Healthy soil the heart of a successful garden

Posted April 15, 2010, at 6:36 p.m.

A successful gardener nurtures healthy soil, developing a keen sense of the garden’s soil as an ecosystem teeming with life, the roots of garden plants exploring a matrix of minerals and organic matter harboring hordes of bacteria, fungi, nematodes, earthworms and other life forms. The more I read and think about soil biodiversity, the less inclined I am to take a spade to the garden beds.

There are between 100 million and 1 billion bacteria in a single teaspoon of productive soil, the equivalent of a ton of bacteria per acre. They live in the water-filled pore spaces of the soil or on the moist surfaces of soil particles and decaying organic debris.

Most bacteria are decomposers, converting the energy in organic matter into forms useful to the rest of the organisms in the soil food web. These decomposers retain nutrients in their cells, preventing the loss of nutrients, such as nitrogen, from the root zone.

Nitrogen-fixing bacteria form mutualistic relationships with the roots of legumes such as peas and beans and trees such as alder. The plant supplies carbon-based energy sources to the bacteria, and the bacteria convert nitrogen in the air to a form the plant can use. When the host plant’s leaves and roots decompose, soil nitrogen levels increase.

Other soil bacteria species, the nitrifying bacteria, convert ammonium to nitrate, a preferred form of nitrogen for most garden crops. And the actinomycetes, a large group of bacteria that are responsible for the “earthy” smell of healthy soil, decompose a wide variety of organic materials, including chitin and cellulose, the tough stuff of plant cell walls.

Joining the bacteria as important decomposers are the saprophytic fungi. They convert dead organic material into threads of the fungal body, called hyphae, that physically bind soil particles together, creating stable soil aggregates that help increase soil water holding capacity and water infiltration. Like bacteria, the fungi also are important in retaining nutrients in the soil.

Other fungi, called mycorrhizal fungi, colonize plant roots in mutualistic relationships. In exchange for carbon from the plant, these fungi bring soil nutrients to the plant.

In addition to bacteria and fungi, a teaspoon (1 gram) of healthy garden soil contains several thousand nutrient-mobilizing protozoa and 10 to 20 species of beneficial nematodes (microscopic worms). Each square foot of healthy garden soil contains up to 100 beneficial arthropods (insects, spiders, mites and others) and between five and 30 earthworms.

Earthworms are particularly important to maintaining healthy soil structure. Their tunnels provide aeration and drainage, and their excretions bind together soil crumbs.

In my mind, no good can come from tilling or digging established garden beds. Each tilling or digging disturbs the natural growing environment of plant roots and disrupts the complex cycling of nutrients through the soil food web. It also enhances soil compaction and erosion, and brings dormant weed seeds to the surface where they will sprout.

No-till systems are freer of pests and disease, likely because of the more balanced soil community of life that builds up in a comparatively undisturbed environment.

When breaking new ground, it may be necessary to till or dig, incorporating copious amounts of organic matter as you go. From then on, however, the garden can be managed with minimal disturbance of the soil.

Mulching is the essence of no-till gardening. Laying down thick layers of compost every year will suppress weeds, shade the soil, reduce water lost to evaporation and allow the complex soil food web to operate undisturbed.

There is in all of this a strong argument for raised bed gardening: heavy foot traffic also disturbs the soil life. You do not need to construct framed beds, simply form walkways between planting areas. Over time, with considerable mulching, the planting areas will be raised above the walkways.

This year, when I sow seeds or transplant seedlings, I’ll gently rake aside the mulch and make a furrow or dig a small hole, do my planting with minimum disturbance of the soil. I’ll spend more time in the garden spreading compost over the soil between my plants and gently pulling the few weeds that crop up. I’ll hang my spade on a nail in the basement.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@shead.org. Include name, address and telephone number.

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