INFLUENCE OF SOIL ACIDITY ON ABSORPTION OF CALCIUM BY ALFALFA AS REVEALED BY RADIOCALCIUM

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Abstract
Radio-Ca was used to label the CaCO3 applied to the soil and the Ca supplied in nutrient solns. in greenhouse expts. When the surface layer (0-3 ins.) of an acid Mardin silt loam (pH 4.8) was limed, as much as 80% of the Ca in the plants was derived from the limed layer overlying the acid subsoil. The relatively great uptake of Ca from the limed surface layer was due not only to the increased Ca supply in the limed layer but also to the deleterious effect of the acid subsoil on root growth and Ca absorption. The plants failed to develop normal taproots in the acid subsoil when only the 0-3.5-in. layer of soil was limed. In another expt. the amt. of exchangeable Ca in the surface layer (0-2 ins.) and the subsoil (2-10 ins.) was kept constant at 2 pH values, 4.8 and 6.5, by diluting the limed layer with sand. Approx. the same proportion of the Ca in the plants was taken up from the surface layer at both pH values when the subsoil was maintained at pH 6.5. On the other hand, the proportion of Ca in the plants derived from the subsoil at pH 6.5 was greater than that at pH 4.8 when the pH of the surface layer was kept at 6.5. When CaCO3 was localized with the seed, 10-30% of the Ca in the plants was found to be derived from the applied lime. The rate of absorption of Ca by alfalfa was markedly reduced in presence of A1+++ and, to a lesser degree, Mn and H+ in nutrient soln. Thus the low Ca content usually observed in plants grown on acid soils may be due to the antagonistic effect of Al+++, Mn++, and H+ on the absorption of Ca++, as well as to the restricted root growth, rather than to the low Ca supply in the soil.