OXALATE EXTRACTION AND DETERMINATION BY IONIC CHROMATOGRAPHY IN CALCAREOUS SOILS AND IN MYCORRHIZED ROOTS ENVIRONMENT

Secreted by mushrooms, oxalic acid plays a basic role in calcareous so ils by trapping calcium (Ca) and releasing phosphorus (P). The assessm ent of this organic anion requires an extraction step before analysis. A theoretical study, with GEOCHEM software, emphasizes the complexing properties of oxalate towards Ca and Al at pH> 1.5, and especially fo r ferric iron (Fe III) at very acidic pH (< 0.5), thus complicating it s quantitation; it also specifies the experimental conditions for extr acting soil calcium oxalate and avoiding the Fe(III) interference. A s imple and quick method is proposed for the calcium oxalate solubilizat ion by mixing for 1 hour 5 grams of soil with boiling 1M HCl, the volu me depending on sample calcareous, Al and Fe content, and adding a red ucing agent (hydroxylamine) of ferric iron. A fast stirring with a Ht- resin puts the resulting solution in good conditions for ionic chromat ography analysis. The efficiency of this method is measured by the rat e of recovery (93 to 101 %) of known amounts of calcium oxalate added (0.8 %) to soils having various calcareous contents (28 to 50 %); it a llows to detect, with a good reproductibility (VC<5 %), amounts of oxa late as low as 0.01% in 30% calcareous soils. This method has been suc cessfully applied to calcareous soils sampled around mycorrhized roots .