The role of soil microorganisms
Although soil organisms comprise <1% of the total mass of a soil, they have a vital role in supporting all plants and thus animals. Some of their vital functions are described below.
Soil is alive
Every gram of a typical healthy soil is home to several thousand different species of bacteria. One square metre of soil can contain about 10 million nematodes and 45 000 microarthropods (springtails and mites). It has more species in it than 1 km2of rainforest. In addition to bacteria, soil is home to microscopic fungi, algae, cyanobacteria, actinomycetes, protozoa and nematodes, and macroscopic earthworms, insects and the occasional wombat.
All of these organisms can be divided between autotrophs (“self-feeders”), such as plants, algae and cyanobacteria, and heterotrophs (“different-feeders”), such as fungi and bacteria, which decompose organic matter.
Soil microbes break down organic matter
Microorganisms play an important role in the decomposition of organic matter. Different types of microbes are specialised to different types of organic matter, between them covering just about everything.
Soil microbes recycle nutrients
Soil microbes play a crucial role in returning nutrients to their mineral forms, which plants can take up again. This process is known as mineralization.
Soil microbes create humus
When the soil microbes have broken down all they can, what’s left is called humus, a dark brown jelly-like substance that can remain unchanged in the soil for potentially millennia. Humus helps the soil retain moisture, and encourages the formation of soil structure. Humus molecules are covered in negatively charged sites that bind to positively charged ions (cations) of plant nutrients, thus forming an important component of a soil’s cation exchange capacity. Humus is also suspected of suppressing plant diseases.
Soil microbes create soil structure
Some soil microbes secrete polysaccharides, gums and glycoproteins, which glue soil minerals together, forming the basis for soil structure. Fungal hyphae and plant roots further bind soil aggregates together. Soil structure is essential to good plant growth.
Soil microbes fix nitrogen
Agriculture depends heavily on the ability of certain microbes (mainly bacteria) to convert atmospheric nitrogen (N2 gas) to ammonia (NH3). Some live freely in the soil, while others live in association with plant roots – the classic example is Rhizobiumbacteria in the roots of legumes. The process of conversion is known as nitrogen fixation.
Biological nitrogen fixation contributes about 60% of the nitrogen fixed on Earth. In contrast, manufactured fertilisers contribute 25%. As the cost of energy continues to rise, so too the cost of manufactured nitrogen fertilisers will rise, so biological nitrogen fixation is likely to have ever increasing importance in food production.
Soil organisms promote plant growth
Some soil microbes produce a variety of substances that promote plant growth, including auxins, gibberellins and antibiotics.
Soil microbes control pests and diseases
The best known example of the use of soil microbes in pest control is the commercial production of the soil bacterium Bacillus thuringiensis (Bt) to control caterpillar pests of crops. Some strains of Bt are used to control beetles and flies as well. Several strains of the fungal genus Trichodermahave been developed as biocontrol agents against fungal diseases of plants, mainly root diseases. Various other genera of fungi are used for the control of insect pests.
My Agriculture Information Bank. 2011. Scope and Importance of Soil Microbiology. Agriinfo, India.
SESL Australia has evolved over 30 years as an environmental, soil, water and plant science laboratory and consultancy service. Established in Sydney in 1984 by Simon Leake, SESL has expanded into the ACT, QLD and VIC. The SESL team of environmental consultants have a clear company objective to provide accurate and timely sampling, analysis and interpretive reporting to solve client problems.
Microbes include fungi, bacteria and viruses. Farmers and ranchers often think of microbes as pests that are destructive to their crops or animals (as well as themselves), but many microbes are beneficial. Soil microbes (bacteria and fungi) are essential for decomposing organic matter and recycling old plant material. Some soil bacteria and fungi form relationships with plant roots that provide important nutrients like nitrogen or phosphorus. Fungi can colonize upper parts of plants and provide many benefits, including drought tolerance, heat tolerance, resistance to insects and resistance to plant diseases.
Viruses are almost always thought of as agents of disease. This is because the ones that cause disease are the ones that have been studied. We have been looking for viruses in wild plants from the Nature Conservancy's Tall Grass Prairie Preserve in northeastern Oklahoma. About half the plants have viruses, but most don't seem to be sick at all. The viruses seem to be living in the plants without doing any harm.
Recently we stressed some plants that were infected with viruses by not watering them. This was part of another experiment, but, to our surprise, all of the plants infected by viruses were much more tolerant of drought. The plants included 10 different species, and we used four different viruses. In all cases, the virus-infected plants did much better under drought stress.
Drought-stressed rice plants after six days without water. The plant on the right is infected with Brome mosaic virus; the one on the left is "healthy" (i.e., virus free).
We also found that viruses can benefit more complicated relationships. In Yellowstone National Park, soil temperatures can get pretty hot in the geothermal areas, but some plants can grow very well in these places with soil temperatures of 115°F. One plant that tolerates the heat is hot springs panic grass. A few years ago, other researchers found that the plant was colonized by a fungus. Without the fungus, the plant could not tolerate the heat. We further found that there was a virus in the fungus. When we were able to cure the fungus of its virus, it could still colonize the plants, but it no longer conferred tolerance to heat. When we reintroduced the virus, it restored heat tolerance.
These studies are making us think very differently about viruses.
There may be many viruses that benefit their hosts and most probably do not harm their hosts. We hope we will find new ways for viruses to benefit agriculture.