Leguminous Cover Crops in Vegetable Cropping Systems and Effects of Seed Inoculation

Legume cover crops such as clovers are planted as winter annuals (6-8 weeks before a killing frost) or as summer annuals (in the spring). In colder climates such as Iowa, summer-annual use of legume cover crops is common as it is a useful tool in vegetable crop rotation planning. The amount of nitrogen a legume cover crop can add depends on biomass generated which in turn depends on temperature, rainfall, and soil fertility status. Also legume cover crops need specific strains of Rhizobium to effectively fix atmospheric nitrogen. In order to generate specific N-credit data under Midwest growing conditions, this study investigated three legume cover crops (crimson, red, and yellow/sweet clover) and evaluated them for the amount of nitrogen they can contribute under inoculated and non-inoculated conditions at the Iowa State University Horticulture Research Station, Ames, IA. Experiment was a randomized complete block split plot design with cover crops as the whole plot and Rhizobium (inoculation or no-inoculation) as the split plot factor. The highest amount of biomass was produced by red clover (5,587 kg/ha) followed by crimson clover (4,591 kg/ha). Yellow/sweet clover produced the lowest amount of biomass of 3,135 kg/ha. Inoculation of seeds with the Rhizobium significantly increased biomass. Inoculated seeds produced almost 1 ½ times more biomass than the non-inoculated seeds. Percentage nitrogen on a whole plant basis was significant for clover species and inoculation treatments. Red clover had higher percentage nitrogen than crimson or yellow clover. Nitrogen percentage in plants was not significantly different between inoculated or non-inoculated treatments. Multiplying biomass generated by N content in the plant provides an estimate of how much N will be added/recycled back into the soil. Nitrogen contribution of red clover was the highest (117 kg/ha) followed by crimson (81 kg/ha) and yellow clover (64 kg/ha). Seed inoculation increased the amount of N contributed by cover crops. Amount of N contributed to the soil by the cover crop is the dependent upon cover crop biomass, amount of N fixed from the atmosphere and N taken up by the plant from the soil. Since percentage N within plants did not differ based on inoculation but yet there was increase in the amount of N contributed, it suggests beneficial effects of inoculation on ability of plants to assimilate nutrients and enhance plant growth.