Part of Soil Science
A teaspoon of healthy agricultural topsoil contains more organisms than there are people on Earth — billions of bacteria, millions of fungi, hundreds of thousands of protozoa, thousands of nematodes, and dozens of arthropods. This invisible community is not decoration. It is the engine that converts dead plant material into plant nutrients, builds soil structure, suppresses plant diseases, and makes farming possible without external inputs. Understanding the soil food web is essential for anyone rebuilding agricultural productivity from scratch.
The Soil Food Web
The soil food web is the network of feeding relationships among soil organisms. It operates in levels:
Primary producers: Plants provide the energy that drives the entire food web, primarily through their roots. Up to 40% of a plant’s photosynthate is released through root exudates — sugars, amino acids, and organic acids that feed the rhizosphere (root zone) microbiome. Plants actively cultivate their own soil microbial community.
Primary decomposers: Bacteria and fungi break down plant residues and other organic matter, converting them from complex organic molecules into simpler compounds and releasing nutrients.
Secondary consumers: Protozoa, nematodes, and microarthropods eat bacteria and fungi. When they consume more nitrogen than their bodies need, they excrete the excess as ammonium — the primary mechanism by which microbial nitrogen is transferred to plant-available forms.
Higher consumers: Predatory nematodes, mites, springtails, and centipedes eat the secondary consumers. Each level of predation cycles more nutrients into plant-available forms.
Engineers: Earthworms, termites, and ants physically restructure the soil — creating channels, mixing horizons, and building aggregate structures. Their effects on soil physical properties are as important as their effects on chemistry.
Bacteria: The Nutrient Cyclers
Soil bacteria are the most abundant organisms in any soil — typically 100 million to 1 billion cells per gram of topsoil. They are microscopic single-celled organisms visible only under high-power microscopes. Despite their tiny size, their collective metabolic activity transforms soil chemistry.
Key Bacterial Functions
Decomposition: Bacteria are the primary decomposers of simple carbohydrates, proteins, and most non-lignin organic compounds. They’re fastest in warm, moist, near-neutral pH conditions.
Nitrogen fixation: Certain bacteria convert atmospheric nitrogen (N2, which plants cannot use) into ammonia (NH3, which plants can). This is one of the most agriculturally important processes in all of biology.
- Rhizobium and related genera (Bradyrhizobium, Mesorhizobium, Sinorhizobium) form nodules on legume roots and fix 50–300 kg N/hectare/year for the host plant.
- Free-living fixers (Azotobacter, Azospirillum, Clostridium, cyanobacteria) fix nitrogen without plant hosts, contributing 5–20 kg N/ha/year to non-legume crops.
Nitrification: Nitrosomonas bacteria convert ammonium (NH4+) to nitrite (NO2-); Nitrobacter converts nitrite to nitrate (NO3-). Nitrate is the form of nitrogen most plants prefer. This two-step process is aerobic — it requires oxygen. Waterlogged soils suppress nitrification.
Denitrification: Anaerobic bacteria (Pseudomonas, Paracoccus) convert nitrate back to N2 gas in waterlogged conditions. This returns nitrogen to the atmosphere and represents a permanent loss from the soil-plant system. Waterlogging can denitrify significant nitrogen within 24–48 hours.
Phosphorus solubilization: Several bacteria (Bacillus, Pseudomonas) produce organic acids that dissolve mineral phosphate, making it available to plants. This is why adding organic matter and maintaining soil biology can improve phosphorus availability even without added phosphate fertilizer.
Pathogen suppression: A healthy, diverse soil bacterial community suppresses many plant pathogens through competition for nutrients, production of antibiotics, and induction of systemic resistance in plants. Soil treated with fungicides or sterilized with heat is much more susceptible to plant disease.
Promoting Soil Bacteria
- Maintain soil pH 6.0–7.0 (most bacteria prefer near-neutral pH)
- Add organic matter regularly — bacteria need carbon to feed on
- Maintain moisture (not waterlogged, not dry)
- Minimize fungicide and antibiotic use (broad-spectrum chemicals kill beneficial bacteria)
- Avoid over-fertilizing with ammonium nitrogen — very high ammonium suppresses nitrifiers
Fungi: Structure Builders and Long-Range Miners
Soil fungi are multicellular organisms that grow as networks of thread-like hyphae extending through the soil pore space. A gram of topsoil contains 100–1000 meters of fungal hyphae. Unlike bacteria, which need liquid films to move, fungi extend physically through air pores, making them effective in dry conditions and excellent at colonizing large volumes of soil.
Free-Living Decomposer Fungi
Free-living soil fungi decompose the tough materials bacteria cannot handle efficiently: lignin (the structural compound in wood), cellulose (plant cell walls), and chitin (insect exoskeletons). White rot fungi break down lignin completely; brown rot fungi remove cellulose and leave lignin (creating the brown, crumbly texture of rotting wood).
This ability to break down recalcitrant compounds makes fungi essential for decomposing straw, wood chips, and other high-C:N materials.
Mycorrhizal Fungi: The Plant-Fungal Partnership
Mycorrhizal fungi form intimate partnerships with plant roots. The plant provides the fungus with photosynthate (sugars); the fungus provides the plant with minerals, particularly phosphorus.
Arbuscular mycorrhizal fungi (AMF) penetrate root cells and form structures (arbuscules) where nutrient exchange occurs. AMF hyphae extend 5–15 cm from roots, mining soil volumes that roots alone could never reach. They are extraordinarily effective at collecting phosphorus, which moves slowly through soil by diffusion. Plants with healthy mycorrhizal associations require 30–50% less phosphorus fertilization.
AMF colonize the vast majority of crop plants, including grain crops, legumes, vegetables, and fruit trees. Notable exceptions: brassicas (cabbage, kale, broccoli), spinach, beets, and most members of the sedge family are non-mycorrhizal.
Ectomycorrhizal fungi form sheaths around root tips without penetrating cells. They’re associated primarily with trees — oaks, pines, beeches, eucalyptus. These associations are critical for forest productivity and tree survival in nutrient-poor soils.
Protecting mycorrhizal fungi:
- Avoid deep tillage (breaks fungal networks)
- Don’t apply phosphorus fertilizer at very high rates (excess phosphorus suppresses fungal investment)
- Avoid fungicides when possible
- Maintain living plant cover — AMF cannot survive without a host plant
Fungal Glues: Glomalin
AMF produce glomalin, a sticky glycoprotein that coats fungal hyphae and persists in soil after the hyphae die. Glomalin acts as a biological glue that binds soil aggregates together. It accounts for 14–50% of the carbon in soil aggregates. No-till soils high in organic matter accumulate more glomalin and better aggregate stability than tilled soils.
Earthworms: The Soil Engineers
Earthworms are the largest and most visible component of the soil food web, and their effects on soil physical properties are dramatic. In productive agricultural soils, earthworm biomass can reach 1–3 tonnes/hectare — more than the weight of cattle stocked on that same land.
What Earthworms Do
Soil mixing: Earthworms ingest soil and organic matter, mixing them internally and depositing the processed material (casts) throughout the profile. A population of 300 earthworms per square meter can process the equivalent of the top 15 cm of soil every few years.
Burrowing: Earthworm burrows create macropores that dramatically improve water infiltration and root penetration. In a good earthworm population, infiltration rates can be 10× higher than in earthworm-free soil. During heavy rain, these channels route water to depth before it can run off — reducing erosion and recharging groundwater.
Nutrient cycling: Earthworm casts have much higher levels of available nutrients than surrounding soil:
- Ammonium nitrogen: 7× higher
- Available phosphorus: 11× higher
- Exchangeable potassium: 5× higher
- pH: closer to neutral even in acid soils
Pathogen suppression: Some earthworm gut bacteria suppress plant pathogens. Earthworm activity is consistently associated with reduced levels of some soil-borne diseases.
Earthworm Types
| Type | Habitat | Effect |
|---|---|---|
| Epigeic (surface dwellers) | Litter layer, compost | Fast decomposers; used in vermicomposting |
| Endogeic (topsoil) | Upper 20 cm | Mix topsoil; create horizontal burrows |
| Anecic (deep burrowers) | 0–2+ m | Vertical channels; pull surface litter deep |
All three types are ecologically valuable. Anecic species (the deep burrowers — nightcrawlers in Europe and America) create the permanent, stable vertical channels that most improve drainage and root depth.
Supporting Earthworms
- Avoid tillage — earthworms and their eggs are physically destroyed by tillage; populations take years to recover
- Maintain soil moisture — earthworms need moisture to move and breathe through their skin
- Add organic matter — earthworms need food; bare, food-free soil cannot support them
- Avoid pesticides — many insecticides, herbicides, and fungicides reduce earthworm populations
- Maintain near-neutral pH — earthworms prefer pH 5.5–7.5; very acid soils are hostile
Field assessment: Dig a spade square (30 × 30 × 30 cm) and count earthworms. Target 10+ per sample. Fewer than 5 indicates poor soil health or pH problems.
Protozoa and Nematodes: The Invisible Grazers
Protozoa — single-celled organisms slightly larger than bacteria — are primarily bacterial grazers. When a protozoan eats a bacterium, it assimilates the bacterial carbon and nitrogen into its own body. Because protozoa have a higher C:N ratio than bacteria, they excrete excess nitrogen as ammonium. This process — the “microbial loop” — is responsible for releasing roughly 30–70% of the nitrogen that becomes plant-available in natural systems.
Nematodes are microscopic roundworms. Most soil nematodes are beneficial:
- Bacterial feeders and fungal feeders cycle nutrients by grazing microbes
- Predatory nematodes control bacterial and fungal populations
- Root-feeding nematodes are the exception and can be crop pests
A diverse nematode community (multiple feeding types present) indicates a healthy, functioning food web. A community dominated by root-feeding types indicates disturbance.
Microarthropods: Shredders and Predators
Mites (Acari) and springtails (Collembola) are the most numerous soil arthropods — often 50,000–500,000 per square meter. They shred organic material into smaller pieces (increasing surface area for microbial attack) and graze on bacteria and fungi, cycling nutrients and regulating microbial populations.
Predatory mites control fungal gnat larvae, nematodes, and other small organisms that might otherwise reach damaging levels.
Their populations crash with heavy tillage, pesticide use, and soil desiccation. Diverse, abundant microarthropod populations are a hallmark of undisturbed or minimally managed soils.
Building a Healthy Soil Food Web
Soil biology cannot be manufactured — it must be cultivated. The steps are simple:
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Feed the system: Add organic matter. Compost, manure, crop residues, cover crop biomass — all of these feed the soil food web. A hungry food web cannot function.
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Minimize disturbance: Every tillage pass disrupts fungal networks, crushes earthworms, and destroys the pore structure that soil organisms live in. Reduce tillage intensity as much as crop management allows.
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Keep the soil covered: Bare soil desiccates, heats to lethal temperatures, and loses organic matter through UV oxidation. Mulch, plant residues, or growing cover crops protect the soil biological community from these stresses.
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Maintain diversity: Rotating crops with different root architectures and exudate profiles feeds different microbial communities. Monocultures progressively simplify soil biology and allow specialist pathogens to build up.
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Avoid biocides when possible: Pesticides, fungicides, and synthetic fertilizers at high rates all suppress aspects of soil biology. This doesn’t mean never use them — but every application has biological costs.
The soil food web took millions of years to co-evolve with plants. It is robust but not infinitely resilient. A farmer who manages for soil biology works with a system far more sophisticated than any technology humans have yet developed.