P buffering power at I o , and the maximum desorbed soil P ( Q max ) were used as desorption intensity, capacity, and quantity parameters, respectively. Intensity and capacity factors derived from sorption and desorption Q/I curves were not good indicators of the P supplying capacity of soils, but the Q max was better related to plant growth and P uptake. These results suggest that a specific quantity parameter gained from P desorption Q/I relationships is more useful and meaningful than any parameter obtained from P sorption curves when evaluating the plant‐available P in soils. No simple relationship existed between capacity parameters obtained from sorption and desorption curves.