Cover crop mulches for no-till organic onion production

Cowpea shows promise as a summer cover crop used as a residue mulch for fall planted crops such as over-wintered onion. COW produced comparable onion yields to BG in both years of this experiment and at commercially reasonable levels. Utilizing cover crops for no-till vegetable production does require some additional management considerations compared to clean till or bare ground cultivation but there are tradeoffs. Many other studies have shown that incorporating cover crops into production rotations provide a variety of benefits, especially over an extended period of time greater than one or two years. This experiment was conducted on two different sites over the two years and therefore did not address the long term affects of these cover crop management options.

Weed control for large scale organic production using in-situ cover crop residues would be a major challenge and expense. Management to reduce weed seed banks and careful selection of low weed pressure fields would be essential to making it commercially feasible. That type of management is not impossible and has been achieved on certain exemplary farms. Even though COW had high weed interference, just two thorough hand weeding events were sufficient to maintain onion yields. Weed control is logistically difficult with surface residue since mechanic cultivation options would disturb residue and tend to catch and drag on equipment. Depending on the field specific weed pressure and types of weed species present, hand weeding through residue is effective but time consuming.

This experiment found that foxtail millet is not a desirable cover crop for this system of no-till overwintered vegetable production. It appears that physical properties of the grass residue negatively affected onion plant stand and overall yield. Millet in a mixture with cowpea either reduced onion yield or had comparable yield to cowpea as a monocrop. While MIL did reduce weed interference in 2006-07 compared to COW there was no significant difference in 2007-08. In addition, foxtail millet appears to be a poor choice for a midsummer seeded cover crop in eastern North Carolina due to its susceptibility to foliar disease.

Onion bolting reduced marketable yield in both years of this experiment. Flower initiation in onions is influenced by onion variety, plant size or age, and several environmental factors including photoperiod, temperature, and plant N status. Onion juvenility lasts until the plants reach a certain critical size and is often measured in terms of plant weight or number of leaves (6 to 9 after the cotyledon). A post-juvenile onion plant is receptive to vernalization, which is essential for floral initiation, and the rate of flower initiation increases with plant size. While not quantified in this experiment, decomposition of cover crop residues could have influenced mid-season onion plant size and in turn differing rates of flower initiation. N immobilization from MIL in 2007-08, which had an initial C:N ratio over 37:1 could have contributed to smaller midseason plant size in MIL which in turn reduced the rate of bolting.

Soybean meal shows potential as an effective source of N even when surface applied in cool weather months. Even though the 105 kg N•ha-1 rate appeared to contribute sufficiently to soil N for use as a starter fertilizer onion yield was higher at the 210 kg N•ha-1 rate. Split applications of soybean meal could be an important improvement on the one application management scheme used in this experiment. This study showed that N would be available for plant uptake in less than two weeks after surface applying soybean meal, which facilitates the use of soybean meal in multiple applications tailored to timing of crop plant demand.