Various studies have been conducted into redox potential (Eh), redox indicators and the
measured soil water contents in soil (Franzmeier et al., 1983; Schwertmann & Fanning,
1976; Veneman et al., 1976). Although a measure of success has come from these studies,
there are still vast knowledge gaps within this field.
The degree of water saturation where reduction in the soil is initiated cannot be determined
from literature, although it was approximated that 70% of water saturation (S0.7) was
sufficient to initiate reduction (Van Huyssteen et al., 2005). This value will vary for different
soil temperatures, varying bulk densities as well as soils with different organic matter
contents.
This study aimed to determine if it was possible to identify a degree of water saturation at
which reduction is initiated for a soil in a closed system. It also aimed to determine the
effect of bulk density on reduction. Reduction was defined by a decrease in pe (Eh) of a soil
and an increase in the soluble Fe2+ concentration. There were three key aims to the study:
to establish the relationship between the degree of water saturation (s) and the onset of
reduction; to establish the relationship between the degree of water saturation (s) and the
duration of reduction and to establish the effect of bulk density on the above-mentioned
processes.
A yellow brown apedal B horizon from an Avalon soil form (profile 234) in the Weatherley
catchment was used in this study. A soil core experiment was carried out to determine the
effect of degree and duration of water saturation on Eh, pH, Fe2+, Mn2+, Ca2+, Mg2+, K+, and
Na+. Soil cores were packed to a bulk density of 1.6 Mg m-3 and individually saturated to
S0.6 (60% of the pores saturated with water), S0.7 (70% of the pores saturated with water), S0.8
(80% of the pores saturated with water), and S0.9 (90% of the pores saturated with water).
Measurements were done in triplicate. The cores were sealed with a double layer of plastic
wrap and stored in a laboratory at 23°C until needed. Analysis started three days after initial
water saturation. A set of cores (four degrees of saturation with triplicates of each) was
analysed every 3.5 days for the first three months after which a set was analysed once a
week for the remaining month of analyses. The experiment was terminated after 121 days. The same soil and experimental setup was used for the bulk density experiment. The
experiment consisted of a set of three cores packed to an initial bulk density of 1.4, 1.6 and
1.8 Mg m-3. The cores were all saturated to S0.8, each packed in triplicate. The bulk density
experiment was terminated after 23 days.
There was a good correlation between an increase in degree of water saturation and pe
(R2 = 0.95); Mn2+ (R2 = 0.91) and Fe2+ (R2 = 0.92) concentrations. Eh, pH, Fe2+, Mn2+, Ca2+,
Mg2+, K+, and Na+ were significantly affected by duration of water saturation and all except
Ca2+ and K+ significantly affected by degree of water saturation. Fe2+ and Mn2+
accumulations and depletions (visible segregations or mottles) occurred within 12 months of
water saturation in a separate experiment where cores were packed to a bulk density of 1.6
Mg m-3 in a core saturated to S0.9. It was therefore evident that this soil with 0.22% organic
carbon and a bulk density of 1.6 Mg m-3 will produce morphological features due to
reduction within a year of water saturation at S0.9.
An experiment was set up with cores kept at a constant degree of water saturation (S0.8) with
varying bulk densities, namely 1.4, 1.6 and 1.8 Mg m-3. All the factors measured (Eh, pH,
Fe2+, Mn2+, Mg2+ and K+) except Ca2+ and Na+ were significantly affected by a variation in
bulk density. In another part of the experiment two different water temperatures were used
to saturated the cores, namely 23°C and 30°C respectively. It was determined that the
temperature difference of 7°C caused the cores to react significantly different to each other..
The higher water temperature caused the Eh to decrease more rapidly and therefore a
higher Fe2+ concentration occurred in these cores.
It was concluded that for this soil at 23°C, Fe3+ and to a certain extent Mn4+ will start to
become reduced at a pe of 6 at S0.78. These findings show that the first approximation of
Van Huyssteen et al. (2005) where S0.7 was found to be sufficient for reduction is very
similar for this soil.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-09052008-064013 |
| Date | 05 September 2008 |
| Creators | Jennings, Kimberly |
| Contributors | Dr CW van Huyssteen |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-09052008-064013/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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