sustaining agricultural productivity and environmental quality for future generations. This review
focuses on lessons learned from long-term continuous cropping experiments. Soil organic carbon
(SOC) is the most often reported attribute from long-term studies and is chosen as the most
important indicator of soil quality and agronomic sustainability because of its impact on other
physical, chemical and biological indicators of soil quality. Long-term studies have consistently
shown the benefit of manures, adequate fertilization, and crop rotation on maintaining agronomic
productivity by increasing C inputs into the soil. However, even with crop rotation and manure
additions, continuous cropping results in a decline in SOC, although the rate and magnitude of the
decline is affected by cropping and tillage system, climate and soil. In the oldest of these studies,
the influence of tillage on SOC and dependent soil quality indicators can only be inferred 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 developed 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 cropping
systems; and the need for sound rotation practices in order to maintain agronomic productivity and
economic sustainability is more critical in conservation tillage systems than conventional tillage
systems. Long-term tillage studies are in their infancy. Preserving and improving these valuable
resources is critical to our development of soil management practices for sustaining soil quality in
continuous cropping systems. 0 1997 Elsevier Science B.V.
Keywords: Sustainable agriculture; Soil management; Crop rotation; Crop