Cover Crop Influence On Nitrogen Availability for Organic Dry Beans.

Michigan is the number one producer of organic dry beans in the nation. With the limited pesticide and commercial fertilizer inputs allowed in organic systems, it is essential to maximize the potential benefit of cover crops for increasing weed control, nutrient availability, and ultimately crop yields. Our objective is to determine the effect of cover crops on nitrogen availability, dry bean population and yield in organic production systems. To meet this goal, a field experiment was conducted at two locations in Michigan. The cover crops studied included: medium red clover, oilseed radish, and cereal rye; a no cover treatment was also included. Within each cover crop treatment there were four bean varieties: ‘Zorro’ and ‘Black velvet’ black beans and ‘Vista’ and ‘R-99’ (non-nodulating mutant) navy beans. Cover crop biomass was recorded at peak production and included both above and belowground growth. Nitrogen availability was monitored using a chlorophyll meter (V2, R1, and R5), soil samples (fall, planting, V2, R1, R5, and harvest), ion exchange resin strips (changed every 2 weeks), and total N and N₁₅ in the beans at harvest.  Dry bean populations (V2 and harvest) and yield were also recorded. The clover cover crop and the no cover crop treatments (weed infestation only) had the greatest peak biomass production (3500 kg/ha) at the Kellogg Biological Station (KBS), followed by oilseed radish (2,200 kg/ha) and rye (1,800 kg/ha). However, at the Student Organic Farm (SOF), rye was unable to be controlled in a timely manner due to frequent spring rainfall events  and reached a biomass of 7,000 kg/ha. At both the V2 and R1 stages, bean chlorophyll florescence was highest following a clover cover crop, though the difference between cover crop treatments was not always significant. The clover cover crop treatment usually had more nitrate and ammonium available to the dry beans. Information regarding total nitrogen and N₁₅ content of the beans will be presented. Beans following an oilseed radish cover crop had significantly higher populations than the no cover treatment at the V2 stage (both locations) and at harvest (KBS only), with 14-35% more plants. At the SOF, bean yields following oilseed radish (2,700 kg/ha), clover (2,300 kg/ha), and no cover (2,200 kg/ha) were higher than beans following rye (1,500 kg/ha). Reduced bean yields following rye may be the result of rye reducing soil moisture early in the season and immobilizing nutrients. There were no differences in bean yield at the KBS. Two more field seasons of this research are planned to clarify the impacts of cover crops on organic dry bean production systems.