NEW TRENDS IN THE STUDY OF PLANT AVAILABLE PHOSPHATE Part I -THE USE OF RADIOACTIVE PHOSPHORUS

Introduction phate (+pCa+pH,P04) as the function most likely to give a numerical index of the condition in the soil Most of the used for the estimawhich mainly controls the availability of phosphate. tion plant phosphate in are a Its evaluation is jndependent of arbitrary or greater or lesser extent arbitrary, involving the use of procedures. extractants which often have a drastic effect on the chemical equilibrium of the soil. The equilibrium mono-calcium phosphate potential of a soil is measured3 by shaking aliquots of Schofieldl in 1955 pointed out that to assess the the soil with O.OIM-CaCl, solutions of increasing availability of phosphate in soil to plants, it was not only necessary to know the quantity of phosphate H,P04-ion concentration, plotting the change in which could be regarded as available, but also its phosphate content of the solution per gram of soil chemical potential (or free energy) and the rate of ( LP) against the final mono-calcium phosphate potendecrease of this ~otential with uhos~hate withdrawal. tial, and interpolating at AP=O. This is analogo<s to the assess&entbf the availability of water in the soil, which requires the measurement not only of the amount of water present, but also its potential or the tension with which it is held by the soil (pF) and the rate of increase of this potential with the removal of water. The use of radioactive phosphorus (P-32) has a demonstrated that there exists in the soil a pool of "labile" or "exchangeable" phosphate from which plants can draw their phosphate requirements. This "labile" pool can only be measured by means of P-32,