Improve Your Soil with Cover Crops

Enlarge Image Image Gallery You can choose colorful cover crops, such as bachelor’s buttons and crimson clover, to build your soil and beautify your beds. ELAYNE SEARS

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There are three main ways to improve your soil — grow cover crops, mulch the surface with biodegradable mulches, and/or dig in organic soil amendments (such as compost, grass clippings, rotted manure or wood chips). All have their advantages and none should be discounted, but cover cropping is the method least likely to be practiced in home gardens. There is a reason for this: Information on using cover crops is tailored to the needs of farmers who use tractors to make short work of mowing down or turning under cover crops. But when your main tools for taking down plants have wooden handles and you measure your space in feet rather than acres, you need a special set of cover crop plants, and special methods for using them.

How Cover Crops Help

A cover crop is any plant grown for the primary purpose of improving the soil. Since the early 1900s, farmers have used cover crops to restore fertility to worn-out land. In addition to helping bulk up soil with organic matter, cover crops prevent erosion, suppress weeds, and create and cycle soilborne nutrients using the power of the sun. Recent advances in soil biology have revealed two more ways cover crops can improve soil.

Rhizodeposition is a special advantage to working with cover crops. Many plants actually release sugars and other substances through their roots. They are like little solar engines, pumping energy down into the soil. With vigorous cover crop plants, this process goes on much more deeply than you would ever dig — 6 feet for oats and rye! If you are leaving your garden beds bare in winter, you are missing the chance to use cold-hardy crops such as cereal rye or oats to solar-charge your soil. Thanks to this release of sugars, the root tips of many plants host colonies of helpful microorganisms, and as the roots move deeper, the microbes follow.

But so much for scientific talk. If you’ve experimented with cover crops, perhaps you have dug up young fava beans or alfalfa seedlings to marvel at the nitrogen nodules on their roots, or watched a stand of buckwheat go from seed to bloom in four weeks flat. Or how about this one: It’s April and the soil is warming up and drying out. After loosening a clump of fall-sown wheat with a digging fork, you pull up a marvelous mop of fibrous roots and shake out the soil. What crumb! The soil’s structure is nothing short of amazing! These are the moments an organic gardener lives for.

Bio-drilling is what happens when you use a cover crop’s natural talents to “drill” into compacted subsoil. For example, you might grow oilseed or daikon radishes as a cover crop where their spear-shaped roots will stab deep into tight subsoil. Bio-drilling action also takes place when deeply rooted cover crop plants penetrate subsoil and die. Then, the next crop grown may actually follow the rooting network mapped out by the cover crop (see illustration in the Image Gallery). Maryland researchers were able to track this process using special camera equipment (a minirhizotron), which took pictures of the interactions between cover crop (canola) and crop plant (soybean) roots. As the canola’s deep roots decomposed, soybean roots followed the trails they blazed in the subsoil, hand in glove. In addition to reduced physical resistance, the soybean roots probably enjoyed better nutrition and the good company of legions of soil-dwelling microcritters, compliments of the cover crop.

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