Let me explain.
An important element of a mixed species pasture, which translates into cover crops or intercropping in broadacre and row-cropping respectively, is diversity. And the more diverse the better.
Keeping the discussion in-house, our ideal pasture mix is comprised of species that provide different leaf shapes and sizes, operate in different zones in the sward and soil, and have divergent root types.
This is all to simultaneously achieve:
Firstly, more efficient interception of light. The name of the game is occupancy. That is, the entire ground should be covered pre- and post-grazing with leaf to always trap much of the incident sunlight possible.
Sunlight fuels growth so the more the better. In this iteration, we can ignore pre-grazing canopy closure (shading which is an anathema to a monoculture) as this will be more than compensated for by my second point.
Secondly, the cooperation created through a community of plants via exudates (C-rich sugars and acids) and trading of nutrients and water. In this new world, plants help each other out so the little guy at the bottom of the sward offers up a trade (for example, minerals brought up from the deep through its taproot) in exchange for liquid sunlight (plant sugar).
Thirdly, occupancy of the root zone is another key goal. For this we need taproots, coarse fibrous roots, fine fibrous roots, rhizomes and so on. These are what tap into nutrient and water sources and build soil structure.
Finally, and more importantly, each root type brings with it, and supports via exudates, a fundamentally different population of microbes which operate in different zones which solubilise, transform, produce and transport, a wide assortment of minerals and other nutrients to the plant. These are directly (while the plant is alive) and indirectly (as it decomposes) traded by plants as currency and communication.
To meet our criteria for an effective mixed sward we need:
- At least 10 species of plant (and maybe up to 17 after which there is little additional benefit) representing a minimum of ...
- Seven plant families. This is easier than you think, and you can look at which families your most common weeds are in and find a domesticated species within that family as your mixed sward representative. And ...
- Five root types. The aim of these is to occupy all the A horizon, a good chunk of the B and penetrate deeply into the C.
Most seed companies are now offering mixed species options which is a great step forward. Just don’t go mad. Once you get past 10 species there is little further gain.
And my other caution would be to not worry about how many species are still in the sward after six or 12 months as long as they all germinated. About half of what we want a mixed sward to do occurs underground and has little to do with what we see above.
Ad each of these things is supported comprehensively by peer-reviewed literature. But more on that next time.
