Reactions between Fulvic Acid, a Soil Humic Compound and Inorganic Soil Constituents

Abstract

AbstractThe order of stabilities at pH 5.0 of complexes formed between a water‐soluble Podzol fulvic acid and nine divalent metal ions was Cu > Pb > Fe > Ni > Mn > Co > Ca > Zn > Mg. Log K (stability constant) values ranged from 8.69 for the Cu‐complex to 2.09 for the Mg‐complex and were higher at pH 5.0 than at pH 3.5. The Irving‐Williams series was not followed. Provisional results indicated that log K values for Fe3+‐ and Al3+‐fulvic acid complexes were higher than those for divalent metal‐fulvic acid complexes. Two types of reactions occurred between fulvic acid and metal ions and hydrous oxides: a major one in which both COOH and phenolic OH groups reacted simultaneously, and a minor one involving less acidic COOH groups only. To develop analytical methods for investigating metal‐organic matter complexes occurring in soils, model compounds, containing varying metal to fulvic acid ratios, were prepared. The methods so developed were later used as diagnostic tools for the characterization of metal‐organic matter complexes extracted from natural soils or occurring as distinct horizons in soil profiles.In contrast to unreacted fulvic acid, metal fulvic acid complexes reacted with phosphate via the metal to form fulvic acidmetal‐phosphate. The interlamellar adsorption of fulvic acid by montmorillonite depended on the pH of the medium, the fulvic acid concentration and the type of cation with which the clay was saturated. Fulvic acid was found to have a positive effect on root initiation, which appeared to be related to its metal complexing ability.