What’s In That Soil?

One Tablespoon of Healthy Soil Contains 50 Billion Busy Microbes

Most of the planet consists of solid rock, upon which most plants can’t grow. Approximately 75 percent of soils are transported soils (such as from windblown sediment, or loess), with only 25 percent forming in place. Soils are incredibly diverse, and different plant communities adapt to different soils.

Soil starts with a mineral source—weathered rock, glacial silt, river sediments, lava flows, sand, etc.—but it isn’t soil until organic matter is added. Organic sources can be living or non-living. Old leaves, dead animals, and tiny living things all enrich the soil with its necessary carbon.

Healthy soil is about 50 percent solids and 50 percent air and water, simply teeming with life—mites, nematodes, protozoa, and a whole menagerie of other organisms, most of them smaller than the head of a pin.

Soil microorganisms are so abundant that 70 to 80 percent have yet to be identified. It’s estimated one tablespoon of soil contains about 50 billion microbes.5 More than 90 percent of land plants are nourished by mycorrhizae, a symbiotic form of fungi that help move nutrients from the soil into the plants’ roots.

But there are also thousands of other microbes playing their parts in this microbial symphony in the soil. According to North Carolina University Cooperative Extension:6

“As soil life forms move through the soil, they create channels that improve aeration and drainage. Nematodes and protozoa swim in the film of water around soil particles and feed on bacteria. Mites eat fungi; fungi decompose soil organic matter.

The microorganisms’ primary role is to break down organic matter to obtain energy. They help release essential nutrients and carbon dioxide, perform key roles in nitrogen fixation, the nitrogen and phosphorus cycles, denitrification, immobilization and mineralization.

Microbes must have a constant supply of organic matter or their numbers will decline. Conditions that favor soil life also promote plant growth.”