EFFECT OF SOIL-PH ON AL AVAILABILITY IN SOILS AND ITS UPTAKE BY THE SOYBEAN PLANT (GLYCINE-MAX)

A greenhouse experiment was designed to investigate the effects of soi l pH and other soil properties on the availability of Al and its uptak e by the soybean plant. Three soils were compared that were developed over three contrasting bedrock types. These soils were (I) a sandy loa m over Devonian shale; (2) a sandy loam over granite; and (3) a loamy sand over the Lower Greensand. The natural pH of the soils ranged from 4.2 to much less than 5.5 and each soil was also amended to give two other pH levels using elemental sulphur and/or calcium carbonate. The solubility of the Al in the soils, and hence its availability to the p lants, was estimated using extraction with 0.02 M CaCl2 and 0.05 M EDT A prior to determination by ICP-AES, The Al concentrations in the plan t materials were similarly determined after acid digestion with concen trated nitric and perchloric acids. In the comparison between two empi rical extractants, the best predictor of soil Al available for uptake into the soybean plant was found to be 0.02 M CaCl2. This relationship appears to be linear. The amounts of extractable Al in the soils and the uptake of Al by the soybean plants both increased as the soil pH d ecreased. These relationships are both non-linear with marked increase s in extractability and uptake when the soil pH falls below 4.4. It is concluded that the speciation of Al in the soils changes at this pH v alue, and that the specie of Al taken up by soybean plants is the Al3 ion, which is the only stable Al ion at a pH of less than 4.4. Soil p H has been identified as the major factor that controls the uptake of Al from soil into the soybean plant.