Phosphorus sorption index in humid tropical soils

•The PSI was highly correlated to the Smax.•As expected, the PSI was higher in the longer contact period.•Soils with higher contents of Alox, clay and C adsorbed higher amounts of P, which may lead to low P lixiviation and less P contamination into the underground water.

The dynamics of phosphorus (P) in soils is strongly influenced by organic and inorganic solid phases, biological activity and environmental factors. Highly weathered soils naturally contain low levels of P available to plants and have high adsorption capacity. The maximum adsorption capacity of P (Smax) has been widely used to evaluate the adsorption capacity of soil P. The P sorption index (PSI) is also used for evaluating the P adsorption capacity of soil from a single concentration of P and is an effective alternative in the estimation of maximum adsorption. We obtained the maximum capacity of P adsorption (Smax) and the P sorption index (PSI) of 29 Brazilian soils with different chemical, physical and mineralogical attributes. The use of the PSI to estimate the adsorption of P in a long-term experiment was also evaluated. For Smax, rates of 10, 25, 50, 100, 200 and 260 mg L−1 of P in 24 h of contact were used. For the PSI, soils were divided into groups because of the variation in their capacity to adsorb P, and each group received an optimal P rate: 200, 500, 1000, 1500 or 3000 mg L−1 of P. The periods of reaction assessed were 1, 3, 7, 21, 42, and 84 d. Smax was affected by amounts of clay, Fe and Al oxides and organic carbon (OC), which ranged from 61.7 (Typic Haploxeralf) to 5459.5 mg kg−1 (Mollic Fluvaquent). In the PSI, the average percentage of P adsorbed at the end of the contact period (84 d) ranged from 23% to 49% of P that was mixed initially and was, on average, four times higher than their values after 1 d of contact. Oxisols, Alfisols and Gleysols had the highest values of PSI. On the other hand, Ultisols and Entisols had the lowest ones. The PSI behaved similarly to the Smax, and the highest values were found in soils with high contents of clay, C, crystalline and poorly crystallized Fe and Al oxides. Furthermore, the PSI was higher in the 84th day, highlighting the influence of the period of contact on P adsorption.