Bacteria outnumber all other types of microbes in the soil. Soil bacteria have major roles in making the soil fertile to provide nutrients for plants to grow. The fertilization of soil is contributed by saprophytes, the bacteria which act as decomposers of decaying or dead organisms and returning materials back to the substrate. Organic materials are also utilized by other soil bacteria to release phosphorus, nitrogen, calcium, potassium, magnesium and iron. There are also soil bacteria that are capable of recycling gas from the air to become nitrogen which is required by plants. Soil bacteria are also beneficial in detoxifying soil and repairing root damage.
Fungi appear like strands of threads under the microscope when maximized to 100 times their normal size. They are not visible to the naked eye until they develop fruiting bodies. Fruiting bodies are attractively visible in the form of mushrooms and molds. Just like bacteria, other species of fungi are also saprophytes. They decompose organic materials from decaying organisms to produce substances which are beneficial to other living organisms. Fungi also aid in the production of plant hormones and are utilized in the making of antibiotics such as penicillin. Mycorrhizae are another type of fungi species and help roots obtain nutrients and water. Fungi also benefit from the plants they are associated with by gaining nutrients and carbohydrates.
The smallest infectious agents are viruses. They are known to be obligate parasites, meaning they are not capable of reproduction or multiplication unless inside a host cell. Viruses in plants are transmitted via vectors such as insects. Other viruses are transmitted by direct feeding of nematodes from infected plants. Tobacco Mosaic Virus (TMV) was the first plant virus discovered (1898) and today it causes yearly loss in crop yields of $60 billion worldwide.
Protozoa are unicellular microbes bigger than bacteria but they are still microscopic in nature. They are known as predators of the microbe world because they eat other smaller microorganisms including some smaller protozoa. Protozoa such as the amoeba primarily feed on bacteria and they produce nitrogen and other nutrients for plants and other soil life forms. Some protozoa are harmful to the plants because they destroy the roots, while other species feed on pathogens in plant roots which suppresses plant diseases.
Algae are microbes which are capable of photosynthesis. They have the ability to convert carbon dioxide, solar energy and other nutrients found in soil into proteins and nutrients. Other forms of algae known as blue-green algae can fix atmospheric nitrogen levels contributing to plant growth. There are many microbes that are capable of nitrogen conversion but algae has further functions. After nitrogen conversion, they excrete adhesive substances which keep the soil intact providing other soil organisms an ideal habitat for growth. Algae have symbiotic relationship with fungi - they are merged together to form composite organisms called lichens. The fungi survive by using sugars from algae while algae are being protected by fungi from weather conditions. Lichens help to break down old woods and provide the nutrients to the soil.