Isotopic equilibration of calcium-45 with labile soil calcium

Summary When soils are suspended in solutions of labelled calcium chloride isotopic exchange with the labile soil calcium occurs rapidly. This may be followed by a slow secondary exchange reaction, but its magnitude is not great and equilibrium is nearly, if not completely, attained within 7 days.When, however, plants are grown in soil throughout which carrier-free calcium-45 has been thoroughly mixed, it is found that the calcium-45 absorbed by the plants has equilibrated with a quantity of soil calcium larger than that which undergoes isotopic exchange when soils are suspended in solutions of labelled calcium chloride. The analysis of plants grown for varying periods shows that equilibration can continue for several weeks, and that the quantity of soil calcium with which the calcium-45 is associated can be increased both by the addition of electrolytes to the soil and by growing plants under exhaustion conditions. In 5 soils the extra calcium which equilibrated with calcium-45 in this way never exceeded 3.5 per cent of the total soil calcium, and was usually considerably lower.The continued equilibration of calcium-45 with soil calcium causes the specific activity (Ca⁴⁵/stable Ca) of the calcium entering plants to decrease. Because the calcium in plant roots has, on average, been absorbed more recently than that in shoots, the latter show higher specific activities.The causes of these effects are discussed and consideration is given to their significance in the interpretation of results of experiments involving the use of calcium-45 as a tracer in soils.