Weathering in a soil chronosequence
Creators: name: Singleton, Glen Allen
Publisher: name: University of British Columbia
Year: 2010

description: A soil chronosequence near Cox Bay, on the west coast of Vancouver Island, British Columbia, was investigated and verified. Soils in the sequence are developed in sandy beach deposits, which were found to aggrade toward the ocean at a rate of 0.26 m per year. Dendrochronology and geomorphic evidence established surface ages for seven selected pedons (sites 1 to 7) across these deposits, that ranged from 127 years at site 1 to 550 years at site 7. Soil classification ranged from an Orthic Dystric Brunisol (Typic Udipsamment) to an Orthic Humo-Ferric Podzol (Aquic Haplorthod) in thes sites, respectively. A variety of soil weathering characteristics, identified by wet-chemical, physical and optical techniques, were used to demonstrate that the soils along this transect have developed in a manner which is compatible with the presumed genesis of Podzols (Spodosols). A 0.17% increase in B horizon, oxalate extractable Al and a 0.22 meq/100 g decrease in C horizon, exchangeable Mg along this transect were plotted with site age to exemplify soil weathering chronofunctions. Soxhlet procedures were developed to artificially weather Cox Bay chronosequence soil materials, in the laboratory. Site 1, C3 horizon soil samples were leached with distilled water in soxhlets containing O₂, N₂, CO₂ and air atmospheres. The CO₂-water system clearly produced allitic weathering trends, whereas, a total elemental balance approach and more intense leaching were required to further interpret the weathering characteristies created by the other atmospheres. Problems of sample erosion and leachate contamination indicated a need for alterations in the design of the commercially available soxhlet. Modifications made, by reducing the height (A) or completely removing (B) the soxhlet siphon tube, rectified these problems and showed promise in simulating pedogenic processes. Acetic acid (0.3 M) was used to leach 220 g samples of the site 1, C3 horizon soil in A-modified soxhlets. High (4.3 litres per day) and low (2.2 litres per day) leaching rates produced 13 106 and 4 750 mg of precipitate, respectively, which accumulated in the leachates during an eight week experiment. Rates of removal of elements from the soil samples were in the order of Ca &gt; Mg &gt; Fe &gt; K &gt; Al &gt; Na &gt; Si for the low leaching rate and Ca &gt; K &gt; Mg &gt; Al &gt; Fe &gt; Na &gt; Si for the high leaching rate. These results are compatible with a podzolic weathering process. Acetic acid and both A- and B-modified soxhlets were used to weather chronosequence soil samples over 4, 8, 12, 20 and 24 week periods. Chemical changes in the resultant weathered products were evaluated. Ca and Mg decreases in the samples from both types of soxhlet were plotted against weathering time and statistically-derived, exponential functions were used to explain the resultant curves. The similarity between these laboratory chronofunctions and the field chronofunctions, derived earlier, was noted. Calcium-bound phosphate (PCa) and Ca chronofunctions from both field and soxhlet weathered soils were compared mathematically. By comparing the slopes of statistically-generated equations, both the A- a B-modified soxhlets were calibrated to pedogenic time. B-modified soxhlets closely simulated the chronosequence weathering and produced consistent calibration results. Statistically-based chronofunctions predicted that 100 years of pedogenic weathering of PCa required 7.4 weeks of B-modified soxhlet weathering. This compared well with the 5.5 weeks predicted for a similar loss of Ca. A- and B-modified soxhlets were presented as valuable pedologic tools for simulating soil weathering in the Cox Bay soil chronosequence.