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Phosphorus Fixation by Soils in Relation to Extractable Iron Oxides and Mineralogical Composition¹

ABSTRACT

The fixation of P was studied for 12 acid soils that contained various amounts of free iron oxides. A significant correlation (r = 0.77) was obtained between P fixation and dithionite extractable Fe, but regression analysis showed that soils which contained kaolinite as the only clay mineral fixed more P and were in a different population than soils that contained both vermiculite and kaolinite. Data are presented which show that Fe in the interlayers of vermiculite is not active in P fixation but is extractable with dithionite.

Amounts of Fe extracted with oxalate were significantly correlated with P fixation (r = 0.95). In this regard, except for one soil that contained moderate amounts of gibbsite, all soils were in the same population. The data suggest that oxalate does not remove Fe from the interlayers of vermiculite but that dithionite does. The oxalate method, as modified, would provide a more quantitative means to determine the relationship between iron-oxides and the P-fixing capacity of soils similar to those studied.

¹ Paper No. 1765, University of California Agr. Exp. Sta. Riverside. Part of the thesis submitted by the senior author in partial fulfillment of the requirements of the Ph.D. degree. Financial support of the Rockefeller Foundation for the senior author is gratefully acknowledged.

² Graduate student, Professor and Associate Professor of Soil Science, respectively, Dept. of Soils and Plant Nutrition, Riverside.

Received for publication July 27, 1966. Accepted for publication December 9, 1966.