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Micromorphology and Mineralogy of Cracking Soils from the Accra Plains of Ghana

Dep. Soil Science, Univ. of Ghana, Legon, Ghana

A. R. Mermut^* and R. J. St. Arnaud

Saskatchewan Inst. of Pedology, Dep. Soil Science, Univ. of Saskatchewan, Saskatoon, Canada. Contribution no. R681.

ABSTRACT

Four soil profiles, uncultivated and cultivated examples of Akuse and Prampram series from the Accra Plains of Ghana, developed in situ from garnetiferous hornblende gneiss, were examined in this study to establish their physical and chemical properties, macromorphology, micromorphology, and mineralogy. Cracking is well expressed in both soils; however, slickenside formation is found only in the deeper and moderately weathered Akuse soils. Although the clay content of the soils is only slightly >30%, the soils are still capable of developing cracks. The cation-exchange capacity (CEC) values are all >32 cmol_c kg^-1 and smectite is the dominant clay mineral. Moderate to minor amounts of vermiculite, chlorite, mica, and kaolinite occur throughout the profile. The amount of kaolinite is largest in the coarse-clay fraction, which also contains considerable amounts of quartz and feldspar, especially in the Prampram soils. Weathering of amphiboles, micas, and feldspars in the solum is responsible for the formation of swelling clay minerals and Fe-Mn nodules. Total chemical composition of both the coarse and fine clays suggests that clay minerals contain considerable structural Fe. The large proportion of vughs and interconnected vughs makes these soils unique and separates them from other cracking soils with higher clay contents. The Akuse soils have greater frequency of voids than the Prampram soils. Carbonate nodules with dendritic Fe-Mn precipitation, and Fe-Mn nodules are particularly distinct pedogenic features. A greater concentration of Fe-Mn nodules in the Akuse than the Prampram soils represents a greater mobility of Fe and Mn because of long-term moisture retention. A majority of the nodules at depth are formed in situ, but those at the surface are probably displaced upwards by the swell-shrink process. Considering the overall soil properties, the Akuse soils may be classified as Typic Haplusterts and the Prampram soils, because of the absence of slickensides, as Vertic Ustochrepts.

^* Corresponding author.

Received for publication December 14, 1990.