THE SIGNIFICANCE OF KCl-EXTRACTABLE Al(III) AS AN INDEX TO LIME REQUIREMENT OF SOILS OF THE HUMID TROPICS

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Abstract
The significance of KCl-extractable Al(III) in acid soils of the humid tropics for determining lime requirement was evaluated by comparing the amounts of Al(III) extracted with 0.1 N KCl and 1 N KCl. The cumulative amounts of Al(III) removed upon successive extractions of the soils with 1 N KCl were found to be greater than those removed by exhaustive successive extractions with 0.1 N KCl. This could, in part, be attributed to the release of H+, owing to the increase of the surface negative charge in the presence of the higher electrolyte concentration at pH values above the zero point of charge, which in turn would be expected to result in an equivalent amount of KCl-extractable Al(III).The value of (pH-1/3Al) for the first and subsequent 0.1 N KCl extracts was found to remain remarkably constant and nearly equal to 2.8, corresponding to the solubility of gibbsite. This would indicate that the concentration of Al(III) in 0.1 N and 1 N KCl extracts was determined by the solubility of precipitated Al(OH)3 and amorphous Al(OH3 originally present in the soil. Gibbsite was found to be virtually insoluble in 1N KCl at pH values above 4. The significant conclusion that may be drawn is that the amount of Al(III) extracted by 1N KCl does not represent the intrinsic property of the acid soils of the humid tropics. The significance of KCl-extractable Al(III) in acid soils of the humid tropics for determining lime requirement was evaluated by comparing the amounts of Al(III) extracted with 0.1 N KCl and 1 N KCl. The cumulative amounts of Al(III) removed upon successive extractions of the soils with 1 N KCl were found to be greater than those removed by exhaustive successive extractions with 0.1 N KCl. This could, in part, be attributed to the release of H+, owing to the increase of the surface negative charge in the presence of the higher electrolyte concentration at pH values above the zero point of charge, which in turn would be expected to result in an equivalent amount of KCl-extractable Al(III). The value of (pH-1/3Al) for the first and subsequent 0.1 N KCl extracts was found to remain remarkably constant and nearly equal to 2.8, corresponding to the solubility of gibbsite. This would indicate that the concentration of Al(III) in 0.1 N and 1 N KCl extracts was determined by the solubility of precipitated Al(OH)3 and amorphous Al(OH3 originally present in the soil. Gibbsite was found to be virtually insoluble in 1N KCl at pH values above 4. The significant conclusion that may be drawn is that the amount of Al(III) extracted by 1N KCl does not represent the intrinsic property of the acid soils of the humid tropics. © Williams & Wilkins 1976. All Rights Reserved.