A decade ago Vermont gardener Dick Raymond began an experiment. He wanted to see if he could grow what he calls an ''eternal yield'' garden. By that, he means a garden that produces year after year without the addition of fertilizers of any kind.
No manures, no compost, and no commercial fertilizer have been added to his experimental beds in 10 years (although he periodically adds a little powdered limestone to counter soil acidity), and yet they yield a better harvest today than in the early years of the experiment.
Soil quality has improved with each season of gardening.
Basic to these experiments is growing nitrogen-fixing legumes: peas followed by beans one year; vegetables, including corn, the next year. In addition, all crop residues are tilled into the soil after harvest and, if there is time in the late fall, a cover crop of annual rye grass goes in to protect the soil over winter. This, too, is tilled into the soil in the spring.
Now all this is contrary to conventional wisdom. Even Dick Raymond isn't sure why it works. But he has a ''few ideas,'' he says.
This is how the eternal-yield system operates: Half of the garden (12-by-24 feet) is tilled and planted to English peas early in the spring. Three pounds of seed are inoculated with nitrogen-fixing bacteria and broadcast over the area. These are raked in or tilled shallowly into the soil. Frost-tolerant, the peas sprout and quickly cover the entire bed with a living mulch.
The Raymonds harvest twice from this patch with the yield generally two bushels of fresh peas or more. They once harvested 75 pounds of shelled peas from 3 pounds of seed.
As soon as the second harvest is completed, the green-pea vines are tilled into the soil and a follow-up crop of beans goes into the garden. Raymond grows yellow and green snap beans, or sometimes shell beans.
Both peas and beans are legumes, and so have that priceless ability to take nitrogen from the air and fix it in the soil. So by the time the beans have been harvested and tilled in, a good deal of nitrogen has been fixed in the soil.
With the beans tilled in, the garden is then sown to annual rye grass.
Rye grass, which does not survive the winter, nevertheless is fairly cold-tolerant and puts on moderate above-ground growth and considerable root growth before the heavy freezes arrive.
This rye, with its deep-probing roots, brings up nutrients that would otherwise be leached away during winter and holds them near the surface. It also protects the soil and much of the soil life over winter.
(Raymond prefers the annual rye to perennial rye that overwinters. Already partly decayed when tilled under in the spring, perennial rye does not borrow any nitrogen from the soil - to the detriment of the new plants - while it decays. Tilling in green-pea vines later in the season to prepare for the bean crop presents no problem because pea vines are rich in their own nitrogen. Also, soil temperatures have warmed up, so decomposition of the living tissue is rapid.)
The following year this half of the garden is planted to other vegetables that thrive on the nitrogen provided by the legumes, and the pea and bean crops grow where the vegetables grew the previous year. In the vegetable beds, too, spent crops are tilled in after harvest, and a cover crop of annual rye grass is sown at the close of the season.
In this system all the nitrogen that the vegetables and corn need are provided by the peas and beans, but the two other principal plant nutrients, phosphate and potash, are not provided. The fact that all spent crops are plowed back into the soil after harvest means that the bulk of these nutrients is returned to the soil to be reused by the next generation of crops. But over the years there should be a steady loss of phosphate and potash in the harvested fraction of the crops (the vegetables we take indoors and eat.) Yet each year the soil tests by the extension service show adequate supplies of nutrients are available. The question is: Why?
Raymond believes the constant tilling in of the spent crops boosts the organic content of the soil which, in turn, helps release phosphate and potash that is normally locked up in soils less endowed with organic matter.
There are, he suspects, large reserves of locked-up nutrients just waiting to be released to the plants by the addition of spent plant materials. In fact most of the phosphate and potash added to a soil in chemical form becomes locked up, combining with other soil chemicals within about three weeks of application.
According to a British study (Struthers and Seiling, published in the British publication Soil Science, 1950) there is enough potash for a thousand years and enough phosphate for 500 years in average farm soils, provided spent crops are returned to those soils.
In addition, the seeds sown represent a small, but significant, input from outside. Each seed comes loaded with energy to give it a start in life. Particularly in the case of rye grass, which is not harvested, all that energy and the nutrients it represents are returned to the soil each year.
Raymond does not know whether his soils will eventually run low on phosphate and potash in the eternal-yield garden.
''I shall just have to wait and see,'' he says. One thing he is certain of: ''The person who gardens this way and also adds household food wastes to the garden in the form of compost, should be able to garden forever.''
His suggestion to anyone wishing to start an eternal-yield garden is to first bring in manure, compost, and perhaps rock powders to build up a rich soil quickly. This would be a one-shot fertilization program.
After that, the eternal-yield system, with the addition of composted food wastes, should work forever.