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

Department of Soil and Crop Sciences, Texas A&M Univ., and Texas Agric. Exp. Stn., College Station, TX 77843

*Corresponding author.

ABSTRACT

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.

Received for publication February 4, 1993.