Don't fire up the tractor just yet. In a dramatic shift that might change the use of clean planting beds on California vegetable farms, scientists are starting to identify the benefits of growing crops amid a tangle of organic matter left over from a cover-crop planting. For decades, it was believed that tilling agricultural fields was advisable to produce well-worked and weed-free growing environments for the next crop. But in recent years, tilling has been implicated in a decline in soil organic matter, which may lead to problems with water infiltration, aggregate stability and nutrient absorption, according to Jeff Mitchell, a UC Davis vegetable-crops specialist based at the Kearney Agricultural Center near Parlier. "There is a great deal of interest in possible benefits of conservation tillage here in California, but there needs to be a lot of work done to evaluate and develop appropriate and workable practices before the concept is widely adopted," Mitchell said. "Whether these systems will work on California's large-scale vegetable farms is something that remains to be seen." The University of California recently hosted experts from throughout the nation and some of the Central Valley's most innovative farmers in regional conferences to discuss reduced tillage systems. The meetings were sponsored in part by the UC Davis-based Sustainable Agriculture Research and Education Program.