Phosphorus sorption characteristics in acid sulfate soils of Thailand: Effect of uncontrolled and controlled soil redox potential (Eh) and pH

Abstract

Two laboratory experiments were conducted to study phosphorus sorption characteristics in acid sulfate soils (para- and actual-acid sulfate soils) of Thailand. In one experiment the soils were subjected to oxidized and reduced conditions, in another they were maintained under different controlled pH (4.0, 5.0, and 6.0) and Eh (+600, +400, +300, +200, +100, and 0 mV) conditions. In both experiments the soils were kept in stirred suspensions with a soil to 0.01 M CaCl2 solution ratio of 1:7 for 6 weeks. After the incubation period, the soil suspensions were equilibrated with KH2PO4 ranging from 0 to 500 mg P kg−1 soil. Sorption isotherms were described by the classical Langmuir equation. The results from the first experiment showed that more native insoluble P was released under reduced than oxidized conditions, with more being released from para-acid sulfate soil than from actual acid sulfate soils. Soil reduction also caused an increase in P-sorption. Less P-sorption occurred under both conditions in para-acid sulfate soil than in actual acid sulfate soils. In the second experiment, the P-sorption of both actual and para-acid sulfate soils was significantly affected by soil Eh, pH, and their interactions. The P-sorption increased significantly with increasing pH and decreasing Eh. At pH 4.0, a considerable increase in P-sorption occurred as Eh decreased from +400 to +300 mV, whereas at pH 5.0 and 6.0 an obvious change in P-sorption occurred when Eh decreased from +300 mV to +200 mV. The actual acid sulfate soil sorbed more P than did para-acid sulfate soil. Significant correlation between P-sorption parameters and iron-oxides indicated the primary role of iron-oxides in P-sorption of acid sulfate soils of Thailand. Aluminum-oxides seemed to play a secondary role in P-sorption of these soils. Manganese also showed a significant effect on P-sorption.