ALUMINUM COMPLEXATION BY HUMIC SUBSTANCES AND ALUMINUM SPECIES IN THE SOIL SOLUTION

Humic substances strongly affect Al speciation in the soil solution of acid soils. Therefore, conditional stability constants (K0) of humic-Al complexes were investigated at pH 4.0, 4.5, 5.0, and 6.0 and were used for Al speciation calculations in the presence of humic substances. The intrinsic logK(o) values, calculated from the Scatchard plot increased between pH 4.5 and 5.0 by more than two units (4.10 and 6.30, respectively). With rising Al coverage of the humics, logK0 decreased up to one unit. The logK0 values were employed for the calculation of the ratio of organically complexed Al/total Al (alpha) at a concentration of 10(-4) eq/1 humics in the soil solution. At pH >5, alpha was greater than 0.9, whereas at pH 4 and 4.5 small variations of logK(o), e.g. as a result of different Al coverage of the humics, strongly affected alpha. Furthermore, a method of calculating the net H+ release per mol organically complexed Al is presented which results from Al hydrolysis at the humic surface and proton consumption by weak acidic groups of the humics. The net H+ release can strongly affect speciation calculations.