Calcium Fluoride Formation During Extraction of Calcareous Soils With Fluoride: II. Implications to the Bray P‐1 Test

Abstract

AbstractCalcium fluoride formed during a 1‐min extraction of calcite (50‐2 µm) with the Bray P‐1 reagent (0.03M NH4F/0.025N HCl) sorbed added P. Because of the problem in identifying minor components of a whole soil matrix by X‐ray diffraction analysis, it was frequently difficult to positively identify CaF2 in residues following a 1‐min Bray P‐1 extraction of calcareous soils; CaF2 was, however, identified in a highly calcareous soil (59% CaCO3) following a 1‐min Bray P‐1 extraction. Added P sorbed by calcite and several calcareous soil horizons (containing more than 10% CaCO3) during 1‐min and 24‐hour extractions with the Bray P‐1 reagent was not recovered in a subsequent 1M NaCl or 0.1N NaOH extraction. By comparison with the data obtained for calcite and noncalcareous soils, the P sorbed during the Bray P‐1 extraction of calcareous soils was shown to be associated with CaF2 and not with CaCO3 or Al and Fe components. Formation of CaF2 and immobilization of P during a 1‐min Bray P‐1 extraction was demonstrated for a wide range of calcareous soils by comparing the amounts of added P recovered in subsequent NaCl and NaOH extractions with the amounts of added P recovered in NaCl and NaOH extractions subsequent to extraction with 0.03M NH4Cl/0.025N HCl. The low Bray P‐1 estimates of “available” P, frequently reported for calcareous soils, may be attributed to the formation of CaF2 when the dilute HCl is neutralized by CaCO3.