Soil Food Web

The soil food web is what makes it possible for plants to grow.

Soil is not simply ground up minerals. We now know that there are gazillions of living things breathing, growing, moving, and reproducing beneath our feet.

Soil food web (USDA) Public Domain

The living things found in soil are mostly smaller than you can see, with only a few exceptions. As these tiny life forms move through the soil, they reduce erosion, impact water and nutrient availability, and aid in decomposition of manure, plants, and pesticides.

Algae 

Algae are single-celled organisms that can form substantial chains. Algae are classified by color. [Did you know that kelp is a type of algae? I didn’t either.] Algae convert sunlight into energy through photosynthesis. They can also fix atmospheric nitrogen. There may be 10,000 to 100,000 algae in a teaspoon of soil. Algae aid in nutrient cycling and help prevent erosion. 

Damp soil encourages algae growth (Anuragi Neri) Public Domain

Arthropods 

Arthropods are bugs. Bugs do not have backbones. Instead, they have exoskeletons. Arthropods include insects, crustaceans, and arachnids. Common insects include ants, beetles, and springtails, while sowbugs are crustaceans. The arachnids include spiders, mites, and millipedes. Arthropods eat a variety of foods. Some types feed on fungi, while others prey upon worms and other arthropods, and yet others are herbivores. As they feed, arthropods aerate the soil, aid in decomposition, and keep other populations in check. At the same time, arthropods can damage root systems. ​​

Sowbug

Bacteria 

Bacteria are one-celled organisms. They are so tiny that they can enter a plant through a broken hair, or trichome. It is estimated that there is one ton of bacteria in every acre of soil. That’s the weight of two adult cows, or half of your car. A teaspoon of productive soil may contain anywhere between 100 million and 1 billion bacteria.

Most bacteria are decomposers that prefer more tender fare. As they breakdown carbon-based life forms, they make those nutrients available to plants and improve soil structure. Other bacteria are mutualists, which means they work together with plants to everyone’s benefit. This group includes the bacteria which convert atmospheric nitrogen into a form available to plants. Another group of bacteria, called lithotrophs, break down hydrogen, iron, nitrogen, and sulfur compounds, rather than carbon, making those nutrients available to plants. The fourth group of bacteria are pathogens. This group includes Erwinia (fireblight) and Xymomonas diseases, and gall-forming Agrobacterium.

Recent research has shown that a certain soil bacteria, Mycobacterium vaccae, improves mood and reduces stress. See, gardening really is good for you!

Digitally-colorized scanning electron micrograph (SEM) of bacterial biofilm (light blue) with amoeba (red) and a microorganism (yellow). Public Domain

Earthworms 

Earthworms are popular decomposers, improving soil structure as they feed on organic matter and move through the soil. Earthworm poop, or casts, is a fermented batch of plant material, bacteria, and fungi that forms nutrient-rich soil aggregates. Earthworms improve soil structure, porosity, infiltration and drainage, as well as make life a lot easier for plant roots. Earthworms are also credited with reducing erosion and runoff.

Earthworm

Fungi 

Fungi are single and multi-celled organisms that grow as long threads, called hyphae. Those hyphae may cluster into groups called mycelium. Yeast is a fungi. So are mushrooms and dog vomit mold. Fungi help bind soil particles together, improving soil water holding capacity and infiltration rates. Most fungi, called saprophytic fungi, are important decomposers that can break down harder materials than bacteria can, such as tree trimmings, and hold those nutrients in the soil, rather than allowing them to be lost to the atmosphere through erosion and runoff. 

Other fungi (mycorrhizae) are mutualists that live on and in plant roots, bringing soluble nutrients to your plants. Plant roots can only come into direct contact with approximately 2% of the surrounding soil. With the aid of fungal threads (hyphae), those roots then have access to all the water and nutrients found in 20% of the soil. There are pathogenic fungi, such as Pythium, which cause blackleg, seed rot, and cavity spot, Rhitozoctonia, and Verticillium, which cause Verticillium wilt, among others.  Parasitic fungi feed on insects and nematodes.

Slime mold on tree trimmings

Nematodes 

Nematodes are worms without segments. Some are large enough to see, at 1/20”, but most are smaller than that. We know surprisingly little about beneficial nematodes. Most research has focused on root feeding omnivores that parasitize our plants. In the world of nematodes, life is hard. There are nematodes that feed on bacteria, fungi, and other nematodes and protozoa, while those same creatures prey upon and parasitize the nematodes. This whole process can either spread or control disease, depending on who wins that particular fight. Nematodes benefit plants by releasing excess nitrogen into the soil, like protozoa. As bacterial and fungal feeders, nematodes keep those populations in check while, at the same time, expanding their ranges by carrying microbes with them. There are usually 100 nematodes per teaspoon of soil.

Colorized electron micrograph of soybean cyst nematode (Heterodera sp.) and egg (USDA) Public Domain

Protozoa 

Protozoa are one-celled organisms that feed on bacteria, fungi, other protozoa, and plant material. These opportunist feeders are significantly larger than bacteria, but you still can’t see them. As they feed, protozoa are unable to digest all the nitrogen in their prey, so they pee it out, releasing nitrogen into the nearby soil in the form of ammonia (NH4+), which plants love to consume.​

Protozoa need moisture to survive and are classified according to their shape. There are hair-like ciliates, foot-shaped amoebas, and whip-like flagellates. Protozoa keep bacterial populations in check. One group of protozoa, the vampyrellids, feed on fungi, vampire-style. There can be anywhere from 1,000 to 1,000,000 protozoa in a teaspoon of soil. Some types of protozoa, such as Paramecium Bursaria actually farm algae on and in their bodies.

Protozoa (Clockwise from top left) Blepharisma japonicum, a ciliate; Giardia muris, a parasitic flagellate; Centropyxis aculeata, a testate (shelled) amoeba; Peridinium willei, a dinoflagellate; Chaos carolinense, a naked amoebozoan; Desmerella moniliformis, a choanoflagellate(Frank Fox, et al) CC BY-SA 4.0

Small vertebrates

Small animals, such as  gophers,  moles, rabbits, snakes, and voles are the giants of this microscopic world. As amphibians, birds, mammals, and reptiles scratch at and burrow through the soil, they help reduce compaction. They can also destroy plant roots. While both predators and prey aid in nutrient cycling, some are more beneficial to your garden than others. In my opinion, snakes, lizards, and toads are preferable. That’s just me.

Alligator lizard

Bottom line, like any other food web, everything gets eaten by something else. Your soil is no different. As things are eaten, the elements that make them up get broken down into smaller bits. These smaller bits get eaten by something else until, ultimately, plant roots absorb those nutrients to help them grow.

The Earth’s crust is a living, breathing entity whose health dictates our own. Do right by your soil, and your garden will thrive. The greater biodiversity in your soil, the healthier we will be.

If you have access to a microscope, I urge you to collect some samples of your garden soil and take a closer look at what is supporting life in your yard. What life forms can you see in your soil?