What is soil Bacteria?
- Soil bacteria and soil fungi are the start of the soil food web that supports other soil organisms and the functions of a healthy soil.
- Diverse populations of soil bacteria and fungi can suppress root diseases.
- Soil bacteria and fungi are encouraged by ground cover and organic matter inputs.
- Populations of soil bacteria change rapidly depending on moisture, time of year, type of crop, stubble management, etc.
- Soil fungi are slower to develop, and are strongly set back by cultivation.
Soil bacteria
Bacteria are the most abundant microbes in the soil. They are single celled organisms, and there can be billions of bacteria in a single gram of soil. Populations of bacteria can boom or bust in the space of a few days in response to changes in soil moisture, soil temperature or carbon substrate. Some bacteria species are very fragile and may be killed by slight changes in the soil environment. Others are extremely tough, able to withstand severe heat, cold or drying. Some bacteria are dependent on specific plant species.
Soil fungi
Soil fungi are microscopic plant-like cells that can be single celled (e.g. yeast) or grow in long threadlike structures or hyphae that make a mass called mycelium. They can be symbiotic with plant roots (figure 1). Fungi are generally not as dependent on specific plant species as some bacteria, and populations are slower to develop.
Figure 1: Hyphae from mycorrhizal fungi emerging from plant roots. Photo: Paula Flynn, Iowa State University Extension
Types of bacteria
Decomposers: play an important role in the early stages of decomposition of organic materials (in the later stages fungi tend to dominate).
Nitrogen fixers: extract nitrogen gas from the air and convert it into forms that plants can use, and can add the equivalent of more than 100 kg/ha per year of nitrogen to the soil. Rhizobium bacteria live in special root nodules on legumes and can be inoculated onto legume seeds. Other free-living nitrogen-fixing bacteria associate with non-legumes, but inoculating with these organisms has not proved effective in increasing nitrogen fixation for non-legume crops.
Disease suppressors: release antibiotic substances to suppress particular competitors. A number of bacteria have been commercialised for disease suppression. Their effect is often specific to particular diseases of particular crops and may only be effective in certain circumstances.
Actinobacteria: help to slowly break down humates and humic acids in soils, and prefer non-acidic soils with pH higher than 5.
