soils and topography Relief or topography is one of the five main factors that influence the way soils develop. The influence of topography on soil formation operates through three main elements: slope angle, slope position, and altitude. Slope angle influences processes such as the movement of the soil and water. Slope position governs whether soils are net receivers or net losers of soil and water. Altitude affects climate, which is also an important soil-forming factor. Areas of great relative relief, such as mountain areas, often experience different climate characteristics from lower to upper slopes, which are usually expressed in terms of different rainfall and temperature regimes. This generally results in a vertical zonation of soil types, which can correspond to a zonation of vegetation types. Thus, a typical sequence on tropical mountains is from lowland tropical forest, through submontane, montane, and subalpine vegetation zones to the alpine zones of grasses and shrubs.
It has been known for some time that soils vary systematically along a transect from the top to the bottom of a hill slope. The effect of topography is seen best on slopes developed on a single rock-type; it may otherwise be difficult to differentiate the effect of topography from that of rock-type. Soil differences are usually related to changes in soil moisture and availability of water. In any explanation of the way in which soils vary with topography, emphasis is placed on the difference between freely drained upper parts of hill slopes and imperfectly to poorly drained lower portions.
There is a continuum between those parts of a slope where the influence of soil moisture is at a minimum and those parts where maximum influence of soil moisture is felt. Slope steepness is one of the most important factors affecting soil moisture, because it influences the balance between the amount of water that infiltrates into the soil and the amount of water that runs off as surface flow. On steep slopes relatively less water infiltrates, thus reducing percolation and the intensity of leaching processes within the soil. On steep slopes more water within the soil will also flow downslope as ‘through flow’. If less water infiltrates, more will flow across the surface, perhaps resulting in soil erosion. Soluble minerals are leached from soils on upper slopes, move down the slope, and are often deposited at the foot of the slope (Fig. 1). This leads to different trends in soil properties on upper and lower slopes. Upper slopes are associated with processes of removal of soil and water, whereas lower slopes are associated essentially with deposition and accumulation. Some workers have made the distinction between ‘non-cumulative’ soils on upper slopes and ‘cumulative’ soils on lower slopes. The operation of such effects across the entire hill slope produces a series of systematic changes in soil properties and soil profiles. The greatest concentrations of certain soil properties, such as the amount of organic matter present, would be expected on the gentler areas at the top and bottom of hill slopes.
Variation in soil colour often provides a clue to the processes that are operating. Colour changes are especially prominent on many tropical hill slopes. Upland, well-drained soils are reddish-brown, the colour indicating the presence of non-hydrated iron oxide. Drainage is slower on middle and lower parts of hill slopes, partly because of moisture seeping downslope from upper soils. Middle- to lower-slope soils remain moist longer and dry out less frequently and less completely. This causes the iron to become increasingly hydrated, and the red colour changes to brown or yellow. Drainage is poor on the lowest slopes and part, or all, of the soil profile may be waterlogged, leading to chemical reduction of iron. Under waterlogged conditions bacteria obtain their oxygen from oxygen-containing compounds, which are then reduced to other compounds (see
soil development). Such waterlogged soils are usually bluish-grey, greenish-grey, or neutral grey in colour, although if the water-table fluctuates, alternate oxidizing and reducing conditions will lead to the formation of red and grey mottles.
The recognition that there are clear patterns of soils on slopes and that these patterns repeat themselves on similar slopes in similar environments led to the formulation of the
catena concept. A catena is a grouping of soils which are linked in their occurrence by conditions of topography and are found in the same relationships to each other wherever the same conditions are met. The concept was first developed in the 1930s by G. Milne, working in East Africa, and was used as a way of mapping soils over wide areas. The assumption made was that where slope patterns were similar, soil patterns would also be similar. Since then catenas have been recognized in a variety of areas and under a variety of climatic conditions, an indication of the strength of the relationship between soil and topography.
John Gerrard
Bibliography
Birkeland, P. W. (1984) Soils and geomorphology. Oxford University Press, New York.
Gerrard, J. (1992) Soil geomorphology. Chapman and Hall, London.
