Health means much more than an absence of disease, although that's a part of being healthy. A healthy organism is active, doing the job it is designed to do and working in concert with the right partners.
Soil health, then, means that organisms in the soil are present and doing the work they are supposed to do to support the growth of plants. Soil teems with microscopic life, each type of creature performing its own function in the soil food web. Plants differ in the way they acquire nutrients—woodland plants tend to rely on soil fungi to cycle the nutrients they need, while grassland ecosystems are driven by soil bacteria—so there is no one set of organisms that creates optimal growing conditions for all plants. In general, a vibrant mix of microbiology is essential for healthy soil and healthy plants.
Bacteria are single-celled microorganisms that are vital for cycling nutrients in the soil. There are thousands of species of bacteria. In general, beneficial bacteria are usually aerobic, requiring oxygen to survive, while disease-causing ones thrive in low-oxygen environments. Many species of bacteria serve as decomposers, eating up dead plant matter and concentrating the nutrients in their bodies—one of the first steps in returning those plant building blocks to the soil. Bacteria bind all kinds of compounds into organic forms that will not leach out of the soil. Another group of bacteria collaborates with legumes to capture or "fix" atmospheric nitrogen and add it to the soil.
Fungi are strandlike microorganisms that help hold soil particles together and improve soil structure. They consume the harder-to-digest organic materials, such as dead leaves, pine needles, and fallen tree trunks. One type of fungi establishes mutually beneficial mycorrhizal relationships with plants, which allows the fungi to hold and transfer nutrients directly to plant roots. While many species of fungi are beneficial, others are responsible for plant diseases.
Protozoa include microorganisms like flagellates, amoebae, and ciliates, all of which eat bacteria. As protozoa consume bacteria, they release excess nutrients in soluble forms that can be utilized by plants. Protozoa also work with bacteria and fungi to build air passageways, letting oxygen, water, and roots move easily through the soil.