The effect of application of liming material (CaCO 3 or CaSiO 3 ) and phosphate, on pH, exchangeable Al, effective cation exchange capacity (ECEC) and P solubility is studied in several variable-charge soils of volcanic origin from Canary Island (Spain). The amendments were added to replace 1.5x or 2.5x exchangeable Al, and P was applied at rates to provide equilibrium solution concentrations of 1 mg 1 -1 . Incubation of the samples with P, had very little or no effect on pH and on the exchangeable Al content of the soils. When P and amendments were combined, exchangeable Al levels declined with no concomitant effect on pH. Phosphate application alone produced a small increase in the effective cation exchange capacity (ranging from 0.2 to 16.1 mmol c kg -1 ). This effect being much more marked when liming materials were previously added. Prior addition of CaCO 3 did not produce any effect on solution P, irrespective of the exchangeable Al content of the soils. On the contrary, in some of the soils CaSiO 3 treatment increased P concentration in solution.
Financed by the National Centre for Research and Development under grant No. SP/I/1/77065/10 by the strategic scientific research and experimental development program:
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