氮、磷、钾是植物营养的三大要素,其中氮肥对作物增产最为显著。空气中约有80%的氮素,据估计每亩地的上空大气层有5300吨氮,这些氮以游离的分子态存在,一般植物不能直接吸收利用。因此,作物所需之氮源主要靠施肥来供给。然而亦有少数植物(主要是微生物和藻类)能 Nitrogen, phosphorus and potassium are the three major elements of plant nutrition, of which nitrogen fertilizer on crop yield the most significant. About 80% of the nitrogen in the air is estimated to be 5300 tons of nitrogen per acre above the atmosphere. These nitrogen molecules exist as free molecular species. Generally, plants can not be directly absorbed and utilized. Therefore, the nitrogen source required for crops is mainly supplied by fertilization. However, there are also a few plants (mainly microorganisms and algae)