COLUMBIA, Mo. — “You don’t need a green thumb to be a good gardener, but it certainly helps to have good soil,” said University of Missouri Extension horticulturist David Trinklein.

“Unfortunately, many of us are burdened with soils that need help,” he said. One of the best ways to help soils be more productive is to add organic matter, Trinklein said. Organic matter improves soil structure, increases nutrient content and exchange, aids in water retention, and enhances the microbial population of the soil. That’s why some call organic matter “a gardener’s best friend,” he said.

Perhaps the easiest and least expensive way to organic matter is to plant cover (or green manure) crops. In vegetable gardens, cover crops frequently are planted as the harvest is completed.

Annual ryegrass is one of the most popular and reliable grasses to plant as a garden cover crop, Trinklein said. It grows quickly, competes well with weeds and does a fine job of building soil structure because of its extensive root system.

If planting a cover crop must be delayed into fall, grasses with greater winter hardiness such as rye and oats are good choices. Both tolerate cold quite well and may grow throughout the winter, weather permitting, said Trinklein.

Gardeners often use cover crops as “catch crops” to take up and fix any residual fertilizer, especially nitrogen that would be lost through leaching during the fall and winter. Add a balanced fertilizer such as 15-15-15 at the rate of about 10 pounds per 1,000 square feet for more vigorous and predictable growth.

Gardeners with ample space might consider rotational planting, with a full-season cover crop as part of the rotation. This type of cover crop is planted in the spring and must be able to withstand the heat of summer.

Good full-season cover crops for gardens include buckwheat, sorghum-Sudan grass hybrids, cowpea and soybean. They are easy to establish, grow rapidly and compete well with weeds. Cowpea and soybean are legumes and add nitrogen as well as organic matter.

Turn cover crops under in early spring when the soil is dry enough to work—preferably, at least three to four weeks before planting. This gives soil microbes adequate time to break down the organic matter in cover crops to a more stable form, Trinklein said.

Turn under cover crops thoroughly. Exposed parts of the plant might decompose slowly or not at all. Partially decomposed organic matter tends to tie up nitrogen. If the cover crop becomes tall, mow before turning it under.