Soil Health · October 2025 · 5 min read
Healthy soil is a living ecosystem containing billions of organisms per teaspoon. Bacteria, fungi, protozoa, and nematodes work together to cycle nutrients, build soil structure, and suppress disease. Understanding the soil food web helps you make management decisions that support rather than undermine this biological foundation.
Bacteria are the most abundant soil organisms and are primary decomposers of simple organic compounds. They thrive in neutral to slightly alkaline, tilled soils with readily available carbon. Fungi dominate in undisturbed, acidic soils and are the primary decomposers of complex materials like lignin and cellulose.
The fungal-to-bacterial ratio shifts with management. Annual cropland tends to be bacteria-dominated, while perennial grasslands and forests are fungi-dominated. No-till systems gradually increase fungal biomass over time.
Protozoa are single-celled organisms that graze on bacteria. This grazing is one of the most important nutrient cycling mechanisms in soil. Bacteria contain far more nitrogen than protozoa need, so when a protozoan consumes bacteria, it releases the excess nitrogen in plant-available ammonium form directly in the root zone.
This bacterial grazing cycle can supply a significant portion of a crop's nitrogen needs in biologically active soils, reducing dependence on synthetic fertilizer.
The soil food web has multiple trophic levels: primary decomposers (bacteria, fungi), grazers (protozoa, nematodes), and predators (predatory nematodes, microarthropods). Each level regulates the one below it, creating a self-balancing system that cycles nutrients in sync with plant demand.
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