Bacteria play an important role in the environment. Many dead materials are decomposed by bacteria. If there were no bacteria, the environment would have been polluted and full of harmful microorganisms. They degrade the dead organic matter and convert it into energy and nutrients. For example, they decompose trees and get their food from them in the form of nutrition.
Organic carbon present in the environment in the form dead organism is able to eat up all the carbon dioxide from the atmosphere if there were no decomposers in the on earth. It can be imagined that if carbon dioxide gets reduced from the atmosphere, there would have been no photosynthesis in plants and as a result no food would have been produced by plants. Decomposers or bacteria help in cycling of minerals like carbon and sulfur. They are also helpful in making, drugs, antibodies and hormones.
Nitrogen is the most important component of plants. Plants totally rely on nitrogen fro their health and growth and they cannot inhale it directly from the environment. They are dependent on soil for the supply of nitrogen. Through the process of nitrogen fixation, nitrogen from the atmosphere becomes available to the plants. This process takes place through bacteria for example Rhizobium and cyanobacteria. These species of bacteria convert the atmospheric nitrogen into nitrates and nitrites which is the part of their metabolism and make it available to the plants. Some plants have modified themselves so well that they are now able to store the bacteria into their tissues.
Using soil rich with beneficent bacteria can increase the productivity, growth and health of the plants. If carbon dioxide is the useful component of plants, similarly oxygen is also necessary for them because plants need oxygen in the process of respiration. They take oxygen by digesting the sugars present in them and use it for their growth. They absorb oxygen from the roots and if the roots will be able to absorb nutrients and energy from the soil only then they will be able to provide large amount of oxygen to the plants. Those soils are not fertile which do not have proper structure and organic matter. If these components will be present in the soil, then plants will get plenty of oxygen for respiration.