Potassium (K) is an essential element for all life. It is abundant in nature and occurs in considerable total amounts in most soils. Forms of potassium available for plant uptake, however, are often deficient in soils. Potassium, nitrogen, and phosphorus are considered the major nutrient elements for plants because their use as fertilizers is more widespread and in greater amounts than other elements. Potassium deficiency in crop plants under different agroecosystems is not as common as N and P deficiencies. Furthermore, K+ deficiency is not as easily identified as N and P deficiencies, which are accompanied by major changes in leaf color and tillering. Figure 4.1 provides a clear example of this. Figure 4.1 shows the response of dry bean to K+ fertilization in Brazilian Oxisol. There was a reduction in growth of bean plants in the pot that did not receive potassium fertilization compared to the pot that received potassium fertilization. However, plants without K+ fertilization experienced reduced growth but did not show any foliar symptoms of K deficiency. The reason for this is that nonavailable or fixed K+ might replenish available K+ when deficiency of this element occurs under high K+ demands by crop plants. However, K+ uptake by field crops is as high as nitrogen uptake. In lowland and upland rice, uptake of K+ is higher than N uptake (Fageria et al., 1997a; Fageria, 2001; Fageria and Baligar, 2001; Fageria et al., 2003).