A THERMODYNAMIC EVALUATION OF THE ADDITION OF CALCIUM AND MAGNESIUM TO AN ULTISOL - I-CA(OH)(2) ADDITION

An experiment was conducted to determine the effects of two calcium sources an the chemistry of an Ultisol at various magnesium levels. A 2 x 4 x 3 complete factorial experimental design had two calcium sources [Ca(OH)(2), CaSO4]; four calcium levels: control (0 Ca added), 1x Ca (1 meg Ca/meq Al), 2x Ca (2 meg Ca/meq Al), 3x Ca (3meq Ca/meq Al), and three levels of magnesium: Control (0, Mg), 1x Mg (10% CEC), and 2x Mg (20% CEC). The addition of Ca(OH)(2) increased the pH of the soil to approximately the following values: Control - 4.40; 1x Ca - 6.40; 2x Ca - 6.70; 3x Ca - 6.90. This rise in pH more than doubled the sites available for exchangeable cations as pH dependent charges were created. A 20% decrease in exchangeable magnesium was observed without a consequent increase in the levels of magnesium in solution. This finding suggests that a fixation mechanism was precluding magnesium from becoming available, a condition which may create magnesium deficiency problems for the crops grown in these soils. The addition of magnesium to these samples increased their levels of soluble and exchangeable magnesium but had little effect on the overall fixation of magnesium. A decrease in pH in those samples previously limed with Ca(OH)(2)d caused a dramatic increase in the levels of calcium and magnesium in solution as a result of the release of cations from pH dependent charges in the case of calcium as well as the liberation of the magnesium possibly being held as a Mg-Al precipitate in the case of magnesium.