Masters Degrees (Soil Science)

Permanent URI for this collection

https://scholar.sun.ac.za/handle/10019.1/570

Browse

Browsing Masters Degrees (Soil Science) by Author "Dinwa, Siziphiwe"

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    Development, optimization and use of a reduced-sample, water dispersible clay extraction technique for taxonomic horizon discrimination
    (Stellenbosch : Stellenbosch University, 2018-03) Dinwa, Siziphiwe; Clarke, Catherine E.; Rozanov, Andrei Borisovich; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: Water dispersible clay (WDC) is defined as the colloid fraction which disperses in water without removal of cementing compounds or the use of dispersing agents. It is a commonly determined parameter and is used in many erosion models and is a proxy for aggregate stability and clay dispersivity. There is no standard method for determining WDC, and although modified particle size analysis (PSA) is the most common technique, numerous other methods are also employed to save time, bench space and reduce sample size. These methods have not been tested against the benchmark PSA method and vary in terms of agitation (time and type), extraction, measurement and expression of WDC. This makes comparison between these methods very difficult. This study aims to develop, test and optimise a simple, reduced sample centrifuge method for determining WDC in order to allow analysis of archive samples and assess the use of WDC as a soil classification discriminator on a limited number of soils. A reliable and calibrated, reduced sample size method will be of value for measuring WDC in sample collections, such as the national profile soil collection housed at the Institute for Soil Climate and Water. This would allow for these valuable collections to be included in erosion models. Archived samples of neocutanic B, yellow-brown and red apedal B horizons and borderline neocutanics/red apedal B horizons were selected for this study. Two reduced sample centrifuge methods (using pipetting and decanting to remove the clay suspension) were examined and their efficiency and accuracy was measured with respect to the sedimentation particle size analysis (PSA) method. For both the centrifuging methods the WDC and chemically dispersed clay, a mixture of sodium hexametaphosphate and sodium carbonate, called CDC were determined. This is the chemically dispersed clay without the removal of organic matter or cementing agents. The effect of ultrasonication and shaking time on WDC was assessed for the centrifuge-pipette method by physically agitating the soil with or without prior sonication, and increasing the initial shaking time incrementally from 1 to 30 hours. X-ray diffraction (XRD) analysis was carried out on the WDC and CDC extracts from the benchmark sedimentation method to establish if the mineralogy of these two fractions differed. The WDC and CDC was measured gravimetrically and by turbidity readings. Water dispersible clay correlated poorly with total clay across all samples. The relationship between CDC and total clay was better, but the extraction efficiency of CDC to total clay was only 54%. The extraction efficiency of WDC is highly dependent on the physical agitation energy exerted on the samples. Increasing the headspace in the centrifuge tube increased the WDC extraction efficiency by 32% (absolute). Shaking time has a major influence on WDC extraction efficiency, with a minimum shaking time of 22 hours required to get maximum extraction. This demonstrates the need to standardise the method as numerous extraction techniques use less than 16 hours shaking time for WDC extraction. Sonication prior to shaking for 22 hours results in a WDC extraction efficiency of 94% for the new centrifuge method compared to the traditional PSA method. The centrifuge-pipette method was shown to be effective in selectively separating the < 2 μm phase, thus reducing the need for sedimentation. Turbidity is not a reliable technique to measure clay in a suspension, due to the clay mineralogy affecting turbidity. Model kaolinite and smectite did not give uniform turbidity readings. This means the gravimetric method cannot be replaced, but centrifugation has both a time saving and sample reducing benefit. Neocutanic horizons tended to have WDCh (the WDC fraction expressed as a function of CDC) content higher than the yellow-brown and red apedal horizons, and were distinguishable from red apedal horizons at a 95% confidence level. However, WDC cannot be used to distinguish neocutanic B from yellow-brown apedals horizons. This supported the tacit knowledge that neocutanic horizons have a less stable clay phase than red apedal horizons, but the distinction is not clear in the case of yellow brown apedals. Borderline neocutanic/red apedal horizons and typical neocutanic proved to have similar WDCh content. Given the importance of clay stability in red apedal horizons, it was recommended they are classified as neocutanics rather than red apedals and a tentative threshold of 47% WDCh be used to differentiate between horizons. The new centrifuge technique for the extraction of WDC is a viable alternative to the PSA method and has the benefits of reducing sample size and extraction time and increasing the number of samples that can be analysed at one time. Standardisation of WDC is important due to the effects of agitation type and duration on the extraction efficiency. Furthermore, WDCh shows promise as a classification aid and should be investigated further.