Topsoil in its natural state contains many microbes and many classes of microbes. They serve important purposes in soil health and the environmental cycling of nutrients, such as nitrogen. Although microbes often have a positive role in the environment, microbes in the wrong places can cause human health problems, such as bacterial infections. This is why washing fruits and vegetables is important for human health.
A single gram of soil contains between 100 million and 1 billion bacteria. A 1,000-square-foot patch of ground contains around 12 lbs. of bacteria. The average acre of soil contains between 300 and 3,000 lbs. of these tiny, single-celled organisms. Most bacteria consume simple carbon compounds like waste plant material and turn them in to organic compounds that can be further broken down by other soil organisms and eventually used by plants growing in the soil. A second type of bacteria, such as nitrogen-fixing bacteria, form partnerships with plants and help the plant convert more complex soil nutrients into forms usable by the plants. A third group of bacteria are pathogens that can cause plant diseases. The fourth group of soil bacteria convert compounds like nitrogen, sulfur, iron and hydrogen to other forms.
Fungi are single-celled organisms that live in topsoil. The are substantially larger than bacteria. Fungi, like bacteria, can not make their own food and rely on outside sources of energy. They get that energy from outside sources of carbon, including decaying plant or animal matter. Fungi are responsible for the earlier stages of decomposition, with bacteria continuing the process once fungi have consumed what they can of the organic matter. A gram of soil contains between 100,000 and 1 million fungi. Because they are larger than bacteria, 1,000 square feet of soil can contain 35 lbs. of fungi. An acre of land contains between 500 and 5,000 lbs. of fungi.
Algae and Cyanobacteria
Cyanobacteria are most often called blue-green algae. A gram of soil contains between 1,000 and 1 million algae or cyanobacteria. A thousand square feet of soil contains about 3 lbs. of these organisms. An acre contains between 10 lbs. and 1,500 lbs. of algae and cyanobacteria. When new, barren soil is formed, such as during a volcanic eruption, algae and cyanobacteria are one of the first organisms to colonize the inorganic rocks, sands and soils. Because these organisms generate energy from sunlight, via photosynthesis, they don't require carbon food sources like other microbes. Although some algae don't require oxygen, most survive on a combination of light, oxygen and inorganic substances. Some forms of algae convert gaseous nitrogen into ammonium and thus help with the beginning of turning inorganic soil into fertile soil that can support life.