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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.