Turning garden soil is meditative, best done in silence punctuated only by reports on the number of earthworms in a spade full of soil. I will say how many, she will respond, “Excellent!”

For as long as I can remember, gardening has been about feeding earthworms. Do all you can to nourish them; do nothing that disturbs their work. Whenever possible, feed them organic matter by digging in compost and growing cover crops, called green manures when they are incorporated into the soil while still green.

Cover cropping is an essential practice in the sustainable garden. I am bothered whenever a garden bed sits fallow, unless it is that two- or three-week period after digging in a cover crop, time allowed for the work of worms.

Which cover crop is best? The answer depends, in part, on the job you want it to do. Cover crops in the bean family, such as peas and hairy vetch, increase soil nitrogen levels. Grass crops, such as annual ryegrass, winter rye and oats, add organic matter to the soil and hold nutrients in the soil root zone. Buckwheat, a fast-growing cover crop used between spring and fall vegetable crops, smothers weeds as it builds biomass. All cover crops serve to hold the soil together, preventing erosion.

Nitrogen-fixing vegetable crops, such as beans and peas, will serve as green manures when you dig the plants into the soil after they have stopped producing and allow two to three weeks for the leaves and stems to decompose, returning the fixed nitrogen back to the soil, before planting another crop. This year, when our Sugar Ann peas stopped producing in early July, we chopped the vines into the soil, waited two weeks, added composted goat manure to the soil, and soon will plant the bed to broccoli for a fall harvest.

Which cover crop to grow also depends on the season. Winter rye, the most common cover crop in Maine, is used to prepare a bed for warm-season vegetable such as tomatoes, peppers or squash. Planted in September, it will resume growth in early spring. Dig it into the soil in mid-May and wait at least three weeks before planting the summer vegetable crop.

A cover crop of oats or annual ryegrass can be grown in the early summer to prepare a bed for a fall-harvested food crop such as broccoli that will be planted in August. These grasses build soil organic matter and serve as “catch crops,” their deep roots scavenging nutrients leached below the root zone of most garden vegetables.

An excellent cover crop for building soil biomass and nitrogen levels from late summer through winter is POV, a mixture of peas, oats and hairy vetch in equal amounts. The peas and vetch are nitrogen-fixing plants while the oats are an excellent catch crop. While the peas and oats will always winter-kill, the vetch may resume growth in spring until incorporated into the soil.

Never allow a cover crop to go to seed and become a weed problem in the garden. Plan your cropping schedule for each bed or garden section early in the year so you can purchase cover crop seeds in spring when they are available at most garden centers and seed stores.

Cover crops will be the hardest-working plants in your garden. They prevent erosion, suppress weeds and build soil organic matter and nutrient levels. They feed the earthworms.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@ptc-me.net. Include name, address and telephone number.