Cover crops are used to scavenge residual soil nitrate, with the goal of reducing N losses from agricultural fields and increasing subsequent N availability. Two experiments were conducted to determine fall‐planted brassica cover crops’ effect on N availability for rotational crops. The first evaluated five species—brown mustard [Brassica juncea (L.) Czern], hybrid turnip (Brassica rapa L. × B. napus L.), radish (Raphanus sativus L.), rapeseed (Brassica napus L.), and white mustard (Sinapis alba L.)—in rotation with oat (Avena sativa L.). The second evaluated radish in rotation with corn (Zea mays L.). End of season cover crop biomass averaged 1160 to 6170 kg ha−1 across experiments, locations, and years. Biomass did not differ by species. Nitrogen accumulation was generally greater than 70 kg ha−1 (range 31–136 kg ha−1). In the subsequent spring and summer, brassica cover crops reduced soil nitrate N levels relative to a no‐cover control by 0 to 132 kg ha−1. In Exp. 1, all brassicas reduced oat N accumulation by ≥77 kg ha−1 and oat biomass by ≥1255 kg ha−1 in June 2012, relative to the control. Mustards reduced oat grain yield by 505 kg ha−1 in 2011, while radish increased oat yield by ≥578 kg ha−1 in each year. In Exp. 2, radish did not affect corn V8 biomass N concentration, grain yield, or response to N fertilizer. Nitrogen taken up by brassica cover crops often is not available when the subsequent crop needs it.

High-tunnel (HT) systems have been shown to effectively improve yields, fruit quality and profitability. In order to maximize returns on investment, HTs are frequently planted successively with both winter and summer cash crops and may include >2 crop cycles per year in some climates. The intense cultivation strategies used in HT systems necessitate increased tillage and nutrient demands posing challenges for soil health, environmental quality and long-term economic sustainability, particularly among organic growers. Seasonal rotations that incorporate fertility-building cover crops, such as legumes and other green manures, have the potential to build soil organic matter, improve crop yield and reduce applications of animal manure and/or compost. The economic impact of cover crop use in HT production systems poses important implications for organic growers. In this study, we present three partial budget analyses to quantify the economic benefits from a leguminous winter cover crop–tomato cash crop rotation in HTs across three regions. Data used in the economic analysis come from multi-year organic HT field trials in Kansas (2016–2019), Kentucky (2016–2019) and Minnesota (2016–2020). Direct financial benefits from hairy vetch (Vicia villosa) cover crop N credits were observed but not sufficient to offset the direct and indirect costs of the cover crop practice. A winter cover crop used in organic HT vegetable systems results in negative financial benefits to producers even with conservation incentive payments. These results highlight challenges for organic growers who are required under the USDA National Organic Program to incorporate soil building practices as part of their rotation schedule. The findings will also be of interest to policy makers as they refine cost-share offerings and programming to incentivize cover crop adoption as a conservation strategy.

Weeds often limit productivity of organic cropping systems. Radish is a fast-growing, potentially allelopathic cover crop that has the potential to improve weed management in organic systems. To evaluate the effect of radish on density, cover, and biomass of weeds in organically managed corn, 2-yr field experiments were conducted over 4 site years. Four cover-crop planting treatments (fall-only, spring-only, fall + spring, and no cover) were tested in factorial with three cultivation treatments (standard [three to four passes], false seedbed [standard with a false seedbed], and reduced [two passes]). All plots were tilled before planting. Shoot biomass averaged 3,057 kg ha−1for fall-seeded radish and 385 kg ha−1for spring-seeded radish. Radish cover crops generally did not improve management of weeds during the corn growing season. However, in the absence of a false seedbed, fall-seeded radish reduced field pennycress density from 9 to < 1 plant m−2and horseweed density from 6 to 2 plants m−2in spring in site years where these weeds were present. Fall-seeded radish also reduced cover of summer annual weeds during the fall cover-crop growing season from 4 to 0% in 1 site year, preventing these weeds from setting seed. Radish cover crops did not affect corn grain yield.

Management of soil microbial communities for enhanced crop disease suppression is an attractive approach to biocontrol, but the effects of agricultural practices on the disease-suppressive potential of the soil microbial community remain unknown. We investigated the effects of long-term nitrogen addition (103 kg ha−1 nitrogen as urea versus no fertilizer) and crop residue incorporation versus removal on in vitro antibiotic inhibitory capacities of actinomycetes from 57-year maize (Zea mays L.) monocultures in southeastern Minnesota. We hypothesized that both nitrogen and crop residue addition would increase inhibitor frequencies by increasing microbial population densities and thus increasing the importance of competitive interactions among microbes to their fitness. We found that although soil carbon and nitrogen and microbial densities (actinomycete and total CFU counts) tended to be greater with nitrogen fertilizer, the frequency of in vitro inhibitory phenotypes among culturable actinomycetes in fertilized plots was approximately half that in non-fertilized plots. Residue incorporation had little to no effect on soil chemistry, microbial density, and inhibitor frequency. These results suggest that density-mediated processes alone cannot explain the effects of amendments on inhibitor frequencies. Fitness costs and benefits of inhibitory phenotypes may vary over time, and may depend on type of resource amendment.

Several brassica cover crops have recently gained popularity in the U.S. Midwest as nitrogen scavengers that are believed to reduce N losses and increase the amount of residual N available to the next cash crop. Field studies were conducted at three sites in Minnesota to determine the effect of fall‐planted brassica cover crops on (i) N availability to the subsequent year's crop of oat or corn, (ii) oat and corn grain yield, and (iii) corn yield response to applied N fertilizer. Earn 0.5 CEUs in Crop Management by reading this article and completing the quiz at http://www.certifiedcropadviser.org/certifications/self-study/755.