Soil Food Web: A Gardener's Primer

By Emma Johnson · October 10, 2026

Beyond NPK: The Living Soil

Conventional gardening focuses on chemical nutrients (nitrogen, phosphorus, potassium), but healthy soil contains a thriving ecosystem of organisms that make those nutrients available to plants. A single teaspoon of healthy soil contains billions of bacteria, miles of fungal hyphae, thousands of protozoa, and dozens of nematodes—all working in an intricate food web.

Trophic Levels in Soil

Level 1: Primary Decomposers

Bacteria and fungi break down organic matter (dead leaves, roots, compost) and incorporate nutrients into their bodies. Bacteria dominate in disturbed, high-nitrogen soils (vegetable gardens). Fungi dominate in undisturbed, carbon-rich soils (forests, perennial beds).

Level 2: Grazers

Protozoa, nematodes, and microarthropods eat bacteria and fungi, releasing excess nutrients as plant-available waste products. This "nutrient cycling" is how organic matter becomes accessible to plant roots—a spoonful of soil with active protozoa delivers more nitrogen than the same soil without them.

Level 3: Predators

Larger nematodes, predatory mites, and beetles eat the grazers, keeping populations in balance and preventing any single species from dominating. This predator-prey dynamic maintains biodiversity and ecosystem stability in the soil.

The Bacterial-Fungal Ratio

Different plants prefer different soil biology compositions. Annual vegetables and grasses thrive in bacteria-dominated soil (bacterial:fungal ratio of 10:1 or higher). Perennials, shrubs, and trees prefer fungal-dominated soil (ratio of 1:1 to 1:10). Compost and mulch type influence this ratio—high-nitrogen compost favors bacteria, while wood chip mulch favors fungi.

How Plants Feed the Web

Plant roots exude 20-40% of their photosynthetic sugars into the soil as "root exudates"—liquid carbon that feeds bacteria and fungi near the root surface (the rhizosphere). In exchange, these organisms mine minerals and water from surrounding soil and deliver them to the root. This barter system is 400 million years old and underlies all terrestrial plant life.

Practices That Build the Food Web

Add diverse organic matter (compost from varied feedstocks)
Maintain living roots year-round (cover crops between seasons)
Minimize soil disturbance (reduced or no-till)
Avoid broad-spectrum pesticides and synthetic fertilizers
Mulch with appropriate materials for your target biology

Signs of a Healthy Soil Food Web

Earthworms visible in every shovelful. Water infiltrates quickly rather than pooling. Soil has a rich, earthy smell (geosmin produced by actinomycete bacteria). Aggregates form naturally—soil crumbles into granular clumps rather than dust or solid clods. Plants show vigorous growth with minimal fertilizer input.

Testing Soil Biology

The Haney test measures soil respiration (CO2 production) as a proxy for biological activity. DIY methods include the bury-your-underwear test (cotton decomposes faster in biologically active soil) and simple earthworm counts (10+ per cubic foot indicates healthy biology).