Clever Clover

"For the home gardener, cover crops can be really useful in adding nitrogen to the soil," says cooperative extension advisor Chuck Ingels. "You can add almost enough nitrogen to meet the needs of the following crop in the summer."

Cover crops also improve soil structure, prevent erosion, attract beneficial insects, improve crop yield, reduce nutrient leeching and keep weeds in check. And in the case of bell beans, field peas and vetch mixture, cover crops can take nitrogen from the air and convert it so it can be used by plants.

"Bacteria on the roots of the legume take nitrogen gas from the atmosphere and fix it onto the plant; the nitrogen is then available to the plant," says Ingels.

Clover is a great cover crop that tolerates many extreme conditions, but make sure it's an annual because perennial clover is extremely invasive and will compete for nutrients in the soil.

If you want to increase the soil's organic matter, grasses and cereals add lots of biomass. Biomass is the amount of living matter in a given area. They germinate quickly and have fibrous root systems for excellent erosion control. "Grasses don't fix nitrogen," says Ingels, " but they help build the soil tilth, the organic matter content in the soil."

Cover crops are started from seed in the garden as soon as the annual plants come out. Ingels wants to replenish the nitrogen so he chooses a blend of legumes. "The atmosphere has about 78 percent nitrogen, and most plants can't use that nitrogen. Legumes can because they have bacteria on their roots which fix the nitrogen, but in order to get the bacteria there, we have to inoculate the seeds."

The crop will continue to grow through the winter and spring, and then right before flowering, till it into the soil. Don't worry if grasses have come up with the cover crop. They'll just add more biomass to the soil. Weeds are okay, too. But you don't want them to go to seed or the weeds will become the next cover crop. (If in doubt, pull them out.)

After the legumes are tilled in, wait two to three weeks for the crop to break down into the soil. Grasses usually take even longer to decompose. Don't plant your spring crop before the green manure has completely disappeared because there still is a lot of organism activity below ground that can damage young crops.

Planting a cover crop can be a safer, more effective way to add nitrogen to the soil than by using fertilizers. "Too much nitrogen can result in too much growth the following year," says James. "So what you want is a balance of nutrients, and a good way to achieve that is by planting legumes as well as grasses in your green manure mixture."