Submitted to: Field Crops Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 29, 2003
Publication Date: N/A
Interpretive Summary: During the "organic transition" period, generally defined as the first three years after switching from conventional to organic management, crop yields are often lower than in conventional systems. Yields may eventually increase, but the expectation of initially lower yields can be a strong deterrent to those farmers considering such a change. Initially lower yields on organic farms have been attributed to lasting negative effects of conventional practices on soil microorganisms that recycle nutrients and control soil-borne pests. Initial inexperience with organic methods, on the part of farmers or researchers, could explain some or all of any yield gap between transitional and organic systems. To eliminate the effects of grower experience and weather, we designed the first replicated, side-by-side comparison between an established organic system and an identically managed transitional system using a tomato-corn rotation. A conventional system with the same crop rotation was included for comparison. By the first year of the transition we had six years of experience with organic management. There was no significant difference in yield between the organic and transitional plots. Contrary to prediction, the soil microbial communities in the transition treatment were not intermediate between those in the organic and conventional treatment. These results suggest that soil quality either very quickly improves under organic management or takes many years longer than the transition period, and that growers' inexperience with organic management may partially explain the loss of yield during the transition from conventional to organic farming.
Reported increases in crop yields with successive years of organic farming have been attributed to improvements in soil properties, such as the capacity of the soil microbial community to mineralize N or to suppress disease. Previous studies were confounded by trends of increasing grower experience, however. We compared identically managed, replicated 0.4 ha plots differing only in the duration of organic management (1 and 2 yr "transitional" vs. >5 yr "organic") with conventionally managed plots. Soil microbial community structure and function in the transitional plots were not generally intermediate between organic and conventional plots. There was no difference in net mineralization of soil N, or in tomato (Lycopersicon esculentum Mill.) growth or yield, between organic and first-year transitional plots. Both organic and transitional systems had higher tomato yield than the comparable conventional system, however, perhaps due to beneficial effects of a winter legume cover crop in a wet year. Results for a second year, in which corn (Zea mays L.) was grown in all systems, were also inconsistent with the hypothesis that soil quality improvements lead inevitably to increasing yields with years of organic management. Yield increases during the organic transition, when they occur, may result mainly from increased grower experience.