They are generally mineral salts, except for some organic chemicals such as urea which are easily converted into salts. In some countries governmental regulations are now being introduced for fertilizer application in general, in order to minimize any avoidable fertilizer-induced pollution or food contamination. Many fertilizer products, however, are not yet subjected to legal control, though there is an increasing voluntary standardization by producers in order to ensure uniformity of quality, especially for organic and organic-mineral fertilizers. Nitrate N in the soil solution is immediately available and thus acts quickly but is most liable to leaching.
Plants take up N mainly in nitrate form.
Ammonium N, although fully available, has a somewhat slower effect, because it is first adsorbed and then only gradually released and nitrified. Amide N, e.g. as urea, must first be transformed to ammonium as a result of microbial action which is dependent upon temperature. Calcium cyanamide, in addition to its fertilizer value, has herbicidal and fungicidal properties due to intermediate decomposition products.