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 (R2 = -0.59) through weathering. The exchangeable potassium percentage (EKP), however, also played a significant role and mica was absent from soils with EKP = 0. 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 l/S phases, are highly K-fixing. 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.