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Nitrogen (N) is quantitatively the most important nutrient for plant growth and productivity. Uptake of inorganic or organic N from the soil, and transport of newly synthesized organic N from so-called source organs (e.g. leaves and roots) to sinks (developing leaves, flowers and seeds) are integral to the physiology of plants and essential for plant development. In addition, reallocation of N stored as protein reserves is important during leaf senescence, seed germination, spring growth of perennials and development of new generations from tubers and other reproductive propagules. Nitrogen assimilation in plants. Plants absorb nitrogen from the soil in the form of nitrate ($N{O}_3^{-}$) and ammonium ($N{H}_4^{+}$). In aerobic soils where nitrification can occur, nitrate is usually the predominant form of available nitrogen that is absorbed.