A micronutrient is an element that plants need in small quantities for growth and metabolism. Common micronutrients are iron, copper, zinc, boron, molybdenum, manganese, and chlorine. Some plants may benefit from small amounts of sodium, silicon, and/or cobalt as well. Additional micronutrients may be added to the list as scientists are better able to detect minute quantities and plant requirements. Micronutrients often act as catalyst in plant chemical reactions. Historically, when animal waste was used as fertilizer, there were few micronutrient deficiencies detected. However, with the use of large amounts of chemical fertilizer, higher yields, and use of monoculture, micronutrient deficiencies in crop plants are becoming more evident. Several plant disorders such as beet canker, cracked stem of celery, and stem end of russet in tomatoes can be related to the lack of certain micronutrients. On the other hand, excess of micronutrients in plants may lead to plant toxicity.