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Chemical and Physicochemical Properties of Soil Humic Colloids: III. Extraction of Organic Matter from Soils¹

ABSTRACT

A study was made of the effects of certain soil treatments on the amounts of humic acids and organic matter extracted by a number of reagents, including sodium pyrophosphate of different pH values, and alkali. The pretreatments were: (1) removal of carbonates and exchangeable bases, (2) removal of free iron and aluminum oxides by mild reducing reagents, and (3) removal of silicate minerals with hydrofluoric acid. The extraction of organic nitrogen was taken as an index of organic matter extracted.

Removal of calcium and free iron and aluminum oxides increased the solubility of organic matter in alkali but had no effect on extraction by pyrophosphate. The removal of silicate minerals by repeated treatment of the soil with hydrofluoric acid was particularly effective in facilitating the extraction of humic acids by pyrophosphate. The proportion of humic acid nitrogen extracted to total organic nitrogen extracted was considerably higher in pyrophosphate extracts than in extracts of other reagents.

Several organic-complexing reagents, including cupferron, 8-hydroxyquinoline, acetylacetone, and disodium dihydrogen ethylenediamino tetraacetate, were also tested and it was found that they were not as effective as neutral sodium pyrophosphate for removing soil organic matter. In addition, a procedure was proposed for mitigating autoxidation reactions during extraction of organic matter.

¹ Contribution from the Department of Agronomy, Illinois Agr. Exp. Sta., Urbana, Illinois. This work includes a part of a thesis submitted to the University of Illinois by the senior author in partial fulfillment of the requirements for the Ph.D. degree, and was supported in part by Regional Research Funds under the North Central Cooperative Regional Project NC-17 on organic matter and soil structure. Presented before Div. III, Soil Science Society of America, Cincinnati, Ohio, Nov. 15, 1956.

² Research Assistant and Assistant Professor of Soil Biology, respectively.

Received for publication April 6, 1957. Accepted for publication May 9, 1957.