Iron (Fe) plays many important roles in the growth and development of higher plants. Iron has many functions in plants; however, its main role is its participation in many plant metabolic functions, and it is also a component of many enzymes. Uptake of iron by crop plants exceeds uptake of all other essential micronutrients except chlorine. Iron deficiency and toxicity (flooded rice) are important yield-limiting factors in crop production around the world (Dudal, 1976; Follett et al., 1981; Clark, 1982; Baligar et al., 2001; Fageria et al., 2003; Naeve, 2006). Iron deficiency chlorosis reduces total soybean production in the United States by several million metric tons (Naeve and Rehm, 2006). Corn grown on calcareous, high-pH soils is susceptible to Fe deficiency, which can reduce grain yield by as much as 20% (Godsey et al., 2003). Any factor that decreases the availability of Fe in a soil or competes in the plant absorption process contributes to Fe deficiency (Fageria, 1992; Fageria et al., 1997, 2006; Mengel et al., 2001). Iron deficiency occurs in a variety of soils. Affected soils usually have a pH higher than 6.0 (Fageria et al., 1994). Iron-deficient soils are often sandy, although deficiencies have been found on fine-textured soils, mucks, and peats (Brown, 1961). Iron deficiency is potentially a problem on most calcareous soils (Marschner and Romheld, 1995; Lucena and Chaney, 2007). Calcareous soils are widespread throughout the world (Lombi et al., 2004). The United Nations Food and Agriculture Organization (FAO) estimated the extent of calcareous soils at 800 million ha worldwide, mainly concentrated in areas with arid or Mediterranean climates (Land, FAO, and Plant Nutrition Management, 2000). These soils are important in terms of agricultural production in many areas of the world. For instance in South Australia, about 40% of the wheat is produced on the Eyre Peninsula, which contains over a million hectares of calcareous soils (Holloway et al., 2001). In addition, in Europe alone, more than 60 million US$ are spent every year on Fe chelates to treat Fe chlorosis (Alvarez-Fernandez et al., 2005).