Soil Tests for Aluminum Toxicity in the Presence of Organic Matter: Laboratory Development and Assess Ment

Al toxicity in plants is related to the activity of Ae+ and AI­ hydroxy monomers in the soil solution, whereas Al complexed with ligands such as fluoride (F), s~lphate (SQ42-), and oxalate is not toxic. Estimation of toxic Al relies on measurement of "labile" Al after short contact times with colorimetric reagents or cation-exchange resins. However, shifts in equilibrium may result in non-toxic forms of Al reacting with the complexing agent or resin. A series of laboratory experiments tested the degree to which labile Al is related to A1+in simplified media and compared methods of estimating labile Al in the presence of organic ligands and in soils. Cation-exchange resins extracted more than the theoretical concentration of A1+from solutions containing a range of concentrations of OR and S04-. More Al was extracted in 15 s by 8-hydroxy­ quinoline than by Chelex-lOO from solutions of AI-humate at pH 4. In sands which had been spiked with Al and organic matter, the estimation of labile Al varied with both the method of measurement and type of extract. The cations present in commonly used soil-extracting chloride solutions can decrease the proportion of organically complexed A