Microorganisms can increase soil fertility by fixing nitrogen from the atmosphere. Some bacteria are called biological nitrogen fixers as they enrich soil with nitrogen. Microorganisms decompose dead organic waste from plants and animals, converting it into simple substances that plants and animals then use. They can be used to degrade harmful and smelly substances and clean the environment. However, plastic materials are not decomposed by microorganisms. Pathogens are microorganisms that cause diseases in humans, plants and animals.