Phosphorus retention in some soils of the Argentinean Mesopotamia

Phosphorus (P) sorption isotherms were derived for 13 soils of the Argentinean Mesopotamia with a wide range of physicochemical properties. Shaking time varied from 1 to 24 hours in order to estimate the phosphate buffering capacity (PBC), maximum P sorption capacity (MPSC), and P sorption energy (PSE). In addition, six rates of P, allowed to react with soil at field capacity for 63 days and labile P, non-labile P, and P in solution, were assessed after that time. The retention capacity of these soils ranged fi om medium to very low and P sorption was correlated not only with the content of amorphous aluminum (Al) and iron (Fe), but with the clay percentage of soils. Organic carbon was a variable not related to P retention. After two months, soils with more P fixation showed about 70% of the added P became non-labile with insignificant amounts in solution. In those showing low retention, P was distributed in the following proportion: 50 to 60% non-labile, around 40% labile, and less than 10% in solution, whereas for sandy soils, of very low fixation, 10 to 20% remained in solution and less than 40% became nonlabile. The increase in the total P due to fertilization resulted in a relative increase in the more available forms for plants. At the same time, a proportional decrease of non-labile P was observed.