THE GENESIS OF STRUCTURE IN TWO CALCAREOUS SOILS

Abstract

The effect of bacteria, actinomycetes, and fungi and of roots of sudan grass on the formation of water-stable structures in 2 calcareous soils, Gila clay and Tucumcari sandy loam, was studied by the wet-sieve method with the following results Each group of microbes formed a distinct type of aggregate. The by-products of these organisms grown in liquid culture formed no aggregates in sterile soil. When water-stable structures were observed in these or any of the other treatments, living organisms were always observed in the structure. Sudan grass grew weakly or not at all in sterile cultures, and the roots produced no soil structures. When grown in soil cultures inoculated with a mixture of the micro-bial groups or contaminated by any one of them, sudan grass produced compound structures comprising microbial structures held together by roots. Total aggregation was not affected by the presence of roots, but the structures were larger when roots were present.