Determination of water-soluble phosphate content of soil using heterogeneous exchange reaction with 32P radioactive tracer

Abstract

The nutrient uptake by plants is significantly influenced by the quantity of phosphate in the soil solution. Optimal fertilization rate demands the determination of water soluble phosphate quantity, the rate of desorption and phosphate exchange between the soil and soil solution at steady state. The aim of this paper was to determine the ratio of water-soluble/exchangeable to mineralized/organic phosphate after incubation of the soil with different phosphate quantities. The sorption process of phosphate on soil was studied by heterogeneous isotope exchange using a radioactive isotope of phosphorous. Using the correct mathematical analysis of the kinetics of heterogeneous isotope exchange, the rate of phosphate exchange was established under steady state condition. The distribution of radioactivity in equilibrium provided information on the reversibility or irreversibility of phosphate sorption. The results revealed that during a relatively short incubation period, the water-soluble/exchangeable phosphorous quantity, namely the portion of phosphate which can easily be taken up by plants, was proportional to the added phosphorous quantity. The tightly sorbed phosphorous quantity, however, reached a limit, suggesting that the reactants leading to tight sorption are consumed. The exchange rate of phosphorous between the soil and the solution was found to be proportional to the phosphorous quantity in the soil solution, showing that the rate-determining step of the heterogeneous isotope exchange process was the diffusion of phosphate ions.