Potassium (K) deficiency is a world-wide problem (Dobermannet al., 1998). In Europe, 25% of soils routinely test low in available K(Johnston, 2003) and the majority of soils in sub-Saharan Africa test low in available K (Smaling, 1998). At the same time, the K status of agricultural soils is also decreasing across the globe, from Europe (Fagerberg et al., 1996; Bengtsson et al., 2003), to Africa (Harris, 1998; Wortmann and Kaizzi, 1998), to Asia (Hedlund et al., 2003; Wijnhoud et al., 2003), and North America (Malo et al., 2005).