Subsurface soil samples contain microbiol. populations that are smaller, less versatile, and less adaptable than surface soil populations. Introduced plant material is decomposed less rapidly and less extensively. Introduction of surface soil populations by addition of inocula of soil or of soil suspensions does not produce the increase in activity that would be expected. The same inocula in sand accomplish rapid decomposition of plant material. Some factor in the subsurface samples prevents the rapid development of the introduced organisms. Evidence is given supporting the view that antibiotic or inhibitory substances are responsible for this effect; these substances may be of microbial origin. Aqueous extracts of soil do not adversely affect the growth of soil bacteria in vitro, but alcohol extracts may reduce the activity of the organisms in soil, the development of bacteria in liquid culture or on plates, and the rate of growth of fungi on plates. The factors affecting the development of subsurface soil populations are discussed. Because the nutritional conditions vary little, and because of the presence of inhibitory agents, subsurface populations tend to stabilize themselves and to resist displacement or colonization by other introduced forms.