THE RELATIONSHIP OF SOIL-PH TO ALUMINUM SATURATION AND EXCHANGEABLE ALUMINUM IN ULTISOLS AND OXISOLS

For soil acidity assessment purposes, the relationship of either percent Al saturation or exchangeable Al to water pH is often misleading for tropical soils. Published data from studies using Ultisols and Oxisols were used to determine whether relationships between Al saturation or exchangeable Al and pH in IM KCl and pHo are better criteria to assess soil acidity for these variable charge soils. For Ultisols, a highly significant negative relationship (R2 = 0.76**) was found between Al saturation and pH in 1M KCl. For Oxisols, soil acidity assessment appears to be better expressed in terms of actual exchangeable Al rather than Al saturation per se. Exchangeable Al correlated best with pHo (R2 = 0.788**). From a practical standpoint, Oxisols with pHo values higher than 4.0 should not contain exchangeable Al which could be detrimental to plant growth. For Ultisols, a threshold pH value in lM KCl higher than 4.0 should indicate an Al saturation less than 15%. The above relationships appear to take into account the effect of the electrolyte concentration and the nature of the soil charge system in assessing soil acidity in variable charge soils.