ABSTRACT
The organic portion of soils includes both soil organisms and the various biological substances and processes that animate soil systems. The connection between the mineral and organic components of soil systems is intimate, converging at the smallest scale of soil structure and function in biofilms and in organo-mineral complexes. Replacing tillage with practices that keep the soil covered and nurture more biotic activity in the upper horizons is one of many biologically based approaches for better management of soil systems. What goes on in soil systems is still incompletely understood because much of our present scientific knowledge has been based on studies that underrepresented or ignored the biological component. Plant roots continually adjust their interactions with the soil environment, particularly with the thin layer of biologically rich soil known as the rhizosphere that envelops the root system and is modified by root exudation.
