EFFECTS OF PH ON PHOSPHATE SORPTION OF SOME PALE-ADULTS OF SOUTHERN NIGERIA

The effect of pH on phosphate sorption by soil samples from two coarse-textured pedons of typic Paleudults in the perhumid tropical zone of Nigeria was studied by equilibrating air-dried soil samples adjusted to pH 3.5–6.5 for 48 h with 10 ppm P and 60 ppm P solutions at two concentrations of CaCl2 and NaCl. Phosphate sorption was generally unaffected by pH at low P additions at the two concentrations of the salts, whereas at high P additions sorption generally decreased with increase in soil pH, except in the presence of the high concentration of CaCl2 (10−2 M) where sorption decreased up to pH 5.0–5.5; beyond this pH range it increased. In both 10−2 M CaCl2 and 3 × 10−2 M NaCl systems, phosphate sorption decreased by 19–89% as a result of dithionite-citrate-bicarbonate (DCB) treatment. The effects of pH on phosphate sorption by treated samples were similar to those by untreated samples. Phosphate sorption increased with increase in depth, probably due to the increase in oxalate and dithionite-citrate-bi-carbonate extractable Fe and Al and the decrease in organic matter content down the pedons. Phosphate sorption by samples from the B horizon increased with increase in period of incubation with lime, whereas in the samples of Ap horizons no consistent trend was obtained. © 1990 Williams and Wilkins.