Differences in mineral ratios between disaggregated and original clay fractions in some South African soils as affected by amendments

Abstract

Conflicting information is available on the propensity different soil clays exhibit to dispersion. We therefore assessed the relative stability of the clay components of 12 soil samples, from various parts of South Africa, to predominantly physical disruption by a mild disruptive treatment [mechanical shaking for 5 min in distilled water (DW), and after the addition of phosphogypsum (PG; 2% by weight) and polyacrylamide (PAM; 0.04% by weight)]. The soils differed markedly in their physical and chemical properties. Clay fractions were of mixed mineralogy and dominated by kaolinite, illite, or smectite. Comparison of the clay mineral composition of the disaggregated clay with that of the original <2 �m fraction indicated that the disaggregated clay composition depended on the amendment. In DW, clay-sized quartz and feldspar were disaggregated preferentially over layer silicates. Within the phyllosilicate fraction, the 2:1 clay minerals (mica, smectite) were on average slightly more easily disaggregated than kaolinite. Goethite was the least easily detached clay component in DW. The increase in quartz and feldspar proportions relative to the other components of the clay fraction was dramatically more pronounced when the soils were mixed with PG. With PAM, however, differences in the nature of the clay fraction between original and disaggregated clay were only marginal. Disruption was not particle-size related, as the minerals of the fine-clay fraction showed no selective increase in any of the treatments. These findings indicate that the most inert members of the clay fraction are most actively involved in the process of disaggregation. PG influenced disaggregation in a manner markedly different from that of PAM. Gypsum preferentially stabilised components with a net negative charge over uncharged species. PAM, in contrast, seemed to affect all clay components equally, independent of charge characteristics.

Soil infiltrability as a driver of plant cover and species richness in the semi-arid Karoo, South Africa

“…However, physical crusting or sealing of soils (and hence infiltration) is a function of many more factors than the density of plants. Such factors include clay mineralogy (Bühmann et al 1996), hydrophobicity (Hillel 1998), silt content, very fine sand content and the dispersibility of the clay fraction (Agassi et al 1981;Mills et al 2006). The consequences for water infiltration can be extreme.…”

Soil mineralogy research in South Africa, 1978 to 2002 - a review

“…Biihmann and co-workers used a set of II soils on which they investigated a variety of erosion-related aspects, i.e., a) differences in mineral associations between dispersed and total clay fractions (Biihmann, Rapp & Laker, 1996). The unexpected finding was that the 'quartz content in the dispersed clay fraction' was m~rkedly higher than that in the total clay fraction.…”

Quantifying the Impact of Off-Road Driving on Root-Area Distribution in Soils

“…It has been found that soils with high quartz contents in their clay fractions are found widespread in South Africa (Laker, 2004). In addition, soils with high quartz contents in their clay fractions are extremely prone to disaggregation, crusting and erosion (Bühmann et al, 1996) and also to sub-surface compaction (Moolman & Weber, 1978). This would then be an important factor at especially Sites 1 and 2.…”

“…Clay mineralogy plays an important role in determining the susceptibility of soil to disaggregation of aggregates, and thus also in its vulnerability to crusting and erosion (Stern, 1990;Bloem, 1992;Bühmann et al, 1996;Rapp, 1998). This would also be the case with vulnerability to compaction.…”

“…Stern, 1990;Bloem, 1992), clay mineralogical analyses were done on the soils (Table 4). The analyses were done at the ARC-ISCW in Pretoria according to the X-ray diffraction method described by Bühmann et al (1996), using a Philips diffractometer and graphite monochromated Co K" radiation, generated at 40 mA and 45 kV. Specimens were scanned from 2 to 35 o 2q.…”