Soil: It’s Alive!

The soil food web consists of a community of organisms that live in soil. Arrows indicate which organisms eat one another. As organisms consume one another, energy and nutrients are exchanged.
(diagram courtesy of http://soils.usda.gov)

If these worms gross you out, they probably won’t after you’ve gardened for a while. You come to find out that they are a gardener’s very good friends.

Earthworms make holes in the soil that help break it up and improve the ability of plant roots to find water and nutrients.

Big black shiny ground beetles such as this one also work underground to make a healthy soil environment for plants.

When you’re in your garden, you can look around and easily see life: a bird singing in a tree, a beautiful flower, a ripening tomato. Life abounds in the average garden, and it doesn’t require a magnifying glass to see. Or does it?

In the soil beneath your feet, there’s an invisible world filled with life-and-death drama. Scientists and garden professionals call this world the soil food web. This web features a diversity of creatures, from microscopic bacteria, fungi, and algae, to mites and nematodes, to earthworms, ants, spiders, and plant roots.

These organisms spend at least part of their lives in soil, enriching and enhancing it as they live their daily lives. Some soil food web members decompose organic matter, such as plant debris or manure. Others convert nitrogen, a basic plant nutrient, into forms plants can readily use. When you apply fertilizer, the soil food web makes that fertilizer available to plant roots. When excess nitrogen is present, the soil food web holds that nitrogen in soil in forms that aren’t readily available to plants, providing a microscopic fertilizer storage facility.

When the soil food web is healthy and active, it changes soil structure. These changes create soil that can hold water and allow air to penetrate to plant roots. Some members of the soil food web also break down pollutants that make their way into soil.

Soil food web components live throughout soil, but they’re typically concentrated where organic matter is—usually in the top few inches of soil. One place organic matter naturally occurs is around the tips of plant roots. Roots release cells, proteins, and sugars, which feed bacteria in the soil. Other creatures, nematodes and protozoa, feed on the bacteria. This part of the soil food web that takes place near plant roots actually helps feed plants and strengthens them to resist disease.

There’s a Fungus Among Us!

Several different types of organisms make up the soil food web. Bacteria don’t move far in soil and tend to digest green matter. Fungi, on the other hand, migrate in the soil. As fungi grow, they create threads that are thousandths of an inch thick. These threads extend through soil, creating channels for water and air. When threads bind together to form a large mass, they increase the soil’s ability to hold water. Some fungi are decomposers, breaking down brown, coarse organic material into forms that other soil organisms can use. This action enhances soil with humic acid-rich organic matter, which resists breakdown and helps soil retain water.

Mycorrhizal fungi grow around or in plant roots, drawing carbon and sugars from the plant and pumping various nutrients and water directly into plant roots. The result is that plants with mycorrhizal colonies grow faster, bigger, and stand up to stress better. Mycorrhizal fungi form specific partnerships with plants, so that there are particular fungi that colonize specific tree, vegetable, or perennial roots. You can buy mycorrhizal fungi in a dry form to mix into soil to increase plant growth and productivity. Typically you add the fungi to planting holes. It is an easy way to jump-start your garden’s soil food web and improve soil health. It’ll be 4 to 6 weeks before you’ll see results in plant growth.

Creatures in Your Soil

Protozoa are single-celled organisms that eat bacteria. As protozoa digest bacteria, the process releases nitrogen in a form plants can use. Nematodes are non-segmented worms that feast on plants, algae, bacteria, fungi, and other nematodes. Their role in the soil food web enriches soil and helps control disease organisms.

Another group of soil creatures, arthropods, includes many insects or close relatives such as tiny invisible mites, springtails, and bigger, easily visible centipedes, spiders, beetles, and millipedes. They shred organic matter in soil so good bacteria can break it down so it releases nutrients for roots to absorb. These bacteria, which don’t move far in soil, tend to attach to insects and other related creatures that are highly mobile and often transport bacteria to their food source.

As arthropods feed on organic matter, they make almost invisible little pellets of poop that enrich soil. Arthropods also compete with disease organisms for food. For instance, some disease organisms feed on plant roots. When arthropods are present, they prey upon those disease organisms, devouring them before they can infect roots.

Earthworms break down organic matter in soil. As organic matter passes through a worm’s gut, it’s enriched with nutrients and minerals. Worm poop, or castings, is sold as a rich fertilizer. When earthworms are active, they increase soil’s ability to hold water, create channels for roots to grow in, and increase microorganism concentrations in soil.

If one component of the soil food web is missing, the whole ecosystem is off balance. Be aware that using salt-based fertilizers, pesticides, herbicides, or miticides, even organic ones, can kill organisms in the soil food web. Compacting soil can also harm the soil food web.

A Few Tips for a Healthy Soil Food Web

To cultivate a healthy soil food web, follow these garden practices:

• Limit pesticide use. When possible, minimize use of pesticides applied to soil or that drip to the ground. Fungicides and insecticides that kill garden pests can have the same effect underground. Even organically approved pesticides kill, so use with caution and be aware that there is a trade-off between protecting your crops and possibly harming critters beneath your plants. Weigh these options when you consider using any pesticides. Generally, the soil will recover if pesticides are used only occasionally, especially the mildest organic products. Healthy soil enables plants to resist problems, so occasional use of pesticides is all that will be needed except under the most extreme pressures.
• Rotate crops. The more crops you rotate over time, the more diverse your soil web will be. For instance, if you rotate corn and tomatoes, you’ll have one level of biodiversity in the soil food web. If you rotate corn, tomatoes, broccoli, and beans, you’ll have more kinds of soil organisms (fungi, bacteria, etc.) present.
• Aim for a little mess. A garden that’s cleanly tilled with crisp edges and no leaf litter offers limited habitat for arthropods. Mulches, fence rows, and deep trench edging provide shelter for arthropods.
• Feed organically. Count on compost, compost tea, and organic fertilizers, such as fish emulsion, worm castings, cottonseed meal, or Bonnie's Herb and Vegetable Plant Food to feed plants and preserve and enhance the soil food web. Our plant food, which is derived from soybean seeds, contains organic compounds that encourage soil acitivity. Avoid any fertilizers high in salts because they can be harmful to creatures in soil. Some fertilizers and soil additives also contain mycorrhizae to add these beneficial organisms to your garden. This is a good thing to get them started, but you won’t need to keep adding. Once your soil is healthy, they should perpetuate themselves.
• Keep soil covered. Mulch is one of the soil food web’s best friends because it adds organic matter and helps keep moisture in the soil. Apply an organic mulch, such as compost, pine straw, or shredded leaves. In vegetable gardens, when crops are not actively growing, cover soil with mulch to encourage the soil food web to remain active. You can plant a "living mulch" of ryegrass, clover, or some other cover crop to keep soil organisms active while vegetables are absent. Some people plant cover crops in the aisles between rows and keep it mowed.