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The Influence of Clay Minerals on the Breakdown of Certain Organic Substrates¹

ABSTRACT

Carbohydrate materials, crop residues and proteinaceous materials were degraded by soil microflora in the presence and absence of clay minerals. The carbohydrates used included two cellulose dextrins differing in solubility and chain length, polysaccharides from sweet corn, starch dextrin, and hydroxyethylcellulose. The crop residues used were alfalfa meal, soybean leaf meal, soybean meal, and oat straw. Casein and gelatin were utilized as proteinaceous materials. A yeast nucleinic acid and a pectin (RX3) also were used in this study.

A calcium bentonite, an illite, and a calcium kaolinite of pH 8.0 respectively, were added to the substrates in a mineral medium and phosphate buffer. Carbon dioxide was collected in standard Baryta's solution and determined by titration. The activity of the enzymes, cellulase and hemicellulase was determined in the presence and absence of two clay minerals using an Ostwald viscometer to measure the time of efflux.

The association of the substrate with the bentonitic clays resulted in an attenuated breakdown of many of these substrates. Very little protective influence was noted with the illitic and the kaolinitic clays. However, the kind and complexity of the substrate influenced the amount of breakdown; the simpler compounds were broken down more completely than the complex. The cellulose dextrins, casein, gelatin, soybean leaf meal, alfalfa meal, soybean meal and oat straw all evolved less carbon dioxide in the presence of calcium bentonite than in its absence.

¹ Published as Miscellaneous Paper No. 210 with the approval of the Director of the Delaware Agr. Exp. Sta. Contribution of the Department of Agronomy. Funds for this research were provided by Andelot Inc. through the Haskell Research Foundation.

² Assistant Professor and Associate Professor in Agronomy respectively, University of Delaware.

Received for publication August 12, 1955.