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 soils. The main objective of the study was to determine the partitioning of adsorbed P between the labile and non-labile phases by some benchmark soils of northeast Brazil for which is little is known. Surface and subsurface samples of several soils: Non-Calcic Brown soil and Planosol (Haplustalfs), Cambisol (Ustropept), Lithosols (Orthents) and Alluvial soil (Tropaquept) were equilibrated with varying concentrations of KH₂PO₄. The readily exchangeable portion of the adsorbed P was determined by anion exchange resin (AER). Considerable hysteresis was observed between adsorbed P and AER-P. To quantify the extent of the hysteresis, a critical P concentration (P_crit), the amount of P adsorbed at zero desorption by AER, was defined. The P_crit of the soils averaged across the soil depths followed the order: Non-Calcic Brown soil > Planosol > Alluvial > Cambisol > Lithosol. The P_crit correlated with clay and oxalate Fe (Fe_o). The P affinity index (K) estimated by Langmuir adsorption model accounted for 66% of the variance in P_crit. A sequential extraction with 0.5M NaHCO₃, 0.1M NaOH and 11.5M HC1 to remove the labile, moderately labile and non-labile P forms respectively, indicated that between 63 and 99% of adsorbed P was in the labile pool (AER-P + HCO₃-P + OH-P), suggesting that the soils might have high potential for residual fertilizer-P responses.