The Orange River is the longest in South Africa. Flowing across almost the entire width of the country, it makes its way from the highlands in the east through the Kalahari depression in the west to empty into the South Atlantic Ocean. Its major tributary, the Vaal River, is one of its northern headwaters; the two rivers together have a combined length of about 1,300 miles. Together with other major rivers on the continent, the Orange–Vaal river system shares the characteristic of flowing over steep gradients for numerous stretches of its course. The largest drop (about 400 feet) occurs at the Augrabies Falls.

Chad basin

The Chad basin constitutes the largest inland drainage area in Africa. Lake Chad, a large sheet of fresh water with a mean depth between 3.5 and 4 feet, lies at the centre of the basin but not in its lowest part. Lake Chad is fed by three major streams, the Komadugu Yobe, Logone, and Chari, but these are in danger of having their waters captured by the drainage systems of rivers that flow in opposite directions. Lake Chad itself, with an area of only some 5,000 square miles, was formerly much more extensive.

For a detailed discussion of Lake Chad, the Congo River, the East African lakes, the Niger River, the Nile River, the Orange River, the Sénégal River, the Suez Canal, and the Zambezi River, see individual articles.

Soil types

In general, soil types on the African continent may be divided into five or six broad categories. There are desert soils; chestnut-brown soils, which border the deserts; and chernozem-like soils (dark black soils rich in humus and carbonates), which are found immediately south of the chestnut soils from Sudan westward to just beyond the Niger Bend (the bend in the middle course of the Niger River) and pockets of which are also found in East Africa, Zambia, Zimbabwe, and South Africa. In addition, there are black soils (often grouped with chernozems), and found on the Accra Plains of Ghana; red tropical soils and laterites (leached red iron-bearing soils), which occur in the tropical wet-and-dry and equatorial climatic zones; and Mediterranean soils, found in the Atlas Mountains of North Africa and the Cape region of South Africa.

The most important factors that affect soil formation are climate, parent material, relief, drainage, vegetation cover, and the passage of time. Where the land has been generally stable and fairly flat for prolonged periods, as in Africa, the climate becomes the major determinant of the soil groups. The different rocks are deeply weathered and are broken down into their common component elements to produce broadly similar soils under the same climatic conditions. Given sufficient time under a tropical climate, the differences in humus content of the great soil groups, which are introduced by vegetation types, are minimized. But within these groups there will naturally be differences in soil types as a function of local differences in physical factors.

Soil problems

Soil is the foundation of Africa’s economic life, and as such its detailed study is most important. Failure to appreciate the physical and chemical properties of the soils has led to disastrous results for several projects for agricultural improvement.

In studying the soils of Africa, it is essential not to lose sight of the importance of such social factors as the ability or inability of mostly uneducated farmers to judge the quality of the soil. Thus, schemes for transforming traditional systems of farming that are based on soil classification but that do not take into account local perception may have little chance of success.

For desert soils to be productive they must be irrigated, as they are on the desert margins of North Africa; their excessive salinity or alkalinity must also be reduced. Compared to desert soils, the chestnut-brown soils are easier to work and are more productive under irrigation. Black soils tend to have a markedly crumbly structure and are sometimes difficult to plow. In the wet season, the black soils of the Accra Plains swell and become slippery, while in the dry season they shrink once more and crack to such an extent that they are said to plow themselves. Red tropical soils need careful handling. Despite their luxuriant vegetation cover, high temperatures coupled with humidity promote the rapid decay of organic matter and keep the humus content low. Erosion is a constant threat if the soils are exposed to the elements for any length of time; the soils remain cultivable only if the sesquioxides remain below the surface.

In the Atlas and Cape regions, there is a clearer relationship between soil characteristics and parent material than in the humid tropical areas. Over expanses of limestone, for example, the soils contain large amounts of calcium compounds, some of which must be washed away or neutralized before the soils can become fully productive.