A greenhouse study involving sequential cropping of two soils of differing cation exchange capacities (CEC) with five Ca-Mg saturation ratios at two K levels was conducted. German millet (one cutting, Setaria ema, German) followed by alfalfa (five cuttings, Medicago sativa, Saranac) was grown with Mg-Ca saturations varied from Mg 5%–Ca 75% to Mg 25%–Ca 55% at two K levels.
German millet yields were not affected by treatment of either soil. Similarly, alfalfa yields were not affected in the above range of Ca-Mg saturations. However, an apparent Mg response occurred when alfalfa yields were more than doubled by increasing initial Mg and Ca saturations from 3 and 18% to 5 and 75%, respectively (concomitant pH increase from 5.4 to 6.8).
Magnesium contents of plant tissue increased with increased Mg saturation, but soil K depressed the Mg content of the tissues more per equivalent than soil Mg increased it. The results indicate that for most conditions, 6–10% Mg saturation (high to low soil CEC) would likely be adequate for maximum yields of crops, although 12–15% may be indicated for supplying adequate Mg when grasses are to be grown for use as the main dietary constituent of ruminants.
Exchangeable Mg and Ca contents in the soils after cropping plus amounts removed by cropping were less than the initial contents in one soil and were more than the initial contents in the other. Evidently some type of fixation mechanism tied up Mg and Ca in the former while solubilization of residual lime released them in the latter.