Ca:Mg RATIOS IN SOIL AND THE YIELD AND COMPOSITION OF ALFALFA

Abstract

When carbonates of magnesium or calcium were added to surface samples of four acid soils to establish a range in exchangeable Ca:Mg ratios, magnesium carbonate reduced exchangeable calcium and, with one exception, calcium carbonate reduced exchangeable magnesium. Corresponding reductions of water-soluble magnesium and calcium occurred in two of the soils. For a given amount of the cation in exchangeable form, there was more magnesium than calcium in water-soluble form. The carbonate treatments had no effect on exchangeable potassium but usually reduced water-soluble potassium. The exchangeable Ca:Mg ratios in the different soils varied from 0.6:1.0 to 1.5:1.0 where magnesium carbonate was applied, and from 4.1:1.0 to 13.5:1.0 where calcium carbonate was used.Yields of alfalfa grown in the soils increased with addition of carbonates. The width of the Ca:Mg ratio in the treated samples had no significant effect on yield except in one soil where the pH was raised to slightly above the neutral point. Magnesium carbonate reduced calcium in the plants more than calcium carbonate reduced magnesium. With addition of potassium, the percentage reduction of plant magnesium was greater than that of calcium. Values for sum of cations in the plants were reduced by the magnesium carbonate treatment and usually by addition of potassium. The carbonate of calcium tended to be slightly more effective than that of magnesium in increasing the phosphorus content of the plants.From the standpoint of alfalfa yield, it is suggested that a magnesian limestone may be expected to be a satisfactory corrective of acidity, even though the resulting Ca:Mg ratio in the soil may be relatively low.