There are between 100 million and one billion bacteria in an average gram of soil, according to Texas A&M University. A single acre of soil can have as much as 3,000 lbs. of microbes in it, or about 12 lbs. for every square foot. These microscopic creatures come in a variety of forms and perform vital functions in the soil ecosystem.
Bacteria species such as Bacillus subtilis and Pseudomonas fluorescens serve as decomposers, digesting organic materials and breaking them down into soil and compost, according to the New South Wales Department of Primary Industries. Decomposers typically take simple compounds like root and plant litter and convert it to forms that plants can use. Some decomposers can break down pesticides or herbicides, and are very good at retaining nutrients within their cells, preventing essential nutrient loss in the soil.
Nitrogen is one of the key mineral nutrients that plants require to grow and prosper. Bacteria play an important role in extracting nitrogen gas from the air and converting it into mineral form that plants use. These bacteria most often live in root nodules of plants like alfalfa, clover and legumes. Rhizobium bacteria serve as nitrogen fixers in many different plants, and are mutualists, forming a partnership with the plants they inhabit.
Aerobic bacteria require oxygen to survive, and are most common in well drained soils where there is no standing water. These bacteria make up the majority of the species found in most soils, and also play a vital role in nitrification. These bacteria turn ammonium into nitrates which are then used by the plants to turn into proteins.
Though its name implies otherwise, green-blue algae is a form of bacteria, specifically cyanobacteria. These bacteria are photosynthetic and transform sunlight into energy and release oxygen as a byproduct.
Actinomycetes are a large and important group of bacteria that give freshly turned earth its recognizable "earthy" scent. These bacteria are decomposers that specialize in decomposing tough materials like cellulose and chitin. Especially active in high pH soils, these bacteria can produce antibiotics, according to the USDA.