COLUMBIA, Mo. – “You don’t need to have a green thumb to be a good gardener, but it certainly helps to have good soil,” said David Trinklein, University of Missouri Extension horticulture specialist. Unfortunately, most Missouri soils are less than ideal for gardening.

One of the most beneficial things a gardener can do to improve soil 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.

Perhaps the easiest and least expensive way to incorporate organic matter into garden soil involves planting cover crops, he said.

Vegetable gardeners frequently plant cover crops in late summer when harvest is complete. “Although the foliage provides valuable organic matter, it actually is the extensive root systems of cover crops that contribute most to soil improvement,” said Trinklein. “Many cover crops produce more biomass below ground than they do above ground.”

Cool-season grasses that thrive in the mild days and cool nights of autumn are ideal candidates as cover crops. Annual ryegrass is one of the most popular and reliable grasses to plant as a garden cover crop. It grows quickly, competes well with weeds and does a fine job of building soil structure because of its extensive root system, said Trinklein.

Choose grasses that show greater winter hardiness, such as rye and oats, if cover crop planting is delayed. Both tolerate cold weather quite well and may grow throughout the winter, weather permitting.

Cover crops often are used as “catch crops” to take up and fix any fertilizer that remains in the garden. This is especially true for nitrogen that would be lost through leaching in fall and winter.

Cover crops should be turned under in early spring, when the soil is dry enough to work. Preferably, this will be at least three to four weeks before planting the garden. Adequate time is needed to allow soil microbes to break down the organic matter in cover crops to a more stable form.

When turning under cover crops, do so as thoroughly as possible. Exposed parts of the plant may decompose slowly or not at all. Partially decomposed organic matter tends to tie up nitrogen when soil microbes complete the decomposition process, Trinklein said.

“In the soil, organic matter continually is broken down in a biological process carried out by soil flora and fauna,” he said. “For this reason, the yearly addition of organic matter to garden soil is considered a best management practice.”

More information: “Cover Crops Improve Garden Soil,” Missouri Environment and Garden newsletter, ipm.missouri.edu/meg/?ID=305.