The primary source of nitrogen for plants is through the processes of nitrogen fixation and decomposition. Atmospheric nitrogen (N₂) is converted into a usable form, such as ammonia (NH₃), by certain bacteria in the soil or root nodules of legumes. Additionally, when organic matter decomposes, it releases nitrogen compounds back into the soil, enriching the nutrient cycle. This nitrogen is then absorbed by plant roots, playing a vital role in the synthesis of amino acids and proteins necessary for growth.