CAB129-37 — Page 354

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This geological diversity naturally complicates the catenary succession. The soil of the crests or ridges, which has been termed the Upland Pallid type, is typically a light grey, yellow, buff or even pale red coloured sandy loam, of clay content insufficient to cause serious compaction on drying. The subsoil is occasionally rather brighter in colour than the topsoil and there is frequently a layer of ironstained quartz about the granite. It would thus appear that the granitic gneiss is relatively impermeable to water. This view is further supported. by the fact that wells, in the Kongwa Region, are entirely confined to upland pallid soil areas, where they tend to occur in groups. Many of these wells, for example those at Hogoro, have been excavated to a depth of forty feet or more, and are used in the dry season by villagers, who often walk several miles to them. The soil of the areas in which the wells occur is often somewhat coarse, with much iron staining of the larger particles and pebbles.

Below the upland pallid soils, in a typical traverse from crest to drainage line, are found the soils derived from hornblende gneiss. They usually exhibit a typical catenary succession. The first product of hornblende gneiss weathering is probably a greyish skeletal soil, but this is seldom seen since outcrops of the rock occur only on sites of geologically recent disturbance. The main products of weathering are a series of rich red loams with sufficient clay to compact them on pressure and drying. These constitute the Kongwa Red soil type, the various. members of which differ mainly in mechanical composition.

The red soils show no marked differences in appearance from surface to parent. rock, although there is an increase in clay content, in some instances at least. Compaction on these types may be very severe, particularly where the soils have been beaten down by rain or otherwise compressed before drying out. The compaction typically affects only the first three to five inches of the soil, but this is sufficient to constitute a major obstacle to dry season working. The effect of compaction can, however, be reduced by breaking the soil before it dries out at the end of the wet season:

Below the red soils, on the long gentle slopes, lie a range of paler types,. in which the top soil is brown, yellow or even grey, and is more sandy in texture than the mature red. These mechanically fractionated and transported types are not very extensive, and are of greater pedological than economic interest at Kongwa.

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The lowest levels of the catena are represented by the Valley soils, a group of drainage line and former lake bed soils, covering many thousands of acres. These range in colour from rich brown to black, and in texture they are heavy silty loams to stiff cracking clays. Many of the black clays show the "crab- hole phenomenon, presumably, where the infilling of dry season cracks with coarse material has prevented them from closing completely when the soils become wet. The pressure of the clay then appears to force up the soil between the cracks so that a very broken appearance of the surface results. The majority of these soils contain calcium carbonate, either as nodules or, in the case of old lake beds, in massive form with fossil inclusions. Some, particularly the black cracking clays, are saline.

It is clear that in earlier times bases have been leached from the soils of higher elevation and redeposited in the drainage lines. There are no indications. of former base accumulation in the upper soils. Thus, although the catena. as a whole was pedocalic in character, the main mature type was subject to leaching and loss of bases.

At the present time with a lower rainfall (indicated in part by the drying up of the former lakes) and heavy transpiration by the Commiphora bush which covers the higher soil types, little or no leaching can occur. The upper soils are of distinctly high base status and crops on them do not in general respond to liming.

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