Bacteria are single-celled organisms that lack a nucleus. Most bacteria are 0.5 to 5 micrometres (µm) long. Despite their small size, they comprise most of the world’s soil life due to their abundance. A teaspoon of productive soil may contain between 100 million and 1 billion bacteria. Most species have not yet been characterised. Roughly speaking, bacteria can be classified into three groups: aerobic, anaerobic and actinomycetes (which cause chitin degradation and give the soil its “earthy” smell).

The main functions of bacteria are:

  • degradation of (simple) organic matter (sugars, proteins, amino acids)
  • fixation and delivery of nutrients in the root zone
  • fixation of nitrogen from the air (see illustration showing nodules on the roots)
  • conversion of ammonia into nitrates
  • formation of stable aggregates
  • disease resistance

Bacteria are eaten by a large number of “grazers” higher up the food web.