939820sare fig 10 N fixation vetch bar graph

1. Figure 10. Nitrogen fixation in vetch monocultures (dark green) and vetch-rye mixtures (bright green) across nine organic vegetable fields arranged based on nitrogen fixed in the vetch monoculture. Small plots were established in fields planted with cover crops to compare vetch growing with and without an interseeded grass. Planting date is a key factor in determining how much nitrogen is fixed. Planting dates which were too late in the season did not permit good establishment of the vetch and performed poorly. Looking at fields 1-5, with similar planting dates and where nitrogen fixation in vetch monocultures was very high (94-106 lbs nitrogen/acre), the variability in nitrogen fixed was much greater in mixtures and ranged from 18-85 lb nitrogen/acre. It is likely that either soil nitrogen fertility or the ratio of grass to vetch seeds resulted in suppression of vetch growth in fields #2 and #6. So far, out analysis of these data indicate that planting date, soil fertility and seeding rates interact in ways that are difficult to predict. Farmers who see that their vetch/rye mixtures are dominated by the rye with very little vetch despite reasonable planting dates can adjust these mixtures by reducing the proportion of grass in the seeding mixture.