ORGANIC-MATTER DELAYS BUT DOES NOT PREVENT PHOSPHATE SORPTION BY CERRADO SOILS FROM BRAZIL

The long-term effect of organic anions on phosphate sorption by soils has not been studied extensively. In this work, we investigated the ti me course of P sorption in 12 Cerrado (mostly Oxisols) soils by measur ing the P concentrations in soil suspensions supplied with various rat es of phosphate over a 256-day period. Phosphate sorption fitted exten ded Freundlich or Langmuir equations that included a potential time te rm. Equation parameters reflecting the significance of slow relative t o fast P sorption were positively correlated with the organic matter ( OM)/specific surface area (SSA) ratio. This suggests that the more ext ensive the coverage of soil-sorbing surfaces by organic compounds, the more significant the long-term P sorption as a result of phosphate sl owly replacing organic anions at sorption sites. The fact that the rel ative affinity for phosphate at a low equilibrium concentration decrea ses as the OM/SSA ratio increases supports the hypothesis that phospha te and organic anions compete strongly for the same sorption sites. Ox alate and malate were the only two low-molecular-weight organic anions that occurred in measurable amounts in the soil suspensions; the amou nt of (oxalate + malate) released from the solid phase was related rou ghly, in a 2:1 ratio, to that of P sorbed after the initial (1-day) P sorption. The results suggest that the blocking action of organic comp ounds on P sorption is only transient. In practice, this fact should b e considered when timing application of P fertilizers.