Yield increases during the organic transition: Improving soil quality or increasing experience?

Reported increases in crop yields over the first few years of organic farming (especially during the 3-year "transitional" period established in US law) have been attributed to gradual improvements in soil properties, such as the capacity of the soil microbial community to mineralize N or to suppress disease. To test the hypothesis that yield increases with years of organic farming are due to improvements in soil properties, we compared identically managed organic and transitional plots differing only in duration of organic management (>5 versus <1 year). Conventional plots were included for reference purposes. There was no difference in tomato (Lycopersicon esculentum Mill.) growth or yield between established organic and first-year transitional plots, but both outperformed the comparable conventional system. Even no-compost subplots within the transitional plots had yields similar to established organic plots, so the yield advantage relative to conventional plots was apparently due to beneficial effects of a winter legume cover crop in a wet year. Soil inorganic N did not differ between transitional and organic plots. Conventional and organic plots differed in soil microbial community composition, but transitional plots were not intermediate between conventional and organic. In the second year of the organic transition, when maize (Zea mays L.) was grown, yields were again not significantly different from the established organic system. This result is inconsistent with the hypothesis that yield-limiting differences in soil quality between organic and conventional systems take at least 3 years to develop. An alternative hypothesis, not tested directly, is that previously reported yield increases result from improved management with increasing experience, not improving soil quality.