Nitrogen is available to plants mostly in the form of nitrogen in the atmosphere nitrite ions in the soil uric acid from animal excretions nucleic acids from decomposing plants and animals nitrate and ammonium ions in the soil

Nitrogen is available to plants mostly in the form of Nitrate and ammonium ions in the soil. Plants absorb ammonium and nitrate during the assimilation process, which are then converted into nitrogen containing organic molecules, such as amino acids, and DNA. Animals are not able to absorb nitrates directly but receive their nutrient supplies by consuming plants or plant consuming animals.