Types

• From a plant's perspective, some of the most important bacteria in soil are nitrogen-fixing bacteria and nitrifying bacteria, which fix nitrogen from the atmosphere and make it available in a form plants can use. Denitrifying bacteria, by contrast, convert nitrates in the soil back to atmospheric nitrogen. Actinomycetes are very common bacteria in soil that help to break down and decompose organic compounds like cellulose.
  
  

Features

• You can identify actinomycetes by the characteristic smell of fresh earth, which is a mark of their presence. If you have a microscope, you can take a pinch of soil, put it on a microscope slide and moisten it. Remove pieces of quartz or rock to see some of the bacteria from the soil, although many of them cannot be distinguished solely on the basis of their shape. Many actinomycetes form threads similar in appearance to the thread-like projections of fungi in soil, albeit far smaller. Like other bacteria, soil bacteria can be divided into bacilli (rod-shaped), cocci (spherical cells), and spirochetes (spiral-shaped).
  
  

Considerations

• Some bacteria like rhizobia and the actinomycete Frankia form nodules on the roots of host plants. Rhizobia form nodules on the roots of legumes, while Frankia is associated with alder trees and sweet ferns.