Total nitrogen encompasses all nitrogen forms in soil, including organic nitrogen in humus and inorganic nitrogen such as ammonium (NH₄⁺) and nitrate (NO₃⁻). Nitrogen is essential for plant growth, being a vital component of amino acids, proteins, and chlorophyll. Total nitrogen is a key indicator of soil fertility and its capacity to sustain long-term crop productivity. Source: Brady & Weil (2017).