Strengthen the Soil Food Web

The recent World Soil Day, observed each year on December 5, provided an opportunity to appreciate healthy soil and the need to sustain it as a living resource. For gardeners, it was a moment to recognize the role of the Soil Food Web—a natural approach to achieving healthy soil that reduces or eliminates the need for synthetic chemicals.

Healthy soil is made up of roughly 50 percent air and water, 45 percent minerals and 5 percent organic matter. That small organic component is the most important. In addition to determining soil structure and the availability of water for plants, it provides nutritional support for the organisms that comprise the living part of the Soil Food Web.

The primary role of these organisms is decomposition. They reduce soil organics into soluble inorganic compounds that can be absorbed by plant roots, and also into long-lived organic material called humus, the foundation of healthy soil. At the base of the web are bacteria and fungi that consume and decompose organic matter directly, converting nitrogen into a plant-usable form and storing it in their bodies. Protozoa and nematodes prey on bacteria and fungi, releasing nitrogen to plants.

Larger organisms also play a role in decomposition. Arthropods—including millipedes, springtails, mites, fly larvae and beetles—break down organic matter while they themselves provide food for other beneficial arthropods. The community also includes insects, earthworms, and mammals such as moles, mice, and rabbits that spend part of their time in the soil, plus the creatures that prey on them.

Soil Food Web, Elaine R. Ingham, USDA.

This web of organisms, combined with air, water and minerals in the greater soil ecosystem, is the essence of nature’s life support system for plants and, for that matter, all living things.

Assess Your Soil Food Web

How strong is the food web in your soil? Find out by digging a one-foot square hole, placing the soil on a tarp and sifting through it. A rich dark color and crumbly texture indicate good organic matter content. If you find earth worms or their castings, it’s a good sign and indicative of the presence of microbes. Earthworms eat bacteria, fungi, protozoans, nematodes and the organic matter these organisms live on. Similarly, the presence of centipedes, millipedes, beetles, spiders and springtails suggests a healthy food web.

The next step is to get a soil test. Request data on pH, nutrients, percentage of organic matter, soluble salt content and a chemical factor known as cation exchange capacity. While certain plants prefer a specific pH, most soils should be in the 6 to 7 pH range. Aim to build organic matter to about 5 percent of soil composition. Soluble salt content indicates whether over-fertilization may be harming soil organisms. Cation exchange capacity indicates whether positive electrical charges carried by clay particles and humus are adequate to adsorb negatively charged nutrient particles to prevent them from leaching away as water passes through the soil.

Improve Your Soil Food Web

  • Add Organic Matter. Compost, whether purchased or homemade, is the best organic material you can add to your soil. It is rich in microbes and is composed primarily of humus, or fully decomposed organic matter. Adding humus helps aerate the soil while holding moisture and nutrients until they can be released to plants. Likewise, natural mulches, such as leaves, leaf mold, pine needles, grass clippings, wood chips, straw or even paper, will strengthen the Soil Food Web by providing organic material and nutrients as they decompose. Chop up the material and let it age before applying. On your lawn, mulch grass clippings and leaves with your lawn mower and leave them there. They will feed microbes in the soil and reduce fertilization requirements.
  • Minimize Inversion Tillage. Turning garden soil over every year disrupts microbial populations, speeds up organic decomposition more than necessary and destroys soil structure. Instead, add a couple of inches of compost to the soil surface and work it into the top 4-6 inches manually or with a chisel plow. Let the food web organisms finish the job.
  • Plant Cover Crops in the Vegetable Garden. During winter and fallow periods, planting a cover crop—annual rye, clover, hairy vetch or field peas, for example—will create pore space in garden soil while reducing the leaching of vital nutrients. Plant the crop a month before frost and cut it in the spring before it goes to seed. The cut vegetation can be used as a mulch or worked into the top layer of soil three to four weeks before planting.
  • Avoid Compaction. Among other damaging impacts, wheel and foot traffic reduces porosity in the soil and reduces the abundance and diversity of food web organisms.
  • Minimize Pesticide Use. Broad-spectrum pesticides can harm beneficial as well as harmful insect populations. Integrated Pest Management is an approach that calls for clear problem identification, using least toxic controls first and using chemicals as a last resort. This will minimize risks to nontarget species as well as human health. Moreover, a strong Soil Food Web and healthy soil will make plants less vulnerable to pests, reducing the need for chemical application.
  • Manage Nutrients. Synthetic fertilizers deposit nitrate salts in soil that can harm soil microbes and create a destructive cycle; the more chemical is applied, the more microbes die and the more chemical is needed to feed plants. Use organic amendments whenever possible.
  • Add Mycorrhizal Fungi to Plantings. Mycorrhizae form symbiotic relationships with plants by either surrounding or penetrating root systems. They draw the carbohydrates they need from the host plants, and they return the favor by giving the plants moisture and nutrients that the root system wouldn’t otherwise reach. There are two types: ectomycorrhizae surround roots of hardwoods and conifers and extend out from them; endomycorrhizae penetrate roots and extend outward, and are preferred by vegetables, annuals, grasses, shrubs, perennials and softwood trees. Buy the spores at garden centers as powders or solutions and follow application directions.

Adopt an Age-Old Process That Works Now

There is a growing community of successful gardeners and farmers who have demonstrated that these natural practices work. Indeed, they have worked for millennia, and they are the solution we need now to sustain the health and biodiversity of our soil.

Graphic from: Soil Food Web | NRCS Soils (usda.gov)