Healthy garden soil is teeming with life: there are earthworms and micro-organisms by the millions, each with a particular function in making soil fertile. Like any living thing, the soil must have food. Without food, the life in soil either leaves or dies. Eventually, the garden itself weakens and dies.

Soil life eats organic matter, decomposing it and creating a crucial soil element called humus. Humus is decayed organic material. The process of decomposition releases nutrients in forms that plants can absorb. In other words, decomposition of organic material has a fertilizing effect.

But fertility is only part of the value of regularly feeding the soil with organic material. Humus also contributes to the sponge-like soil texture that allows air circulation and moisture retention. Loam -- the ideal soil for growing plants -- is a balanced mixture of sand, clay, silt, and organic matter. Humus will bind sandy soil or loosen hard-packed clay.

For these beneficial results (for fertility and texture), the life in soil needs fresh food. Regular doses of organic material will ensure that garden dirt is enhanced rather than depleted over the lifetime of the garden. Every year, a 30 by 40 foot garden needs around 400 pounds (equivalent to 10 bales of hay) of organic material, but it doesn't need to be added all at once.

Additions of organic material take a variety of forms. For starters, chop garden residues into the soil: weeds, mulch, and plants left after harvest. Hauling in compost by the yard from nurseries or hauling animal manures from nearby farms is also an option. But the easiest and most cost effective method of continuous additions of organic material is to grow cover crops, also known as green manures.

Cover crops are grown and tilled into the soil, replenishing rather than removing nutrients. Even in a small garden, this is an effective method when a harvest crop and a green manure are grown in rotation. For instance, plant a late summer green manure after an early crop such as peas or broccoli.

Some suggestions for cover crops include legumes, buckwheat, and ryegrass.

Legumes such as peas and soybeans fix atmospheric nitrogen into the soil when inoculated seeds that attract certain micro-organisms are used. In addition, these legumes are vegetables, making a single planting both a harvest crop and a green manure.

For bulk and quick growth, ryegrass or other annual grains are good choices. In colder climates these are especially good cover crops for the end of summer because they die over the winter and are easy to till in the spring. For the poorest soils, buckwheat is most useful.

Green manures can work with or without using powered equipment, but in larger gardens a roto-tiller certainly makes the process easier. In smaller gardens, the question of whether it makes financial sense to invest in renting or buying a roto-tiller has to be weighed against the cost of hauling in compost and animal manures.

Either way -- hauling or tilling -- some form of additional organic material beyond chopping in garden residues must happen in order for the soil to function and for the plants it supports to thrive.