Crops have the ability to absorb and assimilate inorganic nitrogen compounds. Therefore, in addition to a small amount of soluble nitrogen-containing organic matter in the soil, such as urea, amino acids, ammonium acylamide, etc., the nitrogen absorbed by crops from the soil is mainly ammonium and nitrate, both ammonium nitrogen and nitrate nitrogen. Ammonium nitrogen absorbed into the body can be directly combined with organic acids as products of photosynthesis to form amino acids, which in turn form other nitrogen-containing organic substances. Nitrate nitrogen can be absorbed and utilized only after reducing to ammonium nitrogen in the body. The ammonia absorbed by the plant and reduced to nitrate nitrogen cannot accumulate too much in the body, otherwise it will poison the plant. Ammonia poisoning will reduce the respiration of the plant and hinder the synthesis of protein. Unreduced nitrate nitrogen can accumulate in plants. Dry crops such as wheat, tobacco and salt-tolerant plants growing on saline soil can accumulate more nitrate. Vegetables can also accumulate a large amount of nitric acid in leaves. salt.
Organic acids that combine with ammonia to form amino acids in crops are derived from photosynthetic products, such as pyruvate (to alanine after ammoniating) and Q-ketoglutarate (to glutamate after ammoniating). Therefore, the absorption of nitrogen by plants depends to a large extent on the intensity of photosynthesis, which is consistent with the experience of the people that the effect of fertilization is often better and faster on sunny days.