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A Comparison of Several Chemical Methods With the Bio-assay Procedure for Extracting Zinc from Soils¹

ABSTRACT

A study was made to evaluate the forms of soil zinc removed by the Aspergillus niger bio-assay method and to compare several chemical procedures for extracting soil zinc. Soils used were from humid regions; only one was calcareous.

For 0.1N HCl, the effects of extraction time and ratio of soil to extracting solution were investigated. A ratio of 1:10 and a 45-minute shaking period were considered most satisfactory. Longer extraction periods and wider ratios resulted in removal of larger amounts of zinc, but the rate of zinc release diminished appreciably. The amount of soil zinc removed by 0.1N HCl during a 6-week extraction period, by successive extractions with 0.1N HCl, and the bio-assay values were approximately equal. This agreement suggests the existence of an acid soluble form which made up around one-fifth of the total soil zinc in the soils studied. Very little zinc was found in the exchangeable form, and the organic form, if present, was apparently small.

Amounts of zinc extracted with 0.1N HCl, acetic acid, EDTA, and dithizone were significantly correlated with the bio-assay values. Dithizone and EDTA removed about the same amounts of zinc. The bio-assay, dithizone, and 0.1N HCl procedures were most convenient and rapid. The acetic acid procedures appeared to be the least satisfactory since the range among soils was small at narrow soil to solution ratios, and extracts were too dilute for ready determination at wider ratios.

Zinc fertilization of the soil was reflected in zinc soil test values and plant content.

¹ Contribution from the Department of Agronomy, University of Illinois, Urbana, Ill. Published with the approval of the Director. Part of a thesis submitted to the Graduate College, University of Illinois, by the senior author in partial fulfillment of the requirements for the Ph.D. degree. Assistance of Walter C. Jacob in the analysis of covariance is gratefully acknowledged. Presented before Div. IV, Soil Science Society of America. St. Paul, Minn., Nov. 11, 1954.

² Assistant and Professor of Soil Fertility, respectively.

Received for publication November 6, 1954.