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

Abstract

AbstractAdded 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.