Nickel (Ni) is the most recent addition to the list of essential micronutrients for higher plants. Nickel was suspected of being an essential plant nutrient in the early 20th century, when it was discovered to be a constituent of plant ash (Wood and Reilly, 2007). Foliar Ni sprays were noted to increase yields of wheat, potatoes, and broad beans as early as 1946 (Roach and Barclay, 1946), and responses in other crops were noted in subsequent years (Dixon et al., 1975; Welch, 1981). Its essentiality for higher plants was established in the 1980s (Welch, 1981; Eskew et al., 1983) using soybean as a test plant. Soybean plants were grown in highly purified nutrient solution (without Ni), and urea accumulated in the toxic level in the tips of leaflets, which become necrotic. When Ni was supplied to soybean plants in the concentration of 1 µg L-1, no excess urea was accumulated in the leaf tips, and necrosis was also absent (Epstein and Bloom, 2005). Subsequent research by Brown et al. (1987ab) established the essentiality of Ni for other crop species such as barley. These authors reported that Ni is essential for plants supplied with urea. The Ni-deficient plants accumulate toxic levels of urea in leaf tips because of reduced urease activity (Daroub and Snyder, 2007). Daroub and Snyder (2007) also reported that the essentiality of Ni was established in 1987. In 1992, the U.S. Department of Agriculture’s Agriculture Research Service added Ni to its list of essential plant nutrient elements (Wood and Reilly, 2007). Its essentiality was also recognized by the American Association of Plant Food Control Officials (AAPFCO), the umbrella organization that governs and influences regulation, labeling, ingredients, and amounts of elements in fertilizer products in the United States (Terry, 2004). Ni is now listed on fertilizer labels in the United States, and commercial Ni fertilizer products are now marketed (Wood and Reilly, 2007). Deficiency of Ni in crop plants is rarely observed under field conditions. Even in controlled conditions, creating deficiency symptoms is difficult. However, in nutrient solution experiments, Ni toxicity is frequently observed in crop plants (Table 15.1).