Soil is a mixture of inorganic materials like sand, stones, silt or clay, and organic materials like plant litter (leaves, twigs, moss, etc.), animal litter (feces, insect or arthropod bodies, etc.), and lots of microorganisms (fungi, bacteria, algae, etc.). In many soils the number of microorganisms per gram of soil is so high that it exceeds the number of humans currently living on earth.
Soil is a dynamic environment. Living organisms are constantly degrading the materials in soil to provide themselves with nutrients for growth. If there are no microorganisms, or worms, or other invertebrates in a soil, plants will have a harder time growing. Microorganisms will permit certain nutrients to be released from the soil and be available to plants and animals in the soil.
Even though there are large numbers of individual microorganisms in each gram of soil, the number of unique types of microorganisms is much smaller. There are probably thousands of unique types of microorganisms in a small sample of soil, but the real number is hard to judge because you cannot observe microorganisms without a microscope. Even with a microscope it is very difficult to distinguish one microorganism from another. Therefore to determine the different types of microorganisms in soils, scientists typically isolate into the laboratory as many of the microorganisms that can grow on Petri dishes or flasks of nutrient media. A lesson plan for a school classroom activity that attempts to isolate different types of microorganisms from soil can be downloaded from this website:
Within the soil ecosystem, microorganisms compete for nutrients and space. All organisms have evolved ways to compete with each other. Some microorganisms grow and reproduce very quickly to outnumber other microorganisms. Some microorganisms grow on chemicals which other microorganisms cannot use, and therefore have an advantage. Some microorganisms produce chemicals which inhibit the growth of other microorganisms, and we call these chemicals antibiotics.
Humans discovered that soil microorganisms produce antibiotic substances in the early 20th century, and realized that these compounds could be used to fight diseases produced by other microorganisms. Penicillin was one of the first antibiotics discovered. It is produced by a filamentous fungus called Penicillium, and this compound inhibits the growth of bacteria. Another antibiotic discovered early in the 20th century was streptomycin. It is produced by a filamentous bacterium called Streptomyces, and this compound also inhibits the growth of bacteria. Some soil microorganisms also produce antibiotics which inhibit the growth of fungi. A lesson plan for a school classroom activity to identify microorganisms which produce antibiotics can be downloaded from this website: