THE ROLE OF SOIL ORGANIC-MATTER IN MAINTAINING SOIL QUALITY IN CONTINUOUS CROPPING SYSTEMS

Maintenance and improvement of soil quality in continuous cropping sys tems is critical to sustaining agricultural productivity and environme ntal quality for future generations. This review focuses on lessons le arned from long-term continuous cropping experiments. Soil organic car bon (SOC) is the most often reported attribute from long-term studies and is chosen as the most important indicator of soil quality and agro nomic sustainability because of its impact on other physical, chemical and biological indicators of soil quality. Long-term studies have con sistently shown the benefit of manures, adequate fertilization, and cr op rotation on maintaining agronomic productivity by increasing C inpu ts into the soil. However, even with crop rotation and manure addition s, continuous cropping results in a decline in SOC, although the rate and magnitude of the decline is affected by cropping and tillage syste m, climate and soil. In the oldest of these studies, the influence of tillage on SOC and dependent soil quality indicators can only be infer red from rotation treatments which included ley rotations (with their reduced frequency of tillage). The impact of tillage per se on SOC and soil quality has only been tested in the 'long-term' for about 30 yrs , since the advent of conservation tillage techniques, and only in dev eloped countries in temperate regions. Long-term conservation tillage studies have shown that, within climatic limits: Conservation tillage can sustain or actually increase SOC when coupled with intensive cropp ing systems; and the need for sound rotation practices in order to mai ntain agronomic productivity and economic sustainability is more criti cal in conservation tillage systems than conventional tillage systems. Long-term tillage studies are in their infancy. Preserving and improv ing these valuable resources is critical to our development of soil ma nagement practices for sustaining soil quality in continuous cropping systems. (C) 1997 Elsevier Science B.V.