Cover Crop Mulches for No-till Organically Managed Onion Production

Abstract

Combining the environmentally sound practices of reduced tillage and cover crop use with organic vegetable production systems requires management choices that are tailored to specific climate and crop combinations. In the southeastern U.S. summer cover crops can be grown through the fall until desiccation by frost. At this time a cash crop such as bulb onions (Allium cepa L.) can be grown using reduced tillage for over winter production. Field experiments were conducted on first year transitional (non-organic) land in 2006-07 and 2007-08. Cover crops of foxtail millet ‘German Strain R’ [Setaria italica (L.) Beauv.] and cowpea ‘Iron & Clay’ [Vigna unguiculata (L.) Walp.] were grown as monocrops (MIL, COW) and biculture mixtures and compared to a bare ground control (BG). Mixtures of cowpea and millet consisted of seeding rates aimed at producing 70%, 50%, and 30% millet per total biculture biomass (MIX-70, MIX-50, MIX-30). Cover crop residue treatments were evaluated for weed suppression and N contribution to no-till organic onion production. Supplemental N in the form of surface applied soybean meal [Glycine max (L.) Merrill] was applied to cover crop treatment subplots at three rates: 0, 105, and 210 kg N∙ha-1. Onion yields increased linearly from 0 to 210 kg N∙ha-1 rates of soybean meal. Cover crop treatments COW and BG had the greatest total marketable onion yield and were statistically equivalent both years. Losses in marketable yield were primarily due to mortality in MIL and MIX in 2006-07 and bolting across all cover crop treatments, which averaged 28% in 2006-07 and 72% in 2007-08. Onion mortality was over 50% in MIL and MIX treatments in 2006-07 and was attributed to physical properties of thick surface mulch. Onion mortality in 2007-08 was highest on MIL compared to all other cover crop treatments but remained below 20%. Weed interference during onion production was highest in COW and lowest in treatments with millet (MIL, MIX) in 2006-07. Nitrogen rates of 105 and 210 kg N∙ha-1 increased soil mineral N (NO3- and NH4+) on BG plots two weeks after surface application of soybean meal at the end of November each year but generally ceased to have an effect on soil mineral N by February or March. The 105 kg N∙ha-1 soybean meal rate appeared to be more than sufficient for use as a starter fertilizer. Split applications of soybean meal could be an important improvement in N management to avoid N leaching and better meet increased N uptake demand during bulb initiation and growth in the spring. Overall, this study shows that cowpea cover crops grown preceding an over-wintered no-till allium crop is feasible with appropriate management, and provides onion yield comparable to bare ground production.