We are making a concerted effort to grow more cover crops this year. Cover crops differ from our normal cash crops because we turn them back into the soil, rather than harvesting them for market. After attending the Pennsylvania Association of Sustainable Agriculture (PASA) meeting this winter, we came away with the message, “Treat your cover crops like cash crops,” which has helped us adjust our mindset and actually plan them into our crop rotations. Still, you might be wondering why we would bother growing something we can’t sell.
In the photo, you can see the first summer squash planting on the left, and a field of buckwheat, a quick summer cover crop, on the right. The buckwheat is growing where the strawberries and spinach grew in the spring. In the fall, we plan to plant radishes and turnips in this field. But between strawberries, which finish in early June, and turnips/radishes, which will go in early August, what is to be done with this field? We could totally ignore it, and it would grow up in weeds, which wouldn’t be the worst thing (I’ll explain why in a minute). We could till it every week or so, so that it remains more or less bare soil for two months. Or we could plant a cover crop that fits our time window of about two months.
Tilling frequently to control weeds may be appealing, but it is terrible for soil health. Healthy soil is alive with microbes and fungi, and it has a structure that allows it to hold water and provide a home for all the critters (insects, worms, microbes) that live in it. When we till, we disrupt the ecology of the soil. I attended a talk given by a soil scientist who said, “Bare soil is soil in agony,” meaning that all the underground life dies as it is exposed to the sun after tillage. Of course, soil that is healthy with a diverse ecology to begin with is quite resilient, especially in the summer months. Critters will recolonize quickly if the soil is left to rest after tilling, especially if there is fresh organic matter from a cover crop incorporated into the top layers of the soil.
Another soil scientist who gave a talk that I attended this spring at PASA said, “Grow a root 24/7,” and gave a cool demonstration to illustrate why. He had in a suitcase two dense clumps of grass that he had pulled up from somewhere. One was totally brown and dead and the other had only been pulled out a few days ago, badly wilted, but still mostly green and alive. When volunteers from the audience swished the clumps of grass in a plastic tub with a little water in it, an interesting thing happened. The water from the dead grass just looked like muddy water, but the muddy water from the living grass had a frothy white scum on the surface after about ten seconds of swishing. “What is this stuff?” he said, and I had no idea, but someone from the audience answered, “Exudates.” “Right,” he said, “Roots exude a sugary liquid that feeds the soil.” Normally we think of the purpose of a root as sucking nutrients from the soil to feed the plant, but in fact it is a two way street. Root exudates feed the microbes in the soil. This blows my mind every time I think about it, and leads to the answer of why a field grown up in weeds is not the worst thing. The roots of those weeds are feeding the soil by exuding sugars, organic acids, and other nutrients. Then, we till them under, the soil microbes break down the organic matter and voila! We have a nice healthy soil to plant into.
We like to think of cover crops as the best possible weeds. They break down quickly when you mow and till them at the right time, and they can have other benefits as well. Clovers (there are two side-by-side in the photo above) and other legumes “fix” nitrogen in the soil so that other plants can access it, and they are usually the best choice for farmers who want to graze animals on their cover crops. Sorghum-sudan grass and daikon radish have deep roots that can loosen the subsoil and bring up nutrients from down deep. Buckwheat and rye can suppress the germination of many species of weed seeds.
In nature, you rarely see one species growing by itself; they are almost always mixed in what ecologists call communities. Communities have synergistic properties that make them more resilient than the sum of their parts, so it might be reasonable to think that farmers could replicate some of these effects by combining cover crop species. This is another reason why letting a fallow field grow up in “weeds” is not the worst thing in the world. We don’t begin to understand everything that is going on, but that doesn’t stop us from trying. For example, in one field this year, we combined buckwheat and sorghum-sudan grass (it looks like corn in the photo); the buckwheat germinates quickly and facilitates the establishment of the slower-growing sudan grass. And earlier this spring, we tried oats and peas sown together as a cover crop, which also seemed to work well. We’re far from being cover crop experts, but we do see how cover crops are a powerful tool for the ecologically minded farmer, and we’re looking forward to learning more about them.