Soil‐Conditioning Properties of Modified Agricultural Residues and Related Materials: II. Persistence of Soil‐Stabilizing Activity as a Function of Type and Extent of Modification

Agricultural residues, cotton linters, several of their chemical derivatives, and other related products were tested in the laboratory for their in‐soil stability against microbial degradation for a period of 6 months in a controlled humidity cabinet. A modified Yoder wet‐sieving technique was used to test the stabilizing effectiveness of a given polymer incorporated into Miami silt loam soil. Forty‐ to sixty‐mesh wheat straw, incorporated at a concentration of 0.5%, imparted greater stability to soil aggregates, following the first month of incubation at a temperature of 28° C., than did comparable applications of soybean and cotton stalks. Corn stover applications produced intermediate results. Twenty‐ to sixty‐mesh cotton linters at 0.5% concentration yielded increasingly effective soil aggregation during the first 3 months of incubation, reaching a maximum aggregation value of 93%. Chemically‐oxidized cellulose and corn starch showed surprising in‐soil stability throughout the 6‐month incubation period. In carboxymethyl cellulose samples, a degree of substitution (DS) of 0.7 or less was apparently insufficient to impart resistance to degradation by the microflora of the soil. However, carboxymethyl cellulose with a DS of 1.2 retained approximately 70% of its initial soilstabilizing activity over the 6‐month test period. In the case of hydroxyethyl and methyl cellulose, data are presented which tend to show that a high degree of substitution is not an adequate criterion of resistance to microbial degradation. Rather, the susceptibility of such polymers to enzymatic hydrolysis is apparently attributable to a lack of uniformity of substitution, resulting in a relative abundance of unsubstituted units in the cellulose chain.