PHOSPHATE SPECIATION IN EXCESSIVELY FERTILIZED SOIL - A P-31 AND AL-27 MAS NMR-SPECTROSCOPY STUDY

Both P and Al MAS NMR spectra of samples of excessively fertilized san dy soil provided information about the P and Al speciation. Peak decon volution was used to interpret reliably acid quantitatively the P-31 N MR 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 temperat e climates. A dithionite extraction used to remove interfering Fe from the samples also removed most of the octahedral AI-P phase. After oxa late extraction more than 99% of the original P signal disappeared. Ab out 7.5 to 11% of the total oxalate extractable P of the excessively f ertilized soil was present as a Ca-P phase, even though these soils ar e 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 assess ed 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.