There are more organisms in one tablespoon of healthy soil than there are people on the earth. Soil organisms are an intimate and integral part of a soil. These organisms contribute significantly to soil fertility and structure. Virtually all topsoil has passed through the gut of either microorganisms or macroorganisms.

Microorganisms include fungi, bacteria, actinomycetes and algae. These microscopic animals and microbes play an important role in soil processes.

MicroorganismImportance
Fungi
  • after roots, they make up the largest amount of living material in the soil
  • help make soil nutrients available to plants
  • have in important role in organic matter decomposition
  • are easily destroyed and killed by intensive tillage
Bacteria
  • are important for good soil quality and fertility
  • nitrogen fixing bacteria are important part of a soil's nitrogen cycle; they are associated with plants such as soybeans, peas, clover and alfalfa.
Actinomycetes
  • decompose organic matter
  • are abundant in low pH, droughty soils
Algae
  • decompose organic matter
  • are commonly found in poorly drained soils

Macroorganisms include protozoa, nematodes, earthworms, arthropods (insects, spiders) and rodents.

MacroorganismsImportance
Arthropods (examples: mites, spiders, insects)
  • graze on bacteria and fungi or decomposing plant material
  • help accelerate microbial decomposition
Earthworms
  • burrow extensively, creating macrospores and mixing soil
  • reduce soil bulk density
  • improve air and water infiltration
  • improve soil structure
  • increase the soil's nutrient pool
Rodents (examples: mice, groundhogs, chipmunks)
  • pass organic materials through their gut when the burrow and feed in the soil
  • deposit fecal pellets rich in nutrients such as nitrogen, phosphorus and potassium.

Animals and microbes are not evenly distributed through the soil (Figure 1). Populations decrease quickly just a few centimeters below the soil surface. Intensive tillage can severely impact these populations in a soil's top 20 cm.

Distribution of soil life per square metre of plow layer and depth.
Figure 1. Distribution of soil life per square metre of plow layer and depth.