ALUMINUM AND IRON(III) SPECIES IN THE SOIL SOLUTION INCLUDING ORGANIC-COMPLEXES WITH CITRATE AND HUMIC SUBSTANCES

The solubility of Al and Fe in soil is of relevance for their toxicity and availability, respectively, to plant roots. Humic substances as t he main part of stable soil organic matter and citrate which is often excreted by P deficient plants are strong complexants of Al and Fe(lII ). Therefore, equations were developed to calculate the Al and Fe(III) species distribution in the soil solution in the presence of humic su bstances and citrate as organic ligands. Calculations in the pH range 4.0-7.0 showed that at higher pH humic-Al complexes were the most impo rtant species whereas AlOH-citrate(-) dominated between pH 4.0 and 5.4 . Free monomeric Al and AlSO4+ were of minor relevance. Iron(III) spec ies calculations showed that humic-Fe complexes were the main species in the pH range 4.0-7.0. But if mugineic acid, a Fe complexing phytosi derophore released into the rhizosphere by graminaceous plant species, was present in the soil solution (10(-6) M), Fe-mugineic acid complex es accounted for most of the Fe in solution. Fe-citrate(o) was relevan t at lower pH but contributed little to Fe(III) :species al pH > 6.0. The results demonstrate the strong importance of the considered organi c ligands for Fe and Al in the soil solution.