Someone branded cover crop mixtures as “cocktails” and it has stuck. Even NRCS has adopted the “cocktail” label:
Regardless of what they’re called, the new multi-species mixtures are very exciting, results are intriguing, and these cocktails are probably the cover cropping way of the future. I don’t want to seem old-fashioned (cocktail pun intended), but I do want to take some time to discuss some very simple cocktails and why/how they work.
Even two ingredients can make a darn good cocktail
With the recent release of Rodale Institute’s project report on cover crops and no-till summer vegetables (click image for full report), this seems like a good time to discuss one of the most tried and true two-ingredient cocktails: the grass + legume biculture. This mixture can can facilitate successful high-residue no-till systems by producing enough biomass to suppress weeds and enough nitrogen (N) to reduce fertilizer needs. The Rodale study used rye + vetch, but other grasses can be wheat, triticale, oat etc., and other legumes can be clovers, peas etc.
Dr. Jerry Brust at the University of Maryland did some investigation of weed management strategies in no-till organic high-residue systems, and found some interesting results from simple use of landscape cloth that can be found here.
Competition and stimulation between grasses and legumes
One of the consistent results across highly variable site years in the Rodale study was the greater performance of rye + vetch grown together than grown in monoculture.
The results are not surprising; nearly a hundred years ago, even the USDA was recommending that farmers grow rye and vetch together to increase soil fertility. When interest in cover crops was reignited in Maryland in the 1990s, the increased N uptake from a rye + vetch mixture was once again brought to people’s attention.
Pasture managers have long known that having a mixture of grasses and legumes increases productivity, but why is this? In part, it has to do with maximizing the efficiency of different N acquisition routes.
We may think of legumes as “fixing” N, but it is actually bacteria that fix atmospheric N, then share it with the legume. To maintain this relationship, legumes must give the bacteria some carbon compounds in exchange. In other words, legumes must pay a price for obtaining fixed atmospheric N. If there is N available in the soil, legumes will rely less on the bacteria, instead relying on the cheaper N from the soil. In a soil environment with a lot of available N, the legume may get a high percentage of its total N from the soil, not bacteria.
Grasses, on the other hand, rely entirely on soil N. When grown alone, they can only get as much N as the soil provides. When grasses and legumes are grown together, the grass scavenges for soil N, often outcompeting the legume. The legume is then forced to engage its bacterial partners and start fixing atmospheric N. Some of the fixed N ends up back in the soil as plant parts and bacterial nodules decay and the grass can then benefit from the fixed N as well. All together, these interactions appear to stimulate both the N capture by grasses and the N fixation by the legume/bacteria. There are probably also some additional interactions between the grass + legume + bacteria (not to mention mycorrhizae!), that stimulate biomass production and N uptake.
Additionally, grass can provide a physical support system/trellis for vining legumes, allowing upward growth.
Nitrogen accumulation vs. spring availability
The other aspect of a rye + vetch cocktail that makes it appealing for organic no-till production is the high likelihood that more N in the cover crop biomass will be available to cash crops the following year from the mixture than from a rye monoculture. Ongoing research continues to investigate the N dynamics following rye + vetch cover crops. Understanding how two ingredient cocktails work (they’re not so simple!), may help us understand and plan for more complex cocktails in the future.
Variations on the two ingredient cocktails: adding radish
Lest a blog post on this site go without mention of radish, it is worth noting that the similar competition/stimulation between grasses and legumes in fall may exist between brassicas and legumes. Guihua Chen, the tireless researcher who showed how radish roots can penetrate compacted soil layers, and Cerruti Hooks at the University of Maryland have been investigating a forage radish + red clover combination. A more complete report by Guihua is available here.
One of the main differences between the rye + vetch combination discussed above and the radish + clover combination is the winterkilling nature of forage radish. This changes N dynamics in spring considerably, as N mineralization from forage radish is rapid and early. Of course, radish also leaves little residue in spring, meaning that the radish + clover combination is not well-suited to situations in which a heavy mulch is desired. Mixing radish (or other brassicas) and legumes may be a beneficial fall cover cropping technique when compaction alleviation and deep N capture (among other traits) are desirable , but a radish monoculture is not desirable. A radish monoculture is not well-suited to situations in which fall soil N is somewhat limited (radish can only capture what is there), and when an early spring cash crop is not planned (N from radish is mineralized early in spring and can be lost if a cash crop is not planted).
Start small, and plan ahead
In the Rodale report, there were a couple of points that resonated with our experience in no-till cover crop-vegetable production systems. The first is to start small, and always have a “control” plot. If any of these cover cropping ideas sound good- try them! But keep it small in the first year and always keep an area for comparison of what your usual practice is. It’s a pain for management, but it will give you the information you need to say whether the new system is really working for you.
The second point is to plan ahead. The Rodale folks summed this concept up very well:
…the crop year begins in the fall with planning for the following year. For this reason, organic no-till requires considerable long-term planning.
With spring just bursting, fall may seem like it’s a long way off, but it’s never too early to be thinking about cover cropping for next year.
Oh, and what better way to celebrate Earth Day than to give a toast with a cover crop cocktail?
Clark, A.J., A.M. Decker, J.J. Meisinger and M.S. McIntosh. 1997. Kill Date of Vetch, Rye, and a Vetch-Rye Mixture: I. Cover Crop and Corn Nitrogen. Agron. J. 89: 427-434. doi:10.2134/agronj1997.00021962008900030010x.
Nyfeler, D., O. Huguenin-Elie, M. Suter, E. Frossard and A. Lüscher. 2011. Grass–legume mixtures can yield more nitrogen than legume pure stands due to mutual stimulation of nitrogen uptake from symbiotic and non-symbiotic sources. Agriculture, Ecosystems & Environment 140: 155-163. doi:http://dx.doi.org/10.1016/j.agee.2010.11.022.