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Forms of Phosphorus and Fixation in Soils¹

ABSTRACT

A determination of the main forms of P, P fixation, and a comparison of several methods of determining available P were made on several Minnesota soils.

Inorganic and organic forms of P were characterized and the inorganic fraction was further subdivided into aluminum phosphate, iron phosphate and calcium phosphate. Fractionation data of the inorganic P showed that iron and aluminum phosphates were abundant in acid soils while calcium phosphate was abundant in alkaline soils. In soils having a pH near 7, all three forms of inorganic phosphorus were about equally distributed, though slightly more iron and aluminum than calcium phosphate was present. Organic P agreed, in general, with the content of soil organic matter, but some deviation was noted.

The capacity of the soils to "fix" P from added KH₂PO₄ was observed. It was found that the P-fixing capacity of soils increased with departure from a pH near neutrality.

Bray No. 1 and 2, NaHCO₃, citric acid, Morgan's, and water-solubility methods of determining available P were compared. Available P in the soils varied greatly depending upon the extraction procedure used. Generally the amounts of available P as determined by methods employing strong acids were higher than those using weak acids. Correlation analysis was carried out on forms of P and the amount of P as determined by the various quick methods.

¹ Paper No. 4083 of the Scientific Journal Series, Minnesota Agr. Exp. Sta., St. Paul. Presented before Div. II, Soil Science Society of America, Lafayette, Ind., Aug. 5. 1958.

² Graduate Student and Professor of Soils, University of Minnesota, respectively.

Received for publication March 16, 1959. Accepted for publication June 29, 1959.