As gardens go, the patch next to the garage behind Nicholas Keel's 100-year-old house in Government Hill doesn't look particularly lush or productive. But the scattered seedlings that pushed their way through the crumbled earth in late December grow with purpose. Keel planted the winter crop to benefit the soil.

As the mustard greens and buckwheat shield the bare ground, they also will feed, aerate and potentially cleanse the soil.

Cover crops — plantings typically grown in the garden's downtime — begin their work by protecting the ground from erosion and drying effects of wind and sun. They also help to crowd out weeds.

“Nature abhors bare soil. If you don't at least mulch bare ground, weeds will come in and fulfill a role,” says Michelle Gorham, community gardens program assistant with Green Spaces Alliance.

Cereal rye, a lush, grassy plant that thrives in cool weather, is the cover crop most familiar to area gardeners. Some throw the seeds over their lawn for a verdant view in winter when lawn grasses go dormant. In vegetable plots, though, the grain — not to be confused with winter rye grass — proves it's more than just a pretty plant. Its roots trap root-knot nematodes, microscopic worms that cause the demise of tomatoes and other veggies.

“There is something on the microscopic level that binds up the nematodes in the soil. It's not going to wipe them out or anything, it's just going to lessen the population,” says Brian Wille, general manager at Douglass King Seeds. The San Antonio company typically sells Elbon rye, the best-known cereal rye, in a 50-50 mix with hairy vetch or Austrian winter peas.

Vetch and peas, both cool-season legumes, enrich the soil with nitrogen.

Legumes convert atmospheric nitrogen into ammonia nitrogen, a form that plants can use, Wille explains “That process is known as fixation.” In garden lingo, plants “fix nitrogen” in the soil, and nitrogen feeds foliage.

“I know (farmers) who have sworn they didn't need to fertilize because they put out peas in winter,” says Wille. “Before all the chemical fertilizers, in the pre-World War II era, this is how they did it. They would plant things like alfalfa, peas and vetch for a season and ... follow it with corn or cotton.”

With increased emphasis on organic growing, cover crops are getting more attention in both large and small gardens. “As fertilizer gets more expensive, cover crops get to be a more viable option,” he says.

A 10- to 15-pound bag of rye and vetch seed costs about $15. A pound covers about 1,000 square feet. Wille calculates that 1,000 square feet of winter peas, allowed to grow through the season, could produce the nitrogen equivalent to about 12 pounds of 15-15-15 fertilizer.

Gorham has grown cowpeas in her garden for summer vacation. With just enough rainfall to sustain the cover crop, she pulled some to make room for fall veggies and let the rest grow until frost.

In nature's complex and mysterious way, legumes get help from bacteria to make the nitrogen conversion. A specific species of rhizobia bacteria works with each legume species to accomplish the switch. Seed companies inoculate legume seed with the appropriate bacteria.

The inoculant, a powdery substance that isn't harmful to humans or animals, has a shelf life of one to two years, Wille says, so consumers should ask when the seed was treated. Purchased alone, inoculant costs about $7 for a pouch that treats 50 pounds of seed.

When it's time to plant the next crop, the cover crop goes to work in another way. Once it's cut, the clippings can be tilled into the soil or left on top of the ground like mulch. “It's like using fresh grass clippings and skipping the compost pile,” Gorham says. “You're kind of composting in place.”

Cereal rye and buckwheat provide organic matter, improving the soil structure. Legumes add nitrogen. Growing a mix helps keep balance.

The material can be tilled into the garden, but many farmers and home gardeners opt to plant through the fresh mulch because turning the soil is disruptive. “It takes a year or two for the soil to recover. And when you till, you're also grinding up earthworms,” Wille says. “By keeping (the clippings) on the surface, that allows the soil to mellow.”

In another month or so, when it's time for a different crop, Keel plans to discard the mustard greens and buckwheat plants, and he hopes they will have drawn toxins, especially lead, from the soil.

Because of the age of his house and its location a block west of Interstate 35, he feared lead from paint scrapings and automobile emissions had contaminated the soil. He's being extra cautious because he and his girlfriend, Heather Wenrich, want to grow the majority of the food they eat. The leafy greens that are a staple of Wenrich's vegetarian diet tend to absorb more of whatever is in the soil.

Though it wasn't a scientific measure of lead content in the soil, Keel did use a lead-paint test kit to check the garden soil. It showed positive.

So after scraping off the 2 inches of ground and moving it to areas outside the garden, he put the plants' wicking nature to work. He researched plants used in phytoremediation, a sort of plant eco-therapy, and found mustard greens are best for removing lead. “Plants store lead in their cells,” he says.

So even though his current crop isn't destined for the dinner table, it is serving him well.