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Calcium Fluoride Formation During Extraction of Calcareous Soils With Fluoride: II. Implications to the Bray P-1 Test¹

ABSTRACT

Calcium 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.

¹ Contribution from the Department of Soil Science, Univ. of Wisconsin, Madison 53706. Research supported by the College of Agricultural & Life Sciences, Univ. of Wisconsin, Madison, by the Kellogg Foundation, and by Hatch (Project 1693).

² Kellogg Fellow, on leave from University College, Dublin, Ireland, and Associate Professor of Soil Science, Univ. of Wisconsin, Madison, respectively.

Received for publication January 7, 1971. Accepted for publication September 29, 1971.