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Diversity is a good thing, right? “Respect diversity” ranks right up there with “natural is better.” And both platitudes say that monocultures are bad and polycultures are good. Monocultures are the standard way to grow commodity crops, but it’s voguish for the small/local/organic/biodynamic crowd to talk about the benefits of crop diversity. It’s pretty obvious that a diverse, natural, “wild field” mix is better for a cover crop in a rotating field system or between vine rows than planting just one thing.
The problem with obvious things is that they’re sometimes wrong. Not because they’re “common knowledge” instead of scientifically tested – plenty of old wives weren’t idiots – but because they rely on stereotypes that don’t work when you stop to think them through.
Andrew McGuire at Washington State University’s Center for Sustaining Agriculture and Natural Resources recently published a pair of blog posts (here and here) knocking down the sensible-sounding idea that mixed cover crops are better than carefully selected monocultures. “Better” here means “deliver more ecosystem services,” and ecosystem services are things like improving soil quality, attracting beneficial insects, and fixing soil nitrogen. He reviews a pile of research demonstrating that different cover crop species are best for each of these different services. When you mix them you don’t get more of everything, as you might expect; you just dilute each of their effects. In other words, mixes are great if you want a little nitrogen fixing, a little flowering for beneficial insects, and moderate biomass to fold back into the soil. But, you can’t cheat the system, plant five different species, and expect to reap the full benefits of all five; you’ll just get a little of everything. The more aggressive species in the mix will also out-compete the less aggressive species – your mix won’t remain a nicely balanced mix forever – and the resulting biomass (the mass of plants to be turned under to enrich the soil) will be lower than what the most vigorous species in the mix would have given you on its own. “Transgressive overyielding” – the idea that plants grown in mixtures benefit symbiotically from each other’s presence and outperform plants grown in monoculture – isn’t supported by the data.
But McGuire is talking to annual crop farmers, not to grape growers. He concludes that agriculture isn’t always best-served by mimicking what happens in nature. That begs the question: what if your goal isn’t to best-serve agriculture? And what if you’re planting between rows, not fallow fields? McGuire says that the vineyard situation is probably different because the goals there are different: if “productivity” (plant growth, and how much natural fertilizer you’re giving your next wheat crop) isn’t your main concern, biodiversity might be more important.
Even if vineyard goals are different, it remains that mixes aren’t the clear best option because they’re natural. Needing to think strategically about goals, instead of just defaulting to natural = best, is just as important in vineyards as in wheat farming. What those goals are, though, probably varies more for vineyards than it does for wheat farms. If your vineyard’s wedding venue business outstrips actual wine sales, maybe you’re planting for maximum prettiness to attract that most beneficial of species: the zealous bride with money in her pocket.
One last problem. “Ecosystem services” implies that we’re trying to do something good for the ecosystem. But all of those cover crop benefits describe services for the main crop, not for the environment around it. What if you’re trying to promote biodiversity? To provide habitat for native wildlife? What if you are, in fact, trying to do a service to the ecosystem? Somehow that goal – the one that doesn’t directly increase profits – gets left out of the picture.