MAINTENANCE OF SOIL ORGANIC MATTER

Abstract

The effect of inorganic colloids on the rate of loss of C during the decomposition of several organic materials was detd. A wet combustion method for determining C, not previously used in soil analysis, was employed. This method, which uses a combustion mixture consisting of fuming sulfuric, phosphoric, chromic, and iodic acids, was shown to be as accurate as the dry combustion method. In expts. lasting 1 yr., certain colloids exerted a marked effect in holding the C of decomposing green manures and straw, but had little effect on the C of the more resistant peat, sawdust and cellulose. Mont-morillonite added to sand exerted a nearly 2-fold effect in some instances, kaolin produced a much less effect, whereas Carring-ton soil colloid, which consists chiefly of montmorillonite and hydrous mica, produced an intermediate effect. In a typical 1-yr. expt., where soybeans and corn stover plus urea were added, the avg. % C retention values were for sand, 23; sand plus 10% kaolin, 26; sand plus 30% kaolin,31; sand plus 10% Carrington colloid,33; and sand plus 10% bentonite,41. In soil studies the results were similar except that plant C was retained nearly as well in a kaolinitic as in a montmorillonitic soil. The unexpectedly high values for the kaolinitic soil are due in part to its higher acidity. In general, the work indicates that colloids, especially montmorillonite, protect proteins and their degradation products against biol. attack through formation of organic-inorganic complexes. The C of succulent green crops, added to artificial or natural soils, does not disappear so rapidly as is generally supposed. After 1 yr. the C retention values for green oats, green soybeans, straw, and corn stover, all of which decomposed under conditions of abundant N, were approx. 34, 28, 38 and 38, respectively. These results emphasize the nature of the soil and the composition of the added organic materials as determining factors in the quantity of humus formed and held by a soil; the C/N ratio, as such, has often been over-emphasized.