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Ammonium Fixation in Soils¹

ABSTRACT

Laboratory experiments are reported which show the extent to which representative soils of the United States are able to fix ammonium, and the effect of various factors on fixation and on the availability of fixed ammonium to nitrifying bacteria. Especial attention is given to the comparative values obtained with KCl and CaCl₂ used as the extracting agents.

It is shown that subsoils fix much more ammonium than surface soils. Considerable moist fixation may occur in soil if the predominant clay mineral is illite or vermiculite, and these values are increased by drying and heating. Montmorillonitic clays fix little ammonium unless heated.

If ammonium-saturated soils are leached with N KCl, more fixed ammonium is left than if N CaCl₂ is used. Since potassium contracts some crystal lattices and calcium expands them, the value in the absence of salts would probably be intermediate.

The availability to nitrifying bacteria of fixed ammonium in several soils during a period of 2 months varied between 5 and 24%, being lowest for vermiculite-containing soils and highest for those containing montmorillonite. Leaching with CaCl₂, as compared with KCl, resulted in somewhat higher availabilities of the residual ammonium.

Low nitrification values for added ammonium salts were shown to be characteristic of soils that fix ammonium under moist conditions. The percentage nitrification in such soils is markedly increased if fixation is prevented.

Ammonium fixation is shown to be a factor of importance in agriculture, especially where ammonium fertilizers are added to the plow sole of nonkaolinitic soils.

¹ Contribution from the Division of Soil Management and Irrigation Agriculture, B.P.I.S.A.E., A.R.A., U.S.D.A., Beltsville, Md. Presented before Division III, Soil Science Society of America, Cincinnati, Ohio, November 19, 1952.

Received for publication January 21, 1953.