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The Significance of Apatite Inclusions in Soil Phosphorus Studies¹

ABSTRACT

Apatite inclusions have been identified in several primary minerals, and their abundance and morphology have been described. A comparison of several methods used for the determination of total P in soils showed that Na₂CO₃ fusion is the most reliable method for samples which contain apatite inclusions and that the effectiveness of extraction was usually in the order: Na₂CO₃ fusion > HF digestion > HClO₄ digestion > N H₂SO₄ extraction following ignition. Included apatite was identified using a petrological microscope in residues after digestion with 60% HClO₄. This form was also extracted much more slowly by 0.5N HCl than was nonincluded apatite. Included apatite resistant to attack by 1N HCl was determined as "residual inorganic P" in fractionation studies on weakly weathered materials, and the influence of fineness of grinding on the ratio of "acid-extractable Ca-P" to residual inorganic P indicated the likely dimensions of the inclusions in different minerals.

¹ Contribution from the Dep. of Soil Sci. Lincoln College, Univ. of Canterbury, New Zealand. Supported in part by scholarships to the first two authors from the Commonwealth Scholarship and Fellowship Plan. Presented before Div. S-2, Soil Sci. Soc. Amer., Nov. 5–10, 1967, Washington, D.C.

² Formerly Research Fellow, now Project Associate, Univ. of Wisconsin; formerly Graduate Student, now Scientific Officer, Rothamsted Exp. Sta., Harpenden, Herts., England; Lecturer; and Professor of Soil Science, respectively.

Received for publication May 29, 1967. Accepted for publication July 19, 1967.