A Golden Rule for Gardening: Do No Harm

By Robert Kourik Copyright 1998 New York Times August 9, 1998

WHAT could be more natural than gardening? Most people who grow flowers or their own food think they're "healing the Earth" in some small way. Gardeners may be having fun, getting exercise and harvesting tasty vegetables and fruits, but the best thing, as far as nature is concerned, is probably no gardening at all. In fact, gardening can be downright harmful: tillage releases carbon dioxide, contributing to global warming; cultivation and compaction destroy beneficial soil fungi, and excessive nitrogen fertilizers, even manure, can contaminate water supplies.

While most gardeners assume that a plow, shovel or rototiller is required for a good crop, the only comparable natural model is a landslide. So why have most cultures used cultivation? Mostly to create a zone where domesticated crops have little competition from native plants.

Global Warming

The earth's soil contributes 10 times more carbon dioxide to the atmosphere than all human activity, according to Dr. Tyler Volk, a professor of biology at New York University and an expert on the carbon cycle. It comes from the myriad life forms that inhabit the soil -- microbes, pill bugs, worms and fungi -- as they breathe, process food and die.

Whereas in the past the increase in carbon dioxide gas produced by small-scale tillage was absorbed by plants through photosynthesis, Dr. Volk said, tillage has now become a large contributor to the surplus of carbon dioxide. When soil is stripped of its living cover to grow crops, up to one-fourth of its stored carbon, which contributes to fertility, is lost as carbon dioxide.

Dr. Volk estimates that the rise of the average global temperature of one degree Fahrenheit in this century has expelled an extra billion tons of carbon into the atmosphere from the soil. Currently, he says, the global carbon cycle may be able to absorb it, but carbon dioxide generated by cultivation -- even in gardens -- may contribute to a warmer climate.

Gardeners can compensate for a loss of carbon, Dr. Volk says, by adding compost and manure. And they can minimize the loss of carbon dioxide from their plots by experimenting with "no till" gardening: growing plants in deep mulch, up to a foot thick; sheet composting (thin layers of a variety of compostable materials laid out over the soil like a thick mulch), or by reserving portions of the garden for soil-improving crops like fava beans or vetch.

Good Fungi

In undisturbed soil under many trees, perennials and some vegetables lurks a mostly unknown but beneficial fungus called vesicular-arbuscular mycorrhizae -- VAM, for short -- which can sometimes be spotted because of its above-ground fruiting bodies, including Boletus, Amanita, Lactarius mushrooms and some types of puffballs.

VAM forms a remarkable symbiotic relationship with plants. Its microscopic filaments either actually augment tree root hairs, or they grow into the cells of the root hairs. The filaments provide nutrients to the plant, mostly phosphorous, potash, zinc and copper, and they receive carbohydrates for the fungus.

Many plants grow much more lushly with this association. For example, according to studies at various universities, when VAM was present in otherwise untreated or poor soils, oat plants were nearly twice as heavy and strawberries five times as heavy than they would have been otherwise. Citrus trees growing in soil inoculated with VAM were 16 times as heavy than trees in sterilized soil.

Most natural, undisturbed soils have plenty of VAM. But it can be injured or destroyed by tillage, by removing the natural litter called duff beneath trees, by stripping away topsoil for construction, by compacting the soil (even by walking on it), by fumigation or by overfertilization.

There are several ways of preventing compaction: keeping permanent pathways away from trees, using deep mulches for little-used paths, using some cover crops in your yard and rotating crops with root systems of different depths to help keep untilled soil friable. Keep annual crops away from trees (at least one-half to three times the width of the canopy) so as not to disturb the VAM.

Some garden centers and catalogues are touting new VAM inoculants for all gardens. They may not be required except in two cases: at a new house on a bulldozed site or in sterile potting soil. But if you've spotted the telltale mushrooms in a forest, you can just shovel up some duff there and sprinkle it on your soil or add it to a sterile potting mixture. Then let nature do the work, free.

Surplus Nitrogen

Most gardeners squander nitrogen, even manures. Because of its cost, farmers tend not to waste fertilizer. To equal the amount a farmer spreads on nitrogen-hungry crops like celery, cabbage and potatoes, a home gardener need apply only a quarter to a third of an inch of compost or steer or horse manure. Kate Burroughs of Sebastopol, Calif., a certified crop adviser with the American Society of Agronomy, a professional association, uses the same guideline for home-grown sweet corn and lettuce. Broccoli and pear trees need only a dusting. I often observe gardeners applying manure and compost far heavier than this, wasting both money and nitrogen.

That excess nitrogen leaves the garden as a gas or leaches away with rain or irrigation toward water supplies, and it can set back VAM activity. Surplus nitrogen causes plants to grow more foliage, not stems, tubers or other edible parts, and it stimulates weaker growth more prone to pests and disease.

As it turns out, the ancient Greeks were right: all things in moderation.

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