CAB129-37 — Page 356

The Catena at Urambo

187

Page 356

The soils of the Urambo Region belong to the Plateau soils group of Milne (1947). They are derived from a granitic gneiss of the Basement Complex, in country which was peneplanated in very early times and subsequently uplifted. Slopes are slight, the drainage is almost dead and many of the drainage lines are infilled and wooded. The miombo vegetation (Brachystegia-Isoberlinia open woodland, "trockenwald" of the German authors) is apparently extremely efficient in transpiration and dries out the soil, according to investigations by the Tanganyika Tsetse Research Department, to about 20 feet. In spite of a rainfall averaging 35 in. per year over thousands of square miles, river drainage is relatively small.

The parent granite is not often exposed, but around the rare outcrops grey skeletal soils are found. The mature type on the gentle ridges is a rich red loam, which compacts to some extent on drying. At the foot of the profile massive ironstone is often found lying on the granite and filling in the inequalities. in its surface. Useful deposits of nodular ironstone also occur in this type. Otherwise the appearance of the profile is uniform.

Below the ridge tops the red soil type grades in to a series of buff, yellow and grey sandy soils. When they are first encountered, in a traverse from. crest to hollow, these lighter coloured soils overlie at a few inches only a bright red subsoil essentially similar to that of the higher red soil; but as the drainage line is approached the top soil layer becomes thicker and the subsoil begins to lighten in colour to pale red, to yellow and ultimately to grey. Thus the changes in the top soil are followed lower down the slope by broadly similar changes below. The subsoils appear, however, to be richer in clay materials than. the topsoils.

The drainage line soils present a somewhat bewildering complexity of clays, silts and sands, although their subsoils are all heavy and they are all dark in colour ranging from grey-brown to black. The presence of much sand or silt in the topsoils of some of them may perhaps best be explained by assuming. that these catenas of extremely mature topography have been in process of formation for so long that the clay has been followed to the lower levels by the more slowly moving sand, which now overlies the finer material. Thus the mechanical separation has been followed by a redeposition of the various fractions- in more or less distinct layers, with the finer materials lower down.

The Urambo country is poor in bases and there is no general accumulation of limestone in the soils of lower levels. The great termite heaps which occur in the neighbourhood of the drainage line margins are, however, base accumu- lating, as shown by analysis, and usually have a zone of calcium carbonate nodules below them. This is particularly marked west and south of Ussinge, near the confluence of the two main drainages of the region, the Ugalla and Malagarasi. rivers.

II. SOIL FERTILITY AND FERTILISER STUDIES

Two main lines of activity have been pursued in these fields. Some hundreds of soils were analysed by rapid methods, and 44 field experiments were set out to examine fertiliser and liming effects on the groundnut crop. No fertiliser experiments on other crops were carried out.

Soil analysis

Soil analysis has been confined to the estimation of pH and of certain plant. foods (Ca, P, K, Mg) extracted from the soil in two minutes shaking by O.3N HC1. pH was measured in most cases by means of indicators, using barium sulphate mixed with the soil to speed up sedimentation. A few determinations were made with the glass electrode. Analysis of soil extracts was carried out by the following standard procedures. Ca was determined at first by a rough turbidimetric method, but later a more precise oxalate-permanganate method was used. PO was determined by stannous chloride reduction of phospho-molybdate, the intensity of the resulting blue colour being read on a comparator or on a Spekker absorptiometer. K was determined by a rough

Page 356 125

Page 356

5

11

V81

||||

ER

4