ADSORBED PHOSPHORUS PARTITIONING IN SOME BENCHMARK SOILS FROM NORTHEAST BRAZIL

The partitioning of adsorbed P between labile and non-labile pools by soils is fundamental to the residual effect of fertilizer-P added to s oils. The main objective of the study was to determine the partitionin g of adsorbed P between the labile and non-labile phases by some bench mark soils of northeast Brazil for which is little is known. Surface a nd subsurface samples of several soils: Non-Calcic Brown soil and Plan osol (Haplustalfs), Cambisol (Ustropept), Lithosols (Orthents) and All uvial soil (Tropaquept) were equilibrated with varying concentrations of KH2PO4. The readily exchangeable portion of the adsorbed P was dete rmined by anion exchange resin (AER). Considerable hysteresis was obse rved between adsorbed P and AER-P. To quantify the extent of the hyste resis, a critical P concentration (P-crit), the amount of P adsorbed a t zero desorption by AER, was defined. The P-crit of the soils average d across the soil depths followed the order: Non-Calcic Brown soil > P lanosol > Alluvial > Cambisol > Lithosol. The P-crit correlated with c lay and oxalate Fe (Fe-o). The P affinity index (K) estimated by Langm uir adsorption model accounted for 66% of the variance in P-crit. A se quential extraction with 0.5 M NaHCO3, 0.1 M NaOH and 11.5 M HCl to re move the labile, moderately labile and non-labile P forms respectively , indicated that between 63 and 99% of adsorbed P was in the labile po ol (AER-P + HCO3-P + OH-P), suggesting that the soils might have high potential for residual fertilizer-P responses.