Interactions between calcium amendments and phosphate on the response of alfalfa and barley growing on acid soils

Abstract

Alfalfa (Medicago sativa L. cv Peace) and barley (Hordeum vulgare L. cv Otal) were grown in the greenhouse on three moderately acid soils (pH 4.7 to 5.1). Calcium amendments consisted of adding CaCO₃ to achieve pH values of 6.0 and 6.7, or CaSO₄ (gypsum) equivalent in Ca to CaCO₃ to attain pH 6.7, and a check (nil Ca). Three P treatments were superimposed on each Ca treatment: O, 30 and 60 mg P kg^‐1 soil. Dry matter yields of both alfalfa and barley were increased by CaCO₃ and not affected by CaSO₄. Phosphate increased yields although soil test P values were medium to high. There was a strong CaXP interaction. The yield increase due to CaCO₃ and P additions was greatest for the soil with the least amount of soil test P and organic matter, and relatively small when the soils were limed from pH 6.0 to 6.7. Liming reduced Al concentration and ‐increased P concentration in the soil solution. Gypsum increased Al concentration and had variable effects on solution P concentration. Soil solution P concentration and soil pH accounted for 85 percent of the yield variations in barley.