Nitrogen can be utilized by plants as the ammonium cation or as the nitrate anion. Mineralization is the microbial decomposition of organic matter that releases nitrogen as ammonium ions. Nitrification is the bacterial oxidation of ammonium cations to nitrate anions, which in turn are used by plants, lost by leaching, or denitrified by bacteria to volatile N 2 and N 2 O. Ammonium can be volatilized as ammonia gas. Phosphorus is the second most often limiting nutrient . It is contained in plant cell nuclei and is part of energy storage and transfer chemicals in the plant because mineral phosphate forms are not readily soluble.
Organic phosphates are important phosphate sources in most soils.
Mass flow, the most important of these mechanisms, is the movement of plant nutrients in flowing soil solution. Diffusion is movement by normal dispersion of the nutrient from a higher concentration through soil water to areas of lower concentration of that nutrient. Root interception is the growth of plant roots into new soil areas where there are untapped supplies of nutrients in the soil solution. Mass flow involves large amounts of water flowing to roots as the plant transpires water. Diffusion is the dominant transport mechanism for phosphorus and potassium.