ALLEVIATION OF SOIL ACIDITY IN ULTISOL AND OXISOL FOR CORN GROWTH

Malaysian Ultisols and Oxisols are characterized by low pH, high soil solution Al concentration and Ca and/or Mg deficiencies, which are lim iting to corn growth. An experiment was conducted to determine the cha nges in solid and soil solution phase properties of a representative U ltisol and Oxisol following applications of ground magnesium limestone (GML), gypsum and their combinations, and their effects on corn growt h. A plot of pAl against lime potential (pH-1/2 pCa) showed that the p oints were mostly positioned between the theoretical lines for kaolini te - quartz and gibbsite equilibrium, reflecting the kaolinitic - oxid ic mineralogy of the Utisol and Oxisol. Gypsum application increased A l concentration in the soil solutions of the Ultisol, but had no signi ficant effect on that of the Oxisol. The increase in Al concentration in the Ultisol was due to an increase in ionic strength. Gypsum applic ation increased soil solution pH of the Oxisol due to release of OH as a result of ligand exchange between SO, and OH ions on the oxides of Fe and/or Al. Exchangeable Al in both soils was reduced by gypsum appl ication. The reduction was associated with solid phase immobilization through alunite formation, the soil solutions of soil samples treated with 2 and 4 t gypsum ha-1 were supersaturated with respect to alunite . Application of GML at 2 t ha-1 together with 1-2 t gypsum ha-1 gave high top weight of corn. Relative top weight of corn was positively co rrelated with a soil solution Mg and Ca/Al concentration ratio, but ne gatively correlated with soil solution Al concentration. Foliar Al cor n was positively correlated with soil solution Al concentration. Soil solution Al and Mg concentrations. and Ca/Al concentration ratio can b e used as indices of soil acidity in Ultisols and Oxisols.