Phosphorus retention by tropical soils as influenced by sulfate application
1998
Cajuste, L.J. | Laird, R.J. | Cuevas-G, B. | Alvarado-L, J.
Soils with variable surface charge are reported to have high affinities for the adsorption of anions and also to selectively retain cations. A laboratory study was conducted to determine the influence of sulfate (SO4(2-)) anion on phosphate (PO4(3-)) sorption/desorption from tropical soils. Duplicate 1.0 g soil samples were shaken with 10 mL portions of varying concentrations of calcium sulfate (CaSO4.2H2O) (0.0 to 1.5 x 10(-2) M). Subsequently, the samples were equilibrated with increasing concentrations of monocalcium phosphate [Ca(H2PO4)2.H2O] in 0.01M calcium chloride (CaCl2) to measure PO4 sorption and afterwards with 0.01M CaCl2 for PO4 desorption. Results showed that PO4 sorption maxima increased as concentrations of SO4 in solution increased. In the most acidic soil, SO4 adsorption from the 0.015M CaSO4.2H2O solution increased PO4 sorption by 93%. There was a decrease in pH in the SO4 extracts; this would indicate that calcium (Ca) from the SO4 salt applied to the soils before P addition was specifically adsorbed by the exchange complex and originated new sites for binding PO4. On the average, about 20% of the sorbed PO4 was removed with the CaCl2 treatment, suggesting that PO4 was specifically adsorbed by the variable charge soils.
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