The role of soil microorganisms

SESL Australia

White fungal-like appearance of actinomycetesAlthough 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.

Further reading

My Agriculture Information Bank. 2011. Scope and Importance of Soil Microbiology. Agriinfo, India.

Wikipedia. Biocontrol.

Wikipedia. Humus.