PHOSPHORUS AND ALUMINUM RELEASE FROM A SPODIC HORIZON MEDIATED BY ORGANIC-ACIDS

The ability of organic acids to affect surface and solution reactions of P and Al in soils containing Al-oxide surfaces is related to their ability to form stable complexes with Al. This study was conducted to determine if the amount of P and Al released following the addition of an organic acid to a Bh horizon soil could be estimated based on the Al stability constant (logK(Al)) of the organic acid. The release of Al and inorganic P from spodic horizon material increased in the presence of organic acids that form stable complexes with Al. Overall, the logK(Al) value was a good indicator of the effect of the organic acid on inorganic P and Al release. Among the 16 organic acids studied, release of Al and inorganic P increased exponentially with increasing stability constants. A threshold value of approximately 4.1, however, was required before substantial amounts of inorganic P were released. This value may reflect the stability of P bond to Al-oxide surfaces in this soil. Salicylic acid was a notable exception to the observed trend in release of both Al and P; despite a large stability constant, little inorganic P or Al was released. Although significant amounts of soluble organic P were released in all the organic-acid solutions, the amount of soluble organic P released was not related to the stability constant of the organic acids.