Much of the lowveld is underlain by granites and the soil so formed has both sand and clay particles and minerals. Where the topography is gently sloping the clays and minerals wash out of the soil, move down the hill and accumulate in the valley bottoms. Thus the ridge tops become sandy and nutrient poor, while the “bottomlands” become clay and nutrient rich. The sandy hilltop soils have a low ability to hold water and the underlying granite is poorly permeable. Therefore the excess water draining out of the sands in wet seasons moves downhill below the soil surface until it strikes the interface with the clays of the bottomlands, which are also poorly permeable, and thus force it to the surface. A narrow, seasonally waterlogged band of grassland called a “seepline” forms along the contour of the slope. Below the seepline are dry clay soils. The grassland remains green and palatable for longer than the other areas and makes it a valuable wildlife habitat, especially in winter. During a number of dry years when no waterlogging occurs, tree seedlings become established. These then have to be removed to re-establish the grassland.
The accumulation of minerals in the bottomlands may eventually cause the clay structure to collapse. This becomes hard and impermeable to water, highly prone to erosion and a difficult soil for plants to grow in. Mopane and Euclea species sometimes grow here but in their absence, the thin layer of good soil is stripped off. This bare patch is then called a sodic site, which attracts wildebeest and warthog to the sodium and the wallows. Once formed they are practically impossible to reverse and one can only control the rate of them spreading.
Unfortunately seeplines do not only challenge plants, but also management, and principally road construction and maintenance. Being productive areas for animal activity, it is desirable that roads create access to such seeplines for game viewing purposes. However, due to their clay nature, roads should skirt these areas rather than go through them, which would create an unacceptable degree of environmental disturbance. As part of our ongoing efforts, roads which have been found to encroach on seeplines are being identified and re-routed.