Bacteria and archaea

“We know more about the movement of celestial bodies than about the soil underfoot.” – Leonardo da Vinci


Think about the soil like the web of life in the ocean. The big fish eats the little fish on down to the plankton that represents the base of the oceanic food chain. Microbes are the “plankton of the soil”, or the base of the terrestrial food web.

Bacteria were the earliest form of life, appearing at least 3 billion years ago. They are among the earth’s primary decomposers of organic matter, second to fungi. Bacteria in the soil food web play a crucial role in recycling three of the basic elements needed for life: carbon, sulfur and nitrogen.

Normally bacteria prefers to consume more simple carbon sources like molasses, sugar, milk, seaweed, manure, urine, fulvic acid, fish emulsion.

Nitrogen-fixing bacteria play an important role in the Nitrogen Cycle. Azotobacter, Beijerinckia, and Clostridium and Rhizobium, associated with leguminous plants, Frankia, associated with certain dicotyledonous specie and certain Azospirillum species, associated with cereal grasses, are examples of bacteria that can fix atmospheric Nitrogen.

The symbiotic nitrogen-fixing bacteria invade the root hairs of host plants, where they multiply and stimulate formation of root nodules, enlargements of plant cells and bacteria in intimate association. Within the nodules the bacteria convert free nitrogen to ammonia, which the host plant utilizes for its development.

Bacteria is needed but too much bacteria may lead to tight soils. High bacteria and low organisms that prey on bacteria may tie nutrients and increase nitrates in plants. A balanced fungi to bacteria ratio is important for soil health.

How bacterial activity is essential for plants and soil

Decomposes organic matter
Recycles carbon, sulfur and nitrogen
Convert Nitrogen from the atmosphere into plant plant-available forms.
Lock up nutrients that might otherwise disappear as a result of leaching
Some bacteria can break down pollutants and toxins
Fluorescent Pseudomonads can protect plants against pathogens


Benefits of different probiotics on plants (University of Queensland, 2020)

Pseudomonas fluorescens

Fluorescent Pseudomonads belong to plant Growth Promoting Rhizobacteria (PGPR), the important group of bacteria that play a major role in the plant growth promotion, induced systemic resistance, biological control of pathogens etc.

Many strains of Pseudomonas fluorescens are known to enhance plant growth promotion and reduce severity of various diseases. Fluorescent Pseudomonad has also non-ice nucleating properties and it can be used to protect plants against frost damage.

Soil microbial biomass correlates positively with soil organic carbon. (T.P. McGonigle, 2017)


For long it was thought that archaea are only present in extreme environments like geysers and in hot-water ocean wents. However, it was recently discovered that archaea is also present in agricultural soils.

Archaea play a big role in carbon cycle. Some archaea are photoautotrophic, obtaining energy from light and using carbon dioxide as their source of carbon. Others are chemoautotrophic, using inorganic chemicals as their energy source.

Archaea also participate in nitrogen cycle – archaea are the most abundant ammonia oxidizers of the soil. Like bacteria, archaea are decomposers. They break down organic and inorganic materials, recycling elements needed for plant life. Like fungi and bacteria, archaea form relationships with other organisms in soil.