'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.