Dominant Soils of the Basin

The diversity of rock formations in the basin has not resulted in a comparable diversity in soils as they are very young due to the severe dissection of the landscape. The dominant soil groups are Ferralsols, Arenosols, Luvisols and Lithosols, sometimes with associations of Fluvisols and Vertisols. Ferralsols dominate the highlands in the Upper Kunene while Arenosols and Luvisols are dominant in the Middle Kunene. The Lower Kunene is dominated by Lithosols. Overall, infertile soil groups cover more than 80 % of the catchment.

The dominant soil types in the Kunene River basin.

Source: FAO 1997

( click to enlarge )

The Ferralsols developed from the weathering of basement rocks. Their low nutrient reserves are easily exhausted by crop production. The fine to medium Arenosol sands are more generally called Kalahari sands with sand grain making up more than 70 % of the soil volume and extending often to more than one meter deep. These soils are very poor and at best only marginal cultivation can be carried out on them. The Fluvisols along the main river channels are sediments deposited by high water flows on the floodplains. However, nutrients have not been accumulated sufficiently to create the same rich organic and nutrient content found on floodplains elsewhere in the world. Vertisols occur in large depressions of probably lacustrine origin, and have a high fertility (FAO 1997).

Soil Characteristics

The distribution by soil type (% of surface) in the sub-basins is as follows:

Dominant Soil Types in Sub-basins

The Ferralsols (also known as laterite soils) in the Upper and Middle Kunene are deep, intensively weathered soils and have good physical properties (strong water retention) but are chemically poor. Their low natural fertility is a serious limitation for intensive agriculture. As liming and full fertilization are required for sustainable sedentary agriculture, Ferralsols in the basin are used mainly for shifting cultivation or for grazing.

Arenosols are the dominant soil in the Middle Kunene and are sandy soils that developed from residual rock weathering and have lost all primary minerals other than the coarse grained quartz. The texture is accountable for the high permeability and low water and nutrient storage capacity. However, their ease of cultivation, rooting and harvesting of root and tuber crops, makes them relative intensively cultivated. Under the conditions of the basin, these soils are best left under their natural vegetation as nutrient elements are all concentrated in the biomass in the top 20 cm of the soil.

Luvisols have a clay alluviation horizon formed by the translocation of clay from the surface soil. They have favourable physical properties and are generally fertile soils suitable for a wide range of agricultural uses.

Fluvisols cover only 2 % of the area in the Middle Kunene and are young, azonal soils in alluvial deposits, receiving fresh sediment during floods. The Fluvisols are confined to narrow strips of land adjacent to the actual riverbed. These soils are fertile and are intensively cultivated with food crops and orchards or used for grazing. Flood control, drainage and/or irrigation are often required.

Vertisols are heavy, clay soils with a high proportion of swelling clays and forming deep wide cracks when they dry out. Given their good chemical fertility, these soils have a considerable agricultural potential but appropriate management is a precondition for sustained production. Currently, large areas are still only used for extensive grazing, fuel wood gathering or charcoal burning.

Leptosols cover 75 % of the Lower Kunene. These are generally very shallow soils over hard rock, although they can also be deeper, and are extremely stony. These soils are not used for arable cropping, and have a limited potential for tree crop production or extensive grazing.

Cambisols are young soils with a beginning horizon differentiation derived from slight or moderate weathering of a wide range of rocks. The soils have limited use for agricultural production because of shallowness, stoniness and low base status.