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Soil structure is important in the attainment of adequate aeration for the aerobic microorganisms whose activities maintain such higher plant nutrients as sulfur, phosphorus, and nitrogen in a highly oxidized and thus readily available state. Good soil structure is also necessary for soil and water conservation, Under Nebraska conditions microorganisms and their decomposition products constitute a major source of soil-aggregating agents. The decomposition of crop residues and plant roots is important in the nutrition of these aggregating microorganisms. Other microorganisms, however, also utilize crop residues and plant roots, and they may decompose the aggregating agents as well, so that the aggregation produced may be only temporary. Thus the net effect of the entire soil microflora on soil structure is difficult if not impossible to predict. As a first step in gaining some understanding of the influence of the microflora on soil structure and ultimately in attempting some control of structure, it was deemed necessary to extend previous work dealing with the effects of pure cultures on structure. This paper presents the results of studies in which a number of different microorganisms, which were isolated at random from Nebraska soils, have been compared with respect to their ability to aggregate Peorian loess. In addition, for the better aggregators it presents the results of studies which sought to determine the conditions of time, temperature, moisture level, and the nature and concentration of energy source necessary for maximum aggregation. The results of a similar study dealing with a group of cellulose-decomposing fungi have been reported elsewhere (2).