Healthy Soil is Full of Life!

Illustrations: Kate Solbakk
Text: Josh Weybright & Navona Gallegos
Editing: Isabelle Jenniches & Navona Gallegos

Get to know some of the wondrous creatures that make up our soil ecosystems. These organisms are all interconnected in the soil food web where they maintain pore spaces for water and air, recycle organic matter and make nutrients available to plants. Healthy soil ecosystems are naturally self- regulating, resistant to disease and pests, and do not require synthetic inputs or fertilizer.

Bacteria

Bacteria are single cell organisms that come in many shapes and forms. While they are too small to be seen without a microscope, we can group them by their morphology (defining characteristics, e.g. shape). 

Determining which bacteria are present can tell us about soil conditions:

• Healthy soils are aerobic (oxygen rich) and support beneficial bacteria. These microscopic organisms extract minerals from organic matter and soil parent material (sand, silt, and clay). Some soil bacteria can even break down toxic chemicals. They also make alkaline glues that bind soil materials together into micro-aggregates, the soil structures that allow water, oxygen, and roots to travel deep into the soil.  
• If a soil or compost has become anaerobic (oxygen limited), often bacteria that cause plant or animal disease are found. An abundance of spirilla (corkscrew), spirochetes (snake like) and the vibrio (comma shaped) bacteria indicate anaerobic conditions in soil or compost.

Fungi

Fungi are best known for their fruiting bodies (mushrooms) that grow above the soil, but most of their biomass is underground in the form of hyphae (branching filaments) that make up the network of mycelium that is the body of the fungus. When analyzing soil and compost samples, we look at morphological characteristics such as diameter, color and structure to determine the functional groups of fungi.

• Saprotrophic (decomposition) fungi excel at breaking down high-carbon materials such as wood. These fungi release their stored nutrients when they are eaten by nematodes, arthropods and earthworms.
• Mycorrhizal (root zone) fungi are either endomycorrhizal (growing inside the roots) or ectomycorrhizal (living outside the roots). These fungi deliver water and nutrients to their plant hosts in exchange for carbohydrates and other foods they cannot make themselves. Most food crops depend on mycorrhizal fungi to deliver water and nutrients from beyond where the roots can reach.
• Pathogenic (disease causing) fungi prefer anaerobic conditions and include wilt, blight and mildew species. In healthy soils, beneficial fungi outcompete pathogenic fungi that are harmful to plants.

Protozoa

Protozoa are single celled microbes that eat bacteria and release nutrients into the soil for plants to take up.

Flagellates have one or more flagella (whip-like appendages) that are used for locomotion and to help funnel food into their mouths. You can recognize these protozoa by their bumbling motion. Flagellates are typically aerobic organisms, their presence in soil or compost is a good sign of a healthy soil food web.

Ciliates use small hair-like cilia to zoom around very fast in search of food. Some ciliates are anchored by a long tether, others are free roaming. In compost piles, high dominance of ciliates can signal emerging anaerobic conditions.

Amoeba in the soil generally come in one of two forms, testate amoeba that have a hard shell, and naked amoeba which ooze about unrestrained. Like other protozoa, they prey on bacteria and make nutrients available to plants. Amoeba are aerobic organisms and indicate healthy soil conditions.

Nematodes

Nematodes can be identified by their mouth parts and digestive structures, which are shaped differently depending on their diet. While root-feeding nematodes can be very detrimental to crops, all other nematodes are highly beneficial. Predatory nematodes can help protect plants from disease-causing organisms. Most beneficial nematode groups indicate healthy, aerobic soil conditions.

Micro-arthropods

Micro-arthropods are multicellular life forms that either shred organic matter or are predatory species that consume smaller organisms.

Springtails (Collembola) are tiny invertebrates that feed on organic materials and fungi. Their activity breaks up plant litter and spreads fungal spores. Springtails are good indicators of moist, aerated soils.

Rotifers are sometimes called “wheel animals” because of the crown of tiny hairs that they use to whirl bacteria and microalgae into their mouths. Their populations rise quickly in moist soils and drop during dry periods.

Tardigrades or “water bears” are microscopic animals that live in thin water films on soil and leaf litter. They mostly graze on algae and microbes and help recycle nutrients. Tardigrades can survive extremely dry or freezing conditions by entering a dormant state and rehydrating when conditions improve –famously even after space travel!

Soil mites are not pests, most of them are helpful. They shred leaf litter and build soil aggregates, recycle nutrients and hunt small insects, springtails, and other mites. Predatory red velvet mites often appear after rains.

Earthworms are extraordinary soil builders. As they travel through the soil consuming organic matter, they produce castings (droppings) that concentrate nutrients and redistribute carbon. Their burrows increase water infiltration, soil aeration and aggregate stability, and reduce surface crusting. 

Most disease-causing microbes are destroyed in their digestive tracts. Earthworms only survive in aerobic conditions, so their presence indicates healthy soil.