Materials and Methods

2.2.1 Soil samples

The soils used in this chapter were archived samples house in the Department of Soil Science. The majority of soils were collected and described by Le Roux (2015), but other soils were also included from other studies. Only subsoils from the B1 horizon were selected for this study. The locations of the soils used in this study are indicated in Figure 2-1. The samples were collected in close proximities and because the map is big they appear as one point in the Figure. For this chapter, only twelve samples (all from the study of Le Roux (2015) were used to develop and test the method. The physicochemical properties and soil classification for these soils is indicated in Table 2-2. In Chapter 2, which involved improvement of this method, only nine samples were used because of low sample volumes.

2.2.2 Textural analysis

Le Roux (2015) determined total clay using a laser particle size analyser. Total clay was also determined using the PSA method after removal of all aggregation agents (Gee and Bauder, 1986). The Fe and Al content were determined using atomic absorption spectrophotometer (AAS) and added to the colloidal fraction.

Figure 2-1: South African map indicating the (A) Western Cape and (B) Mpumalanga province from which the soil samples used in this study were obtained

Table 2-2: Physicochemical properties and the soil classification systems used to describe the selected B1 horizons in this chapter

Soil sample pH

(water)

pH (KCl) Sand Silt Clay OC Fe citrate-

bicarbonate- dithionite (CDB)% SA diagnostic horizons1 WRB FAO diagnostic horizons 2 USDA Soil Taxonomy diagnostic horizons3 % Classification systems Br 1.2 5.4 4.3 77.8 14.5 7.7 0.9 1.7 re ferr oxic Pb 2.2 4.8 4.3 66.4 19.8 13.8 0.7 1.1 ye ferr oxic Us 1.2 5.0 4.3 57.7 25.3 17.0 1.0 2.9 re ferr oxic Bp 1.2 6.2 5.1 56.2 25.5 18.3 0.9 3.2 ne arg arg/kan Hh 1.2 5.9 4.7 38.0 43.2 18.8 0.6 2.1 ne arg arg/kan Hh 3.2 5.1 4.1 35.6 37.8 26.6 1.2 3.7 ne arg arg/kan Hh 4.2 5.0 3.9 34.0 44.5 21.5 1.3 2.6 ne arg arg/kan Mb 1.2 5.7 4.5 67.4 18.9 13.7 0.6 1.2 ne arg arg/kan Pb 3.2 4.7 3.8 25.0 60.0 15.0 0.9 2.3 ne arg arg/kan W3(1.2) 5.4 4.2 38.8 19.8 39.6 0.4 3.0 ne arg arg/kan Bp 3.2 7.2 6.3 74.2 15.7 10.1 0.8 1.5 ye argic/ferr arg/kan

W2 5.4 4.0 10.2 22.1 65.7 0.8 Not available pr arg arg

1 _{South Africa’s (SA) taxonomy where: re – red apedal B, ye – yellow-brown apedal B, ne – neocutanic B and pr – prismacutanic B (Soil Classification Working Group, 1991)}

2 _{World Reference Base (WRB) taxonomy where: fe – ferralic, arg – argillic, orc – ochric (IUSS Working Group WRB, 2015)}

3 _{United States Department of Agriculture’s Soil Taxonomy where: arg – argillic, kan – kandic (Soil Survey Staff, 2014)}

2.2.3 Water dispersible clay and chemically dispersible clay extraction methods

2.2.3.1 Sedimentation methods: pipette and hydrometer measurements

The adjusted PSA method was used as a benchmark method for both WDC and chemically dispersible clay (CDC). The method is the same as particle size analysis (PSA), however there is no removal of binding agents. The WDC treatments added only deionised water while the CDC treatments had a 10 ml mixture of sodium hexametaphosphate and sodium carbonate (10%) added in each 1000 ml water volume prior to dispersing samples. Two separate sedimentation mixing methods were used to disperse the WDC and CDC, an overhead electrical mixer (Hamilton Beach HMD200 Single-Spindle Drink Mixer 120V) and a reciprocal shaker supplied by Scientific Manufacturing Paarden Eiland Cape Town. The soil was mixed for 5 minutes using electrical mixer and 24 hours when the reciprocal shaker was used (Soil Cassification Working Group, 1991). A mechanical impeller physically mixes the soil in a mixing machine in the electric mixer. While a horizontal motion mixes the soil in reciprocal shakers. The sand was separated from silt and clay using a 2 mm sieve. The WDC and CDC were extracted from each agitation method by pipette, and the hydrometer readings were taken for both treatments at the same time intervals. All extractions were done in triplicates.

2.2.3.2 Reduced sample centrifuge method

The reduced sample centrifuge method was based on the centrifugation method of Seta and Karathanasis, (1996) but modified in terms of sample size and extraction procedure. Two extraction treatments were used for the centrifuge method. In one treatment clay was extracted after centrifugation by decantation (centrifuge-decant) and in the other treatment the suspension was accurately pipetted (centrifuge-pipette). For both the centrifuge-decant and centrifuge-pipette methods 2.5 g soil was added into centrifuge tubes, with or without dispersing agent. Chemical dispersing agent was made up using the guideline specified by the Soil Classification Working Group (1991). A 0.5 ml mixture of sodium hexametaphosphate and sodium carbonate (10%) aliquot and 49.5 ml distilled water was added into 50 ml polypropylene tubes for the CDC samples, while 50 ml deionised water was added with no chemical dispersant for the WDC samples. The centrifuge tubes were placed horizontally on a high-speed reciprocal shaker (ca. 148 rpm) for 20 hours. The samples were monitored in random intervals to ensure that mixing was effective. All samples were centrifuged (Sigma 2-16P, Germany) at 800 rpm for 3.5 minutes as described by Le Roux (2015). For the decanted samples, the supernatants were carefully poured out up to the 7.5 ml mark whereas for the pipette method a fixed 42.5 ml was extracted using a Lowy pipette. Three replicates were extracted for both WDC and CDC, oven dried at 105 ℃ (overnight) and weighed using a 0.001 g decimal place scale.

2.2.4 Mineralogy for the WDC and CDC suspension

The method proposed by Harris and White (2008) was used to determine the mineralogy of the WDC and CDC separated for XRD. No cementing agents were removed during sample pre-treatment. The WDC and CDC mineralogy was determined on ten samples (included in Table 2-2 excluding samples W2 and W3). For each sample Mg-saturation were prepared. The K-saturation was not performed because the soils analysed did not contain any 14 Å peaks. The clay fraction was pipetted and flocculated by adding 1M HCl. The Mg-saturated slurry was agitated on reciprocal shaker supplied by Scientific Manufacturing Paarden Eiland Cape Town and centrifuged (Sigma 2-16P, Germany). The samples were washed twice with 0.5 M MgCl2 and distilled water to remove excess Cl- (AgNO3) and

then oven dried at 105 ºC (overnight). Finely ground clay powders were sent to iThemba Laboratories in Cape Town for XRD analysis at angles ranging from 4 to 60 degrees.