There is a lot of hype around mixed-species mixed cover crops with three, eight, or even 30 species mixes often promoted as the silver bullet for improving soil health and productivity.
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The key is to know when to stick with single species cover crops and when to jump into mixed species cover crops.
Are mixed species cover crops a good fit for vegetable production?
To answer the question, you need to be clear about your main aim and how long you have for the cover crop.
I like to keep it simple with single species cover crops.
If a single species cover crop delivers, stick with it.
They are cheaper, easier to manage and often deliver more specific benefits.
If you are new to cover cropping, start with single species cover crops.
When managing soilborne disease pressure, carefully chosen single species cover crops are a clear winner.
Non-hosting cover crops, or break crops, are an important way of reducing soilborne disease pressure.
Typically, cereals (oats, cereal rye, sorghum, millet) are used in the vegetable rotation as a break crop.
Moving to a mixed species cover crop greatly increases the likelihood that one of the additional species will be an alternate host to the soilborne disease.
This complicates rotation and can increase losses caused by soilborne disease in the following crop.
The amount of biomass produced by cover crops is important for biofumigation efficacy, nitrogen fixation by legumes and suppressing weeds.
By adding in more species, the effectiveness of a biofumigation cover crop is probably reduced as there is less biofumigant produced.
Maximum biomass incorporated at the right time is required for a biofumigant to be most effective.
Carefully chosen (right season and sowing rates), single species cover crops will usually produce more biomass than mixed species cover crops.
For both nitrogen fixation and weed suppression, biomass is king.
Maximum biomass is particularly important for biofumigants that rely on high biomass and rapid incorporation into the soil for their efficacy.
Adding in other species will reduce the biomass of the biofumigant and hence the biofumigation efficacy.
If single species cover crops are your choice but you want to introduce more diversity, consider changing your cover crop species occasionally to provide more diversity.
Where do mixed species cover crops fit in?
If you have long cover crop growing time and no soilborne disease issues, then mixed species cover crops can be a good option.
The mixes can provide a greater variety of rooting structures to help build soil structure (including some ryegrass with cereal and adding in a deeper rooting brassica), or a wider range of flowering plants to help maintain beneficial insects.
There is also evidence that nutrient cycling is more efficient in mixed-species system.
A longer growing time and different management practices (grazing or mowing) will be required to allow different species to contribute to mixed species cover crops.
In general, the most successful mixed species cover crops have been in grazing systems - and then I would call them pastures!
There is also a lot of talk about mix species cover crops being better for soil biology - greater diversity in the cover crop creates greater diversity in soil biology is the logic.
However, the research is not clear cut on mixed being better than single species cover crops.
But what is clear, is that any cover crop is way better than a bare fallow.
If you want to try mixed species cover crops, then start building with one or two additional species and make sure the benefits you want are being delivered, and that you are not compromising or complicating your money-making vegetable crop.
- This article was produced by Ausveg.
