An incredible diversity of organisms make up the soil food web. They range in size from the tiniest one-celled bacteria, algae, fungi, and protozoa, to the more complex nematodes and micro-arthropods, to the visible earthworms, insects, small vertebrates, and plants… As these organisms eat, grow, and move through the soil, they make it possible to have clean water, clean air, healthy plants, and moderated water flow.
USDA, “The Soil Food Web”
The soil food web refers to the relationship between sunlight, the vast populations of organisms in the soil, and the higher forms of life like birds and mammals that feed on or rely on soil organisms. You may be more familiar with the food chain, which is a linear relationship; the soil food web is a holistic relationship in which every organism is dependent on every other organism.
A large and robust community of organisms brings the soil to life, living their entire lives, or at least part of it, in the soil. Some of these organisms create problems in plants like root rot and blight. Others, called beneficial organisms, help to build the soil and prey on unwanted organisms, thereby protecting plants. Both types of organisms are needed to create a healthy cycle of growth and decay in the soil. It’s the ultimate food chain, as larger organisms eat smaller organisms and create dynamic interactions which builds a healthy soil. Above ground animals like mammals and birds also depend on this interconnected, living system.
Organisms in the soil food web do many jobs, including:
- Decompose organic compounds like manure, plant residue, and pesticides, preventing them from entering the water and becoming pollutants.
- Sequester nitrogen and other nutrients that might otherwise enter groundwater.
- Fix nitrogen from the atmosphere, making it available to plants.
- Improve the quality of the soil, which increases water filtration and reduces runoff.
- Prey on crop pests.
- Serve as food for animals.
How the soil food web works
Every plant, including grass, trees, shrubs, and food crops, depends on a healthy soil food web to survive. As plant roots grow and plant parts decay, soil organisms receive food. As soil organisms flourish, they decompose organic matter, pass nutrients between plants and soil, improve soil structure, and keep crop pests in check.
Organic matter in the soil is made up of equal parts humus and active organic matter. Active organic matter is the material available to organisms; humus is the material already broken down by soil organisms and available to feed plants.
The first step in the soil food web is sunlight. Solar energy triggers photosynthesis in plants, which pulls carbon dioxide (CO2) from the atmosphere to build plant material. This starts the buildup of humus and active organic matter in the soil.
The second step is the decomposition of plant material by simple soil organisms: Bacteria, parasites, root feeders, and fungi. Bacteria feed on fresh plant residues and substances the roots produce. Fungi feed on wood, soil humus, and fibrous plant tissue.
In the third step, larger soil organisms such as protozoa, nematodes, and arthropods feed on the smaller soil organisms.
In the fourth and fifth steps, ever larger organisms, including above ground organisms like birds and mammals, feed on or rely on smaller organisms.
How to improve the soil food web in your garden
In our gardens, the goal is to boost the number of beneficial organisms and reduce unwanted organisms. Improving the soil with compost and other amendments, provides what’s needed for beneficial organisms to thrive. With a robust community of these organisms, the soil food web thrives, and plants flourish. Practicing gardening techniques that build and improve the organic matter in your garden is critical to maintaining a healthy soil food web.
Don’t rototill or dig your garden beds
Tilling or digging garden soil causes bacteria to rapidly multiply, consuming active organic matter. They convert it to carbon dioxide (CO2), which is released into the atmosphere. Sell your rototiller.
Use cover crops
Cover crops are useful to protect the soil food web from extreme weather, stabilize and nourish the soil, and continue photosynthesis when your garden would otherwise be fallow.
Regularly adding organic matter like compost, and using no-till methods, increases the proportion of organic matter in your garden soil. As this organic matter increases, more humus is created by soil organisms, sequestering carbon in the soil and making nutrients available to plants.
Add organic mulches
Organic mulches like shredded autumn leaves, leaf mold, pine needles, pine straw, pesticide-free grass clippings, wood chips, seaweed, or straw, smother weeds, moderate soil temperature, retain moisture, reduce soil erosion, and add organic material as they decompose. All of this benefits the soil food web.
Avoid compacting your garden soil
Compacted soil reduces soil food web activity and restricts water drainage and air storage. Avoid stepping on a flat earth bed, or better yet, build raised garden beds.
Eliminate or reduce pesticides
Non-specific pesticides, called broad-spectrum pesticides, harm beneficial organisms. A garden with a thriving soil food web needs no or few pesticides, including organic-approved treatments.
Limit or eliminate synthetic fertilizers
Chemical fertilizers leave behind nitrate salts in the soil, which harm soil microbes. The more you apply, the more microbes die, and the more chemicals are needed to maintain fertility. It’s a destructive cycle. Use organic amendments instead, like compost, fish emulsion, composted manures, or guano.
Add Mycorrhizal fungi
Mycorrhizal fungi are beneficial fungi that form a symbiotic relationship with plants. They surround roots or penetrate and extend roots to help plants easily take up nutrients.
A healthy and thriving soil food web makes for a healthy and thriving garden!
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