Saturday, December 5 was World Soil Day, celebrated since 2014. It was formally adopted by the 68th UN General Assembly in December 2013. The theme for World Soil Day 2020 was “Keep soil alive, protect soil diversity.”

What does it mean to keep soil alive and protect soil diversity? Why are these goals important? If you are like me, you probably pay more attention to what you can see than to microscopic organisms that are not visible to the naked eye. As I read articles on soil diversity, this one statement kept popping up: “There are more microorganisms in one teaspoon of soil than there are people on earth.” That is a mindboggling claim. How is this possible, and why are these microbes (and larger soil organisms) important to soil health and the entire ecosystem?

All of the organisms that live in the soil make up the soil food web. Some organisms, including green plants, moss, lichens, algae, and some bacteria use the sun’s energy to fix carbon dioxide from the atmosphere through the process of photosynthesis. Microbes and insects in the soil consume organic compounds found in plants and other organisms to obtain energy. All soil organisms interact with one another. Plant root residues feed soil organisms, which in turn improve soil structure by decomposing organic matter and storing and cycling nutrients.

Pretty heady stuff, but I found the soil food web easier to understand by examining each group of organisms and their relationships to each other and to the soil.

Bacteria are microscopic, single-celled organisms. Between 100 million and 1 billion bacteria live in that teaspoon of soil. Four groups of soil bacteria exist. Most bacteria decompose and consume carbon compounds from plant material. Nitrogen-fixing bacteria form symbiotic relationships with legumes like peanuts and clover. The plants provide carbon to the bacteria and the bacteria convert atmospheric nitrogen to a form the plants can utilize. Pathogens, or disease-causing bacteria, make up the third group. Members of the fourth group are called lithotrophs. They obtain energy from iron, sulfur, nitrogen, or hydrogen, rather than carbon.

Fungi are microscopic cells that grow into hyphae, long strands that penetrate the soil. Hyphae sometimes form masses called mycelia. Mushrooms are the fruiting bodies of fungi. They are composed of hyphae, spores, and the gills where the spores form.

Saprophytic fungi are decomposers. They convert dead organic matter into soil nutrients. Mycorrhizal fungi colonize plant roots. They provide essential nutrients to plants and receive carbon in return. Some fungi are pathogens that cause plant diseases like Verticillium wilt; others feed on insect pests or parasitize nematodes that cause plant root disease.

Protozoa are one-celled animals that consume bacteria, other protozoa, organic matter, and fungi. As they eat, protozoa release nitrogen that can be used by plants. Protozoa are classified by how they move. Ciliate protozoa propel themselves by hair-like cilia. Amoeba move by a temporary foot called a pseudopod. Flagellates are the smallest protozoa. They move by whip-like flagellae.

Nematodes are tiny, non-segmented worms. Some species of nematodes attach to plant roots and cause diseases, but most nematodes are beneficial. The four groups of free-living (non-root) nematodes include bacteria feeders; fungal feeders; predatory nematodes that feed on small organisms and larger nematodes; and omnivores that consume many species of organisms.

Arthropods are invertebrate organisms with jointed legs and an exoskeleton. Insects such as beetles; crustaceans such as sowbugs; arachnids such as mites and spiders; scorpions; and myriapods, which include millipedes and centipedes, are all classified as arthropods. Arthropods are grouped as herbivores, fungal feeders, predators, and shredders, which consume plant material along with the bacteria and fungi on the plant surface. Most arthropods are beneficial to the soil.

Earthworms decompose dead or dying plant material while feeding on the bacteria and fungi on plant surfaces. Their waste products, called casts, improve soil quality. Earthworm movements increase soil porosity and improve drainage.
Go to the Natural Resources Conservation Service (NCRS) Soils “Soil Food Web” for detailed information on soil biodiversity.