Phosphate speciation in excessively fertilized soil: a 31P and 27Al MAS NMR spectroscopy study

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Abstract
Summary: Both P and Al MAS NMR spectra of samples of excessively fertilized sandy soil provided information about the P and Al speciation. Peak deconvolution was used to interpret reliably and quantitatively the 31P NMR spectra recorded. Most of the P was found to be associated with Al. Part of the P exhibited a chemical shift that could be attributed to octocalcium phosphate, amorphous calcium phosphate or apatite. Apatite has, however, never been reported to occur in sandy soils of temperate climates. A dithionite extraction used to remove interfering Fe from the samples also removed most of the octahedral Al‐P phase. After oxalate extraction more than 99% of the original P signal disappeared. About 7.5 to 11 % of the total oxalate extractable P of the excessively fertilized soil was present as a Ca‐P phase, even though these soils are slightly acid to acid. The estimated size of the Ca‐P phase roughly corresponds to the size of the labile P pool of these soils, as assessed in long‐term batch desorption experiments. It still remains unclear whether the labile P pool should be attributed solely to such a Ca‐P phase.

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