Effect of the Level of Microbial Population on Isotopically Exchangeable Phosphate in Soil

Abstract

IF a soil is shaken with a phosphate solution labelled with phosphorus–32 the specific activity (phosphorus–32/phosphorus–31) is reduced in consequence of isotopic exchange with the ‘exchangeable’ phosphate in the soil. At equilibrium the following relationship holds: phosphorus–32/phosphorus–31 in the solution=phosphorus–32/phosphorus–31 in the exchangeable form in the soil. The specific activity in solution can be determined by assay while the phosphorus–32 in the soil can be calculated by the difference between the initial and final phosphorus–32 contents of the solution. The amount of phosphate in the soil which has exchanged isotopically under the conditions of the experiment can readily be calculated^1–4. It is also possible to calculate the quantity of phosphate which is removed or ‘sorbed’ by the soil from the change in concentration of inactive phosphate in the solution. The amount of phosphate which plants will absorb does not always bear a simple relationship to these quantities; but knowledge of ‘exchangeable’ or ‘sorbed’ phosphate in the soil can be of value in the study of factors which control the availability of phosphate to plants.