'n Studie van die reaksiekinetika van fosfaatsorpsie deur twee grondseries

Abstract

It is a well-established fact that a significant portion of the phosphorus in soils, added bv means of fertilizer materials or released from rocks by weathering, is rendered unavailable to plants by means of chemical and/or physical bonding forces. In an attempt to describe and characterize the extent as well as the reversibility of this bonding process, it is necessary to follow one of two lines of thought,i.e. to see it either as a precipitation of phosphatic compounds or as an adsorption of P on the surface of soil particles. A study of recent research findings suggests that looking at the process a s an adsorption process, and more particularly as a process of chemical adsorption, offers better possibilities to explain and describe the whole phenomenon. In this study an attempt has been made to determine the P-adsorption maxima of samples of two soil series and to study the influence of the original P-status of the soils as well as the length of equilibration time on the Padsorption maximum. It was found that the adsorption of phosphate by the soil from a solution was not complete after 64 hours shaking. Since a true equilibrium was not established, it was not possible to determine the adsorption maxima using the concentration range of the original phosphate solutions chosen for this study. The existence of two populations of adsorption sites reported by research workers like Seyers, Browman, Smillie & Corey (1973) was not confirmed in this study, probably as a result of a too low and a too narrow concentration range of the original P-solution. A better fit of experimental data on the Langmuir isotherm was found with Hutton soil than ·with Avalon soil - as was also found with longer shaking periods with both soils. It is, however, not certain whether the better fit with longer shaking periods was a result of the fact that fewer data points had been used on the curves since the phosphate in the low concentration· solutions was depleted at the end of 48 hours' and 64 hours ' shaking periods. A study of ~he kinetics of P-sorption by a Hutton and an Avalon type of soil suggests that the adsorption process can be seen in terms of more than one first order reaction taking place simultaneously. A log-log plot of the sorption data of phosphate from a solution by both the Msinga series of the Hutton soil form and Soetmelk series of the Avalon soil form f i ts a two constant equation of the form: (a - x) = k(t + 1)-b where a = original concentration of p in the solution. (a - x) = concentration of p in solution at time t. k & b = constants. It appears therefore that the use of the above formula presents challenging possibilities for further investigation. The measure of fit of the experimental data on the double log plot of the formula seems too exact to be incidental.