Magnesium (Mg2+) is an essential macronutrient for all plant growth and development. Its adequate level in the soil is important for producing maximum economic yields. Like deficiency of calcium (Ca2+), deficiency of magnesium in crop production is more common on highly weathered acid soils (Fageria and Souza, 1991). Deficiency of Mg2+ may also occur in coarse-textured soils of humid regions with low cation exchange capacities. Data in Table 6.1 show significant increase in root and shoot dry weight of dry bean and cowpea, except cowpea root dry weight, to increasing Mg2+ concentration in the range of 0.30 to 6.22 cmolc kg-1 of soil. The increase in root and shoot dry weight was quadratic in fashion with increasing Mg2+ level in the soil. More than 80% variability in root and shoot dry weight of both the crops was due to variation in Mg2+ level in the soil. This means that for annual crop production in Oxisols, application of magnesium is necessary for maximizing crop yields. The low Mg2+ content of such soils is associated with leaching of this element from the soil profile.