Nutrient deficiencies in plants are widespread throughout much of the agricultural regions of the world and can result in reduced growth or a decline in yields of many crop species (e.g., Graham, 1984; Sillanpaa and Vlek, 1985; Welch et al., 1991). Losses from environmental stresses, such as water and nutrient deficiency stresses, in agricultural production are enormous and have been estimated at 75 percent of the potential yield in U.S. agriculture (Boyer, 1982). Plant responses to a range of nutrient deficiencies have been discussed in a number of recent reviews (Graham and Rengel, 1993; Rengel et al., 1994; Randall, 1995; Welch, 1995; Jolley et al., 1996; Jungk and Claassen, 1997; Pearson and Rengel, 1997b; Cianzio, 1999; Crowley and Rengel, 1999; Rengel, 1999a, 2000, 2001; Engels et al., 2000; Jungk, 2001).