Secrets of building a fertile garden with compost alone

For 10 years Vic and Betty Sussman lived close to the banks of the Potomac River on a 2-acre ''homestead'' surrounded by rolling farmlands.

The couple raised goats and gardened intensively in what they say, was ''a deeply satisfying learning experience'' which Sussman chronicled in his book, ''Never Kiss a Goat on the Lips'' (Rodale Press).

But eventually encroaching suburbia from nearby Washington, D.C., surrounded the homestead. So the Sussmans moved here to northern Vermont with but one regret: they had to leave behind the soil which a 10-year program of consistent composting had turned into a rich, fertile loam. Now the couple must start all over . . . and compost will again be the key to a quick buildup of intensive vegetable-producing beds.

In this respect the Sussmans are fortunate. Their new homestead boasts some moderately good soil to start with and, in addition, a wealth of ready-made compost. Apparently previous owners of the one-time farm had regularly thrown garden waste, spoiled hay, and manure onto a vast pile and used very little of the resulting compost.

Vic Sussman began delving into all facets of compost and composting from the moment he started gardening a little more than a decade ago. In the process he acquired a good deal of knowledge on the subject which he drew into his book titled ''Easy Composting'' ($6.95 Rodale Press, Emmaus, Pa.). The volume is loaded with practical advice on how to make and use compost.

Mr. Sussman admits to wondering at one time if compost really could compete with artificial fertilizers in a garden. Experience quickly told him that it could; later he found out how it does this.

In simple terms, compost (partly decomposed organic matter) releases plant nutrients steadily as it continues to decompose. So even if all water-soluble nutrients were to be washed out of a batch of compost in a heavy rainstorm one day, they would return as the compost continued to decompose.

How quickly they returned would depend on the rate of decomposition - slowly in the cool spring, rapidly during the heat of summer.

This decomposition continues until there remains only a very black humus, which is composed of the most highly rot-resistant lignins. This humus is not particularly rich in plant nutrients but remains an outstanding soil conditioner until it, too, decomposes.

Along with water-soluble nutrients, compost also contains water-insoluble nutrients which plant roots are able to extract by what is known as cation exchange, in which negative and positive ions are exchanged between root and organic matter.

Finally, compost helps to release a number of nutrients that, in the absence of organic matter, remain chemically locked up in the soil and unavailable to the searching plant root. Tests at the Rothamstead research station in England have shown that highly soluble chemical fertilizers applied to a field are largely lost to a plant in three to four weeks - largely by leaching in the case of nitrogen, and chemical change in respect to phosphate and potassium.

Not to be underestimated are the improved aeration and moisture-holding qualities which compost imparts to soil and, in turn, raises the ''comfort level'' of the plants.

Compost can be turned into the soil before planting or spread on top of the soil where it acts as a mulch, allowing the rain and earthworms to take the nutrients down to the plant. Or, as the Sussmans prefer, the compost can be placed at the bottom of each planting hole.

In this latter method, compost is placed at the bottom of a hole, a little soil scattered on top, and the transplant set on top. The amount of compost used would depend on the needs of the plant: a loose-leaf lettuce may get a trowelful; a cabbage with its more hearty appetite, two trowelfuls; and a tomato , a shovelful.

With direct-sown plants (beans or peas, for example) Sussman open up a V-shaped trench which he partly fills with compost. He then covers this with about an inch of soil, sows the seed, and pulls the remaining soil on top of the seed, firming it down. By placing the compost below the plants or seeds, new roots quickly find a rich source of nutrients in which to grow.

One way to give plants a quick lift is to make compost ''tea.''

Simply stir a shovelful of compost into a bucket of water. Pour off the water and feed it to the plants. You can repeat this process for about three days by which time all the immediately available soluble nutrients will have been removed.

What remains of the compost should be returned to the heap to continue decomposing.