Do you ever wonder what's really growing in your soil beside the things you planted there? You might find it shocking to learn that there are literally billions of microbes hard at work in every square inch of your garden and landscape.

Microbes are microscopic organisms that include bacteria, fungi, protozoa and plankton. They transform minerals and organic matter into soil that supports an abundance of life. They are so tiny that millions can fit into the eye of a needle.

They are in the air we breathe, the ground we walk on and even inside ourselves.

There are 10 billion bacteria in a single gram of soil. Bacteria are single-celled organisms that help break down and recycle decaying organic matter in our soil and convert nitrogen from the atmosphere into organic compounds that can be used by our growing plants. Actinobacteria are critical in decomposing organic matter and turning it into humus. They are responsible for the sweet "earthy" smell we associate with healthy soil.

Fungi are strandlike microorganisms that improve soil structure and help hold our soil together. Many fungi are parasitic and can cause disease to plants and animals. However, there are many beneficial fungi -- some create mutually beneficial relationships with plants wherein they store and transfer nutrients directly to the roots.


Fungi spread underground by sending long threads of mycelium throughout the soil. The mycelia fungi are able to create visible fruiting bodies -- mushrooms, toadstools, puffballs -- that contain millions of spores. When the fruiting body bursts, spores can be dispersed over wide areas through the air. They may lay dormant for many years before the right food and conditions are available to allow them to take hold.

Protozoa are unicellular mobile organisms that are classified into three groups: ciliates, amoebae and flagellates. They primarily feed on bacteria and each other. As they eat they release excess nitrogen that is used by plants and other members of the food web. Bacteria, fungi and protozoa work together to build airways that allow oxygen, water and roots to move freely throughout our soil.

There are many things besides microbes moving around underground. Nematodes, or roundworms, are one of the most diverse species of all animals. Some studies estimate that there are more than a million species of nematodes. They have adapted to nearly every ecosystem from marine to fresh water and to all soil types from the polar regions to the tropics. They account for 90 percent of all life-forms on the ocean floor and nearly 80 percent of all individual animals here on Earth. And although most people think of them with a negative connotation, when it comes to soil health they can be beneficial. Not only do they help build soil structure, there are predatory nematodes that actually kill gardens pests such as cutworms, weevils and grubs.

Microarthropods are small invertebrate animals that includes mites, spiders, springtails, minute centipedes and millipedes. They break down organic matter into a form that bacteria can eat and are fundamental to the creation of humus and the formation of soil.

Many species of microarthropods thrive only in the layer between leaf litter and the soil, where they help break down the mulches that we apply.

To help nurture and support these essential work horses in our soil it is important to avoid using toxic chemicals that not only kill the intended pests but wipe out all of our allies as well. Even horticultural soaps destroy many desirable organisms. Avoid compacting or disturbing your soil unless absolutely necessary.

The best way to nurture your team of soil builders is to apply annual or biannual layers of locally grown organic compost topped with 3 to 4 inches of mulch. You can grow your own by creating a composting pile, setting up vermicomposting (worm) bins or purchasing compost from local garden centers.

Rebecca Jepsen is a Santa Clara County Master Gardener.

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