The Dirt Issue 38 – Cover Cropping

Posted on April 24, 2019
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Keeping Soil Healthy
When most of us think about farmland, we think about crops planted, harvested and marketed. After all, it’s these cash crops that—ideally—make farming economically viable. But, to a farmer, the cash crop is just one part of a far larger endeavor: the responsibility of ensuring that the condition of the farmland is the best it can be to produce that crop.

It starts with soil tilth, a term which describes the attributes of the soil which, taken as a whole, make the soil suitable for seeding and growing the crop. These attributes include soil aggregation (how crumbly it is), moisture content both in terms of retention and drainage, and how well aerated the soil is.

Achieving good soil tilth is one of the many benefits derived from cover cropping.

Part of a whole cropping system
Farmland fertility is a constant concern for any farmer. What the farmland requires to maintain or enhance its fertility varies over time depending on what’s been planted and on what is planned next. Here in the Skagit Valley farmers use a variety of means to accomplish this. Crop rotation (sequential planting of different cash crops over several growing seasons), amending the soil through composted organic material, and cover cropping are each important in an overall cropping system.

A cropping system is a long-term commitment that requires constant tweaking. Cover cropping itself is under constant assessment because different crops bring different benefits and what works one time doesn’t necessarily meet the needs the next time.

Cover cropping defined
Cover crops are sown for their benefits to the soil itself rather than for their possible commercial value. In most instances the cover crop is killed before it can set seed and it’s either left on the field as mulch or turned into the soil as green manure.

A long list of benefits
First of all, cover crops effectively limit soil erosion. If a field is left bare, high wind and rain can strip the surface of precious topsoil. Heavy rain also causes compaction of the soil surface. This compaction pools rainwater on the surface and prevents absorption of both water and oxygen into the deeper layers of soil. To quote SARE (Sustainable Agriculture Research & Education), “Cover crops are an indispensable tool. They are planted to slow erosion, improve soil health, enhance water availability, smother weeds, help control pests and diseases, increase biodiversity, and bring a host of other benefits.”

Further, “Cover crops have been shown to increase crop yields, break through a plow pan, add organic matter to the soil, prevent leaching of nutrients and attract pollinators. There is a growing body of evidence that shows cover crops improve resilience in the face of erratic and increasingly intensive rainfall, as well as under drought conditions. Cover crops help when it doesn’t rain, they help when it rains, and they help when it pours!”

Specific crops for specific benefits
Cover crops fall into four broad categories: grasses, other non-legumes, legumes and mixtures. Each type of cover crop has distinct advantages for a cropping system. If a farmer needs to increase bioavailable nitrogen in the soil, legumes (fava beans, field peas, hairy vetch, clovers, and others) will effectively fix nitrogen directly from the atmosphere and introduce it into the soil, even to the point of reducing or eliminating the need for further nitrogen amendments.

Among the grasses used as cover crops in the Skagit Valley are annual ryegrass and sudan grass. According to Building Soils for Better Crops: Sustainable Soil Management, annual ryegrass “develops an extensive root system and therefore provides very effective erosion control while adding significant quantities of organic matter,” and sudan grass is “especially helpful in loosening compacted soil and at suppressing weeds.”

Other non-legume cover crops used here include several brassicas such as mustard and forage radish. Citing the previous source, forage radish “develops a large taproot—1 to 2 inches in diameter and foot or more deep—that can break through compacted layers, allowing deeper rooting by the next crop. Forage radish will winterkill and decompose by spring, but it leaves the soil in friable condition and improves rainfall infiltration and storage. Further, certain brassicas “may function as biofumigants, suppressing soil pests, especially root pathogens and plant-parasitic nematodes.”

The fourth category of cover crops, mixtures, is just what it sounds like, a combination of crops that planted together provide a mix of valuable benefits. If all this sounds complicated, it is. A farmer deciding what benefits are most needed is just the beginning. There’s also issues such as when to plant, how to plant, how long to leave the crop in the field, and when to kill the crop or incorporate it into the soil.

It all comes back to tilth
Achieving optimal soil tilth is a matter of knowing the land, the soil profile, and all the variables that can affect that ideal, either positively or negatively. As a tool, cover cropping is effective in year one. It is even more effective in year two and continues to produce greater benefits the longer it is part of a system-wide approach. Cover crops simply improve overall soil health. As one local farmer puts it, “I’m learning that the soil biological system is much more important than just about anything else. The soil tilth is my favorite reason to grow these crops and I am on a continuous search for a better plant system.”