How to Use Cover Crops to Build Your Soil

There's a lot more to the stuff most people consider just dirt. A healthy handful of soil has billions (yes, billions) of microorganisms in it, members of a truly humbling food web below our noses that cycles nutrients and essentially makes life on Earth possible. Keep that in mind next time you consider doing a two-step on your garden bed to snatch some rabbit food for dinner. Just a little high-stepping compacts soil, depriving your microscopic garden partners of vital oxygen and moisture, held in minute soil spaces. "Cover crops play a critical role in maintaining soil fertility. I never leave bare soil in my community garden," explains Andy Clark of the U.S. Department of Agriculture's Sustainable Agriculture Network, who earned his doctorate in cover crop science. "The cover crop gets planted as soon as the main crop is harvested -- sometimes sooner."

Cover crops and green manures have different functions, depending on whom you talk to. Here's the purist view: Cover crops are living mulches designed to cover soil, sometimes an entire inactive bed or just parts of a bed, such as the spaces between rows of other crops. Here, the cover crop smothers weeds, adds nitrogen, extracts nutrients from deeper soil and reduces erosion and compaction. Green manures, conversely, are grown, cut down (before flower) and incorporated (once brown) into your bed. Hence, the term green manure. This adds organic matter to your soil, improves soil quality or tilth, and stabilizes it as well. The new organic matter also boosts earthworm and beneficial microbe activity, which improves soil fertility. As these decomposers feed they play an essential role in the mineralization (breakdown) of organic and inorganic matter, making nitrogen, phosphorus, potassium, sulfur, calcium, iron, etc. available to your ornamental plants and edibles. When you cover-crop, in other words, you feed your soil organisms, which feed your plants. Both cover crops and green manures also increase air and water penetration in your soil. Additionally, your microbe engine produces humus and soil aggregates that increase your soil's relative moisture-holding capacity. Finally, cover crops prevent unused soil nutrients from being leached out of your soil after you've harvested your edibles and emptied your beds. In fact, cover crops and green manure are often used interchangeably. Alfalfa, vetches and clover are generally the most popular pure cover crops because they are low-growing and also fix nitrogen. At the end of the day, though, all of these crops get mowed, dug under or composted.

Cover Crop Selections

There are a great number of cover crops, which have different applications and benefits. Knowing every one is less important than understanding a few basics, namely that there are annual and perennial species and cool- and warm-season varieties. It's also critical to differentiate between nitrogen-fixing legume covers and non-legume varieties, which include grasses and some broadleaf species. Other than alfalfa, perennial cover crops are used more frequently in commercial settings, such as vineyards and orchards. Generally, annual cover crops are the best choice for home gardeners, as they can be grown to maturity from seed in a single season (six to eight weeks), giving you time to cut them down for mulch or use them as green manure in your soil before planting warm-season veggies such as tomatoes, zucchini and cucumbers in the late spring or early summer, and cool-season greens such as broccoli and spinach in the fall. Warm-season cover crops are able to grow in the heat of late spring and summer, while cool-season or winter cover crops are suitable for cool temperatures in the fall before and after hard freezes. Some varieties, such as summer buckwheat, even have beautiful flowers which attract beneficial insects. Legumes fix their own nitrogen and release it to your edibles. Grasses and broadleaf cover crops do not add nutrients; they recycle nutrients that might otherwise be lost through leaching. Grasses also generally add more organic matter, an essential component of any healthy soil.

Be sure to identify your main reason for planting a cover crop. Do you want to add nitrogen? Plant a legume? Is your soil compacted, or too sandy or too hard to work? A grass adds organic matter and improves your soil faster than a legume. Sometimes a grass-legume mixture is the best option.

Planting Time

The easiest way to sow a single variety of cover crop or a blend is to just broadcast it across your bed by hand with some fluid sweeps of the arm. It's important to evenly disperse the seed. Before you sow the seed, rake your bed and remove any rocks and debris, so the seed has a nice landing pad. Once the seed is down, gently rake it under, about twice the diameter of the seed. Another route: Sift some nice planting soil or compost over the top so birds don't see the seeds and go to town. Keep the bed wet initially to help the seeds germinate. Also, check if your legume seeds are pre-inoculated. If not, inoculate them with rhizobacteria. Rhizobia, as they're called, help legumes convert atmospheric nitrogen into organic soil nitrogen.

Other tips: Rotate your cover crops just as you rotate vegetables to improve their performance and pest resistance. Make sure to till your crop under before it goes to seed and at least 4-5 weeks before planting for decomposition to take place. If planting a legume, make sure to till under before it even flowers to benefit most from it’s nitrogen fixing capabilities.

Recommended Cover Crops Available at Anderson’s Seed & Garden

Warm season legumes:

Alfalfa (Medicago sativa), legume, perennial

Warm season grasses/broadleaf:

Buckwheat (Fagopyrum esculentum), broadleaf

Cool season legumes:

Red and White clovers (Trifolium spp), legume, perennial

Hairy Vetch (Vicia villosa), legume Best Choice, hardy annual

Fava beans (Vicia faba), legume

Cool Season Grasses

Ryegrass (Lolium multiflorum), organic matter

Winter Wheat (Triticum aestivum) – organic matter

Brome grass (Bromis mollis), organic matter