The determination of exchangeable aluminium in soils

A leaching procedure employing 1N potassium chloride was found to yield satisfactory estimates of exchangeable aluminium in subsoil samples. The use of 1N ammonium acetate solutions as extractants was not recommended. When buffered at pH 4.8 these solutions gave low results, whereas at pH 4.0 there was some attack on hydroxy-aluminium polymers, which led to erroneous results, particularly in sesquioxidic soils. Satisfactory estimates of exchangeable aluminium in subsoils were obtained by potentiometric titration of soil suspensions in 1N potassium chloride with 0.1N sodium hydroxide, provided allowance was made for the contribution of pH-dependent charge between pH 4.0 and 5.5. This procedure gave values consistently lower than those obtained by leaching with 1N potassium chloride, and of the order of 81 % of these. This was consistent with the formation of an aluminium compound with the formula Ak6(OH)153+ whose existence has been postulated by other workers.