Extraction of Aluminum from Aluminum-Organic Matter in Relation to Titratable Acidity
1984
Hargrove, W. L. | Thomas, G. W.
This study was conducted to further characterize aluminum (Al)-organic matter complexes and their role in soil acidity. The objective was to determine the extractability of Al from organic matter and to relate it to titratable acidity. Two samples of muck were treated with Al³⁺ in such a way as to achieve a range in Al saturation. The titratable acidity [determined conductimetrically with Ca(OH)₂] of selected samples was compared to the Al extracted with 1 M KCl, 0.5 M CuCl₂, 2 M HCl, 1 M NH₄OOCCH₃ (pH 4.8), 0.5 M CaCl₂, and 0.33 M LaCl₃. Results show that NH₄OOCCH₃ (pH 4.8) is a very poor extractant of Al associated with organic matter. For unbuffered salts, the effectiveness of the extractant generally decreased as the valence of the replacing cation decreased (i.e. La³⁺ > Ca²⁺ > K⁺). Copper is an exception to this rule as a result of its ionic structure and its tendency to form “inner sphere” complexes with organic matter, displacing a disproportionate fraction of the Al. Lanthanum extracts an amount of Al which seems well related to the titratable acidity and, thus, to the lime requirement. On the other hand, K⁺ extracts far too little Al, while Cu²⁺ extracts all of the Al present. The use of LaCl₃ to predict lime requirement on field soils (especially where organic matter is increasing such as for no-tillage, etc.) looks promising but needs widespread evaluation.
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