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This article in SSSAJ

  1. Vol. 36 No. 1, p. 20-25
    Received: Jan 7, 1971
    Accepted: Sept 29, 1971

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Calcium Fluoride Formation During Extraction of Calcareous Soils with Fluoride: I. Implications to Inorganic P Fractionation Schemes1

  1. J. K. Syers,
  2. G. W. Smillie and
  3. J. D. H. Williams2



Added inorganic P sorbed during 0.5M NH4F extraction of calcite or aragonite was recovered in subsequent citrate-dithionite-bicarbonate (CDB) and HCl extractions of the Williams et al. (1967) inorganic P fractionation scheme. The sorbed P was shown to be associated with CaF2 formed by the replacement of CaCO3 during the NH4F extraction. Similar results were obtained for soils which contained calcite but no P fertilizer reaction products. Thus, the amounts of non-occluded Al- and Fe-bound P (NH4F- and NaOH-extractable) were underestimated whereas reductant-soluble P (CDB-extractable) was overestimated and acid-extractable Ca-P frequently overestimated. In contrast, magnesite, dolomite, and a dolomitic soil sorbed only a low proportion of added P during NH4F extraction. A modified fractionation procedure, developed for calcareous soils, omitted the NH4F extraction and involved sequential extraction with (i) NaOH to remove nonoccluded Fe- plus Al-bound P, (ii) NaCl and citratebicarbonate (CB) to remove P sorbed by carbonates during the preceding NaOH extraction, (iii) CDB to remove occluded P (reductant-soluble P), and (iv) HCl to remove Ca-bound P. Phosphorus added with the NaOH reagent to carbonate minerals and calcareous soils was recovered during the NaOH, NaCl, and CB extractions, enabling differentiation of nonoccluded Fe- plus Al-bound P from occluded P. The procedure also permitted a more meaningful separation of nonoccluded Fe- plus Al-bound P and Ca-bound P, particularly in calcareous soils which contained P fertilizer reaction products.

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