下DPEa O horizon Layer of undecomposed plant materials A horizon Surface soil (high in organic matter,dark in color,is tilled for agriculture;plants and large numbers of microorganisms grow here;microbial activity B horizon Subsoil(minerals, humus,and so on, leached from soil surface accumulate here;little organic matter;microbial activity detectable but Lower than at A horizon C horizon Soil base (develops directly from underlying bedrock; microbial activity Figure 19-6b Brock Bi generally very low) ©2006 Pearson Prenti Q6310 上海交通大学通识教育核心课程《微生物的世界》 Lower than at A horizon