Soil biology encompasses the diverse community of organisms that live in the soil: bacteria, fungi, protozoa, nematodes, earthworms, insects, and small mammals. A single gram of healthy soil can contain billions of bacteria, millions of fungi, and thousands of protozoa. These organisms drive the decomposition of organic matter, nutrient cycling, soil structure formation, and suppression of plant diseases. The living component of soil is essential for maintaining [[Soil Composition]] and fertility.
Bacteria are the most numerous soil organisms, performing critical functions including nitrogen fixation, decomposition, and nutrient mineralization. Actinobacteria, a group of filamentous bacteria, are responsible for the characteristic earthy smell of soil and produce many antibiotics. Fungi break down tough organic compounds like lignin and form networks of hyphae that stabilize soil aggregates. Mycorrhizal fungi form symbiotic associations with plant roots, extending the effective root surface area and improving phosphorus and water uptake in exchange for carbohydrates.
Earthworms are the most visible and well-known soil animals. A healthy population of earthworms can process several tonnes of soil per hectare annually, creating casts that are rich in nutrients and improving soil structure through burrowing. Nightcrawlers (Lumbricus terrestris) create deep vertical burrows that improve [[Drainage]], while red worms (Eisenia fetida) are surface dwellers that process organic matter. The introduction of earthworms can significantly improve soil fertility in degraded soils.
Practices that support soil biology include adding organic matter through [[Composting]] and [[Mulching]], minimizing soil disturbance through [[No-Till Gardening]], maintaining continuous plant cover through [[Cover Cropping]], and avoiding broad-spectrum pesticides and fungicides. Synthetic nitrogen fertilizers can suppress microbial activity over time, while organic amendments tend to increase microbial biomass and diversity.