Organic Broadcaster

 

Research compares summer cover crop options for organic vegetable farms

By Naomy Candelaria, University of Minnesota

Naomy Candelaria earns first place for her research project in the juried poster session of the Organic Research Forum at the 2020 MOSES Organic Farming Conference. Photo by Laurie Schneider

As a woman and Latina, my particular experiences have truly shaped the person I am, and the one I aim to become. I am originally from a small, rural town in Puerto Rico, where natural environments constantly surrounded me. It is not a surprise that how nature works and interactions with nature have always drawn my attention. My undergraduate degree in sustainable agriculture gave me a unique vision of how the world works, how the food system works, and how important those concepts are for having a strong society. It also taught me the importance of our environment, and our pivotal role in preserving it. My passion to learn more about agroecology motivated me to pursue a master’s degree in applied plant sciences at the University of Minnesota-St. Paul and to further understand how agroecosystems can support biodiversity. 

Premise for Research

Summer cover crops have been proposed as a tool for increasing resilience and efficiency in organic systems and may provide ecosystem services that benefit both agroecosystem health and crop productivity. However, knowledge of specific summer cover crop species’ effectiveness at providing ecosystem services, such as beneficial insect habitat provision and nutrient delivery, is not well documented.

Summer cover crops can supplement fertility inputs necessary to maintain crop health in organic systems. Nonetheless, it is crucial to narrow the purpose and use cover crops will have in order to determine what sort of management strategies are necessary to obtain such services, whether it is to provide nitrogen (N) to the subsequent crop, suppress weed growth, add floral resources, restore soil composition or some other intention. 

Warm-season cover crops offer a range of unique contributions. For example, they could accumulate more organic matter than over-wintering cover crops, leading to increased soil N contribution. Additionally, summer cover crops have the capacity to provide floral resources. These floral resources are vital for attracting and sustaining beneficial insects, such as pollinators, parasitoids, and predatory insects. In an agroecological sense, beneficial insect refers to any species that contributes to the conservation, protection and enhancement of an agronomic crop and farming system (Landis et al., 2005). Research shows that many beneficial insects are highly dependent on these resources for nesting and forage (Hogg et al., 2010). The use of summer cover crops may provide alternative food sources to maintain beneficial species at key points in the season when resources are intensely needed. Supporting the presence of these organisms in organic systems is fundamental considering the limitations of organic pesticides.

Synthetic fertilizer restrictions in organic farming make conscientious nutrient management decisions crucial. All cover crops obtain N through scavenging (grasses) and fixation (legumes), the latter achieved through a symbiotic relationship with N-fixing bacteria (rhizobia) located in the roots. Although both types of summer cover crops can be planted as single species, mixes are often recommended. This is most applicable to soils with high organic matter concentrations; because N fixation comes at a high energetic cost for the plant, the legume will primarily use what is already available in the soil (Kiers, 2003). Grasses are more intense N scavengers than legumes; by adding them to the mix, legumes are pushed to use other resources such as nitrogen fixation. Circumstances that could promote single species planting could include fields prone to N leaching that merits efficient scavenger species.

Experiment

In May-September 2019, we evaluated a range of summer cover crops (see Chart 1) at the University of Minnesota-St. Paul campus to determine feasibility in organic rotation systems. We also wanted to assess the capacity of these treatments to provide two fundamental ecosystem services: N contribution and beneficial insect attraction—two services that may possess trade-offs. 

The Xerces Foundation, an organization involved in the conservation of invertebrates and their habitats, recommended species based on capacity to attract pollinators. Choices included nine treatment mixes of monocultures and polycultures (2-7 species), with a total of 16 species evaluated. Initial trade-offs between N provision and beneficial insect services associated with flowering cover crops were explored. 

Collected data consisted of flower counts, non-observational insect collection, soil sampling and cover crop biomass (biomass, hereafter). Flower count data and insect collection occurred once a week. Soil sampling was taken twice: at the time of biomass collection (before termination), and two weeks after biomass incorporation, to capture the effect of biomass on soil N. Termination was conducted when most summer cover crops had reached flowering peak, and at least 10 seed pods had started to develop and mature. The C:N ratio and percent nitrogen in cover crop biomass and amount of biomass produced were quantified. Insect species were identified to family taxonomic level. Inorganic nitrate nitrogen (NO3-N), a form of nitrogen readily available to plants, was evaluated in the soil before and after biomass incorporation. 

Results

Our study showed that a mix of summer cover crops containing oats, field peas, and clovers (OFC) contributed the most N (237 pounds of N per acre) relative to other species combinations. Other studies using field peas in monoculture have reported up to 161 pounds of N per acre (Beckie et al., 1997). Biomass contribution by summer cover crops and cover crop mixes was primarily supplied by the OFC mix (7,804 lb. acre), followed by phacelia (Ph) (4,492 lb. acre) and sunflower (S) (3,792 lb. acre) monocultures.

Soil inorganic NO3-N increased in all treatments following biomass incorporation, supporting the contribution of N from the evaluated species. The OFC mix showed highest overall soil NO3-N after biomass incorporation with a mean of 10.98 NO3-N mg per kg of soil.

Of the species that flowered by termination time, buckwheat demonstrated the fastest time to flowering at 42 days, followed by field peas and phacelia (56 days). Sunflower time to flowering, at 76 days, was the longest of all evaluated species. Flower duration was maximized by phacelia with 27 days of total flowering period, followed by field peas (20 days), buckwheat (13 days) and sunflower (7 days). Insect data collected showed presence of beneficial families such as Apidae (bees), Coccinellidae (lady beetles), Syrphidae (pollinator-predator syrphid flies), Pteromalidae (parasitoids), and common pest families like Miridae (plant bugs) and Thripidae (herbivores). A mixture containing legumes (peas), grasses (oats and millet), brassicas (canola and radish) and buckwheat (BKRFMO) had the highest presence of syrphid flies, while the phacelia monoculture attracted most bees. Herbivore pressure by thrips was predominant in the OFC treatment. 

Conclusion

Maximization of ecosystem services provided by warm-season summer cover crops relies on thoughtful management. The total amount of nitrogen contributed from both legume and grass cover crops depends on the amount of biomass produced. Cover crops with capacity to produce greater amounts of biomass will subsequently provide more N to organic systems. Similarly, vigorous growth and flowering provides the desired forage and habitat for beneficial insects. Therefore, management should focus on considerations such as planting time and establishment, species selection, planting density, weed control capacity, and termination strategies. Even though trade-offs may exist between ecosystem services in many summer cover crop species, if combined and managed properly, single species as well as polycultures can provide a variety of agroecological benefits. 

Naomy Candelaria is a CFANS Diversity Scholar and graduate student in the Organic and Sustainable Horticulture and Soil Agroecology Labs at the University of Minnesota. Her research is funded in part by the North Central SARE Research and Education Grant (LNC19-423) and the Xerces Foundation. 

To discuss her research, contact her at cande036@umn.edu.

 

From the May| June 2020 Issue

 

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