abstract: The potential oxygen flux density in soil is one of the parameters describing the
availability of oxygen for plant roots. The parameter can be estimated with the help of
two electrochemical methods – the amperometric method and the voltammetric
method.
The primary objective of this work was to improve the voltammetric method
for the determination of the potential oxygen flux density in soil through the
estimation of the selectivity of the method and of the possibility of the application
of the two-electrode symmetrical measuring setup. Attainment of the objective
was possible due to the realization of the successive stages of the study:
1) validation of the thesis that oxygen reduction is the sole source of electric
current flow in the measuring setup;
2) selection of proper measurement procedure in the aspect of maximizing
the repeatability of results obtained for a given soil sample;
3) comparison of measurements of the potential oxygen flux density taken
with a three-electrode potentiostat-system setup and a two-electrode
symmetrical setup.
4) investigation of the effect of the selection of integration range in the
voltammetric method for the estimation of potential oxygen flux density
in soil on the level of uncertainty of the results obtained.
An additional objective was validation of the thesis that the current-voltage
characteristics recorded during the measurements of the potential oxygen flux
density in saturated soil may provide a basis for the estimation of salinity as
expressed by the electric conductivity of soil solution.
Summarizing the results of the study one can state that:
- the proposed method of measurement and interpretation of results
constitutes an improvement of the voltammetric method for the estimation
of the potential oxygen flux in soil by reducing the measurement
uncertainty and providing a deeper-than-hitherto assessment of the
uncertainty and factors affecting the uncertainty. Many of the results
obtained may also help improve the amperometric method;
- voltammetric measurements of the potential oxygen flux in soil can be
conducted with the help of a two-electrode symmetrical measuring setup (with
both electrodes made of platinum), which permits the elimination of measurement environment disturbance due to soil compaction with the
relatively large saturated calomel electrode, the avoidance of technical difficulties
involved in the installation of the electrode encased in a glass casing,
especially in compacted soils, and undertaking further work on the development
of an integrated probe for the measurement of soil moisture with the TDR
technique and of soil oxygenation with the voltammetric method;
- the value of background current which results from the reduction of
substances other than oxygen in the course of measurements with both the
methods under anaerobic conditions may be approximately twice the value
in “chemically pure” solution of potassium chloride. At the same time, the
value of measurement error decreases with increasing oxygenation and may
be up to 20% of the value determined in amperometric measurements and
up to 10% in voltammetric measurements. These values should be taken
into account in the method uncertainty budget;
- the uncertainty of soil potential oxygen flux density determination with
the voltammetric method can be minimized through the adoption, for the
integration, of the current-voltage relations of the generalized integration
range. One of the methods for the determination of the limits of the range
can be the averaging of the upper and lower limits of quasiplateau area
determined individually for each measurement;
- the platinum electrode which acts as the measuring electrode should be
re-installed before each successive measurement. This procedure will
allow the electrode to be cleaned of residues from the preceding
measurement that might block the electrode for the next measurement;
- the current-voltage characteristics recorded in the course of the voltammetric
measurements of the potential oxygen flux in saturated soils may be
interpreted in terms of soil salinity (as expressed by electric conductivity);
- the analogue electrical model of phenomena taking place during the
electrochemical estimation of potential oxygen flux in soil may be used as
a tool for the validation of measurement apparatus.
)