Clay mineral associations in soils formed under Mediterranean-type climate in South Africa

Abstract

The objective of the study was to determine clay mineral associations in soils of the winter rainfall area of South Africa and to relate them to dominant soil-forming factors. Clay mineral compositions from 69 profiles, involving 190 samples, were determined by means of X-ray diffractometry. Mica proportions decreased predominantly (R 2 = -0.59) through weathering. The exchangeable potassium percentage (EKP), however, also played a significant role and mica was absent from soils with EKP = O. Smectite, vermiculite and hydroxy-interlayered vermiculite (HIV) occurred in 60, 28 and 62 samples respectively, and illite/smectite (I/S) in 44. The number of samples containing swelling clays decreased with increasing rainfall while the percentage of HIV increased. The most important agricultural aspect is that many, if not all, vermiculites, smectites and lIS phases, are highly Kfixing. As a result of the geothermal history of the area, chlorite and mica must have been by far the dominant minerals in the parent rock. Chlorite was absent from all soil samples and is therefore the most unstable precursor mineral. The kaolinite content was virtually unrelated to weathering (R2 = 0.1) but probably reflects the original amount of chlorite.

Effects of Weathered Soil Parent Materials on Merlot Grapevines Grafted onto 110 Richter and 101-14Mgt Rootstocks

“…Because quartz is chemically inert, its role in soil is that of a diluting agent rather than a reactive component. Kaolinite predominates in the clay fractions of the soils in the winter rainfall region (Bühmann et al, 2004b). In the current study, kaolinite was more abundant in the shale than the granite-derived material (Table 2), perhaps reflecting the former abundance of mica and chlorite in the metasediment (Bühmann et al, 2004b).…”

“…Kaolinite predominates in the clay fractions of the soils in the winter rainfall region (Bühmann et al, 2004b). In the current study, kaolinite was more abundant in the shale than the granite-derived material (Table 2), perhaps reflecting the former abundance of mica and chlorite in the metasediment (Bühmann et al, 2004b). Smectite was absent from the silt fractions of both soils, but present in the clay fractions ( Table 2).…”

“…Even after intense weathering, granite-derived soils may retain sufficient feldspar and mica in the coarser fragments to provide a limited supply of poorly buffered K (Wooldridge, 1990). However, in the shale soils of the western coastal foreland, most of the original mica structures have degraded and their constituent K has been lost (Bühmann et al, 2004b). Shale soils may nevertheless have the capacity to fix applied (fertiliser) K where layer structures persist (Bühmann et al, 2004b), or to provide a sufficient degree of buffering to reduce luxury consumption and loss through leaching (Wooldridge, 1988).…”

“…Feldspar is broken down by carbonic acid from rain, releasing K. Under severe weathering conditions, kaolinite, a near chemically-inert clay mineral, may form from weathering products (Wilson, 1999). Micas (sheet silicates) also occur in metamorphic rocks, such as shale (Bühmann et al, 2004a(Bühmann et al, , 2004b. On weathering, mica degrades into interstratified forms such as chlorite and vermiculite, releasing K in the process (Wilson, 1999).…”

“…However, in the shale soils of the western coastal foreland, most of the original mica structures have degraded and their constituent K has been lost (Bühmann et al, 2004b). Shale soils may nevertheless have the capacity to fix applied (fertiliser) K where layer structures persist (Bühmann et al, 2004b), or to provide a sufficient degree of buffering to reduce luxury consumption and loss through leaching (Wooldridge, 1988). The ability to supply, or fix, limited amounts of K therefore is a soil characteristic that may differ between conferred mineral assemblages in soils even after severe weathering.…”