The relationship of polysaccharides to soil aggregation

Abstract

Studies were conducted on two soil types to determine whether variations in level of aggregation, induced by adding wheat straw were accompanied by significant changes in the polysaccharide content of the soils. The addition of finely ground wheat straw to the soils, followed by varying periods of incubation, markedly improved the level of aggregation in both soils. Where nitrogen was added together with the straw, the percentage of aggregates > 0.5 and 0.1 mm. diameter was reduced significantly as compared to the straw treatment alone. The data indicate that maximal structural improvement due to straw additions can only be attained where N is maintained at a relatively low level. The level of aggregation of the variously treated soils was shown to be approximately 35 per cent dependent on the 'microbial gum' content of the soil samples. This low level of correlation was due m part to the fact that the maximum level of aggregation was attained at a different sampling period than maximum accumulation of 'microbial gum'. Also, appreciable contents of polysaccharides were found in the humin and humic acid as well as the fulvic acid fractions of the soil organic matter. It is concluded that level of aggregation is a function, not only of the microbial gum content of the soil, but also of the carbohydrate carbon content of the numic acid-humin fraction.