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 µm 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.