There are two ionic forms of nitrogen that can be root adsorbed, the anion nitrate and the cation ammonium. Ammonium is more quickly simulated metabolically, while the nitrate anion must be reduced before metabolic simulation occurs, a reduction process that requires energy. Therefore, some have suggested that the ammonium form of nitrogen would be the more desirable form made available to plants. Research has found that ammonium can adversely interfere with various metabolic processes in plants, resulting in what some have identified as a form of toxicity. Blossom-end rot in tomato and pepper, for example, and short-life in citrus and peach trees have been traced back to ammonium nutrition when it was the major form of nitrogen available to the plant, or even when greater than 25% of that form of nitrogen absorbed. Ammonium will interfere with calcium function in the plant, resulting in the breakdown of vascular cell walls, thereby restricting the movement of water up this conductive tissue. One of the early signs of ammonium toxicity in some plants is wilting when the plant is under moisture stress, particularly during periods of high atmospheric demand. Therefore, the absorption of the ammonium form of nitrogen at high, or even at moderate levels may result in poor plant growth and fruit quality, and can even lead to the death of plants, such as fruit trees, when exceeding a certain concentration for root absorption.
