The main source of nitrogen in the soil for plants is through the process of nitrogen fixation, primarily carried out by certain bacteria, both free-living and in symbiosis with legumes. These bacteria convert atmospheric nitrogen into ammonia, which becomes part of the soil nutrient cycle, available for plant uptake. Additionally, nitrogen can be supplied through decomposing organic matter and fertilizers. Nitrogen is crucial for plant growth as it is a key component of amino acids and proteins, essential for overall plant health.