Whatever else you cultivate, don't forget compost

While I was extolling the virtues of soil building the other day, it came out sounding like a promotional for my own ability to turn nearly sterile sand into fertile garden loam, according to my wife.

''Showing off,'' I think she called it.

She was right, of course (she usually is). Gardeners do not build soil; the bacteria, molds, and earthworms do that. We can help by providing the food - organic matter - which the soil organisms then turn into humus-filled soil.

One of the richest sources of soil-building materials in towns, particularly in the tree-filled regions of the country, are the leaves which all our nongardening neighbors dutifully sweep up, bag, and leave for us - if we beat the garbage crew to the pickup.

This is all very well in most suburban situations, but what about those regions where few trees grow and such free pickups are not available?

Conceivably, the time will come when everybody who can will grow some of his own. In such a situation there will be few, if any, excess bags of organic material available to build up or even sustain soil quality.

What options will the gardener, seeking to sustain yields through organic means alone, then have? Quite simply, he will have to grow his own soil-building , or composting, materials.

Fortunately, it can be done even in small gardens.

A couple of years ago, I was pulled up short by John Jeavons, whose Ecology Action group in Palo Alto, Calif., had been researching this very subject. His statement was powerfully blunt: ''Organic gardening or farming methods are unsound if they use more organic matter than the garden or farm can produce.''

Those gardeners who mow a field year after year and use the hay for compost or as a permanent mulch on their gardens may achieve spectacular results, but they are doing so at the expense of the field. ''Unless the field is being manured or otherwise fertilized, it is being mined of its nutrients,'' Mr. Jeavons pointed out.

At the time the Ecology Action folks were working on this problem. They had built up their garden from the poorest of subsoils (C horizon soil) into very fertile topsoil by incorporating organic materials and fertilizers which were brought in from outside. A good case can be made for building up all new garden soils this way. But ideally the gardener or farmer should not rely on outside sources to maintain soil fertility indefinitely.

Now Ecology Action researchers feel they have learned enough from ongoing trials (it will take 50 years of research before any soil-building program can be fully tested, in Jeavons's view) to say that they need not.

Their self-teaching mini-series, Pamphlet No. 10, ''Grow your compost materials at home - an approach to sustainable organic matter production and soil fertility,'' explains the findings to date. I find the message encouraging.

It points out that soil quality in a closed system can be readily maintained, not just for centuries but permanently.

Any gardener who regularly returns all spent crop material to his soil, either by plowing it in or returning it as compost, goes a long way toward retaining soil fertility. But he does not maintain it completely, because he removes the edible portion of his crops for his family's use. It is to make up for this small annual loss that some portion of the garden must be given over to producing plant material purely for the compost pile.

Legumes - peas, beans, and the like - that take nitrogen from the air and fix it in usable form in the soil play an important part in any closed system.

Ecology Action lists 21 crops that produce impressive amounts of vegetable matter for this purpose. Surprisingly, until you think about it, two of the best are radishes and beets. Small though these plants are, they grow so rapidly that many succession crops can be harvested in a season. Depending on soil quality, intensively grown radishes produce between 1.82 and 5.09 pounds (dry weight) per square foot of composting material in six months; beets produce between 1.82 and 4.37 pounds per square foot in the same period.

Irish potatoes are also good organic-matter producers, but I suspect it would take considerable self-discipline on the part of the gardener to dig up some Idaho-type bakers and feed them to his compost heap rather than his family.

Ecology Action has found that, once garden soil has been built up to a satisfactory level, 0.94 pounds of organic matter (dry weight) per square foot added each season should be enough to maintain soil fertility.

Spreading a one-inch layer of compost (made in the classic manner with soil layers every six inches) over the garden provides the soil with roughly this weight of organic matter. If your compost heap was made without any soil (with pure organic matter, in other words) a three-quarter-inch layer should prove adequate.

(In this respect it is interesting to note that Britain's Good Gardeners Association, which follows a largely no-dig policy, has found that a one-inch layer of finished compost, made with soil, and applied to the vegetable garden each spring, is enough to produce satisfying crops year after year.)

In regions with a 4-month growing season (between last and first frosts), as little as one-fifth to as much as half the garden would have to be given over to growing plants for composting, depending on soil quality and the gardener's skill. In regions with a six-month growing season, between one-eighth and one-third of the garden would be given over to this activity.

These recommendations are based on converting all crop residues and the specially grown crops into compost before returning them to the soil.

Growing cover crops, such as winter rye, to protect the soil over the winter would provide additional organic matter that would somewhat reduce the need for compost. Much research is still to be done, Ecology Action points out, and the group would welcome any input from organic gardeners everywhere.

Obviously, growing plants purely for the compost pile is not yet vital in most urban and suburban settings. If your neighbor is discarding his leaves anyway, you might as well make use of them. But it is useful, as well as comforting, to know that it is possible to maintain soil fertility without being dependent on outside sources.

Self-Teaching Mini-Series No. 10 is available for $1.40 postpaid from: Ecology Action of the Midpeninsula, 2225 El Camino Real, Palo Alto, Calif. 94306 .