Soil Technology最新文献

The weathering mantle of the Middleveld of Swaziland consists of thick soil-saprolite complexes. The isovolumetric chemical weathering of the saprolites has led to mass losses of more than 50%. Compared with saprolites from quartz-diorite and granodiorite, those from diorite have higher portions of easily weatherable plagioclases and amphiboles and 20–30% greater total pore space. The macro pore space reaches a maximum (4.6–7.0%) in the central saprolite zone, corresponding to saturated hydraulic conductivities of 6.02–11.81 × 10⁻⁷ m/s. Similar to the saprolites, the overlying ferrallitic soils show total pore volumes ranging from 39 to 52%. Compared to the soils, the available water capacity of the underlying saprolites is two to four times higher and the saturated hydraulic conductivity is about two times higher due to the high portion of medium pores which amount to 70% of total pore space. In the areas affected by sheet erosion, most of the soil cover is denuded and the underlying saprolites essentially determine the site properties. The hydrological properties of the saprolites are therefore of great importance with respect to erosion during wet periods and plant growth during drought periods. The low structural stability of the saprolites, indicated by shear strength values < 5 kPa, results from a silty texture, absence of organic matter, and low contents of Fe- and Al-oxides. As a result, saprolites are highly susceptible to erosion and represent an essential precondition for the development and rapid expansion of deep incising erosion gullies in areas with magmatic rocks. In contrast, the clay-rich ferrallitic soils developed from saprolite are comparatively stable, indicated by shear strength values ranging from 7 to 12 kPa. The inherent stabilizing properties of the soil are altered by overgrazing and unwise land use leading to infiltration capacities below 65 cm/day and high overland flow potentials at low rainfall intensities.

Quartz-diorite, diorite and granodiorite of Archaic ages are the most common rock types in the Swaziland Middleveld. The composition of the regolith cover is dominated by soil-saprolite complexes. The genetic and mineralogical gradient within the soil-saprolite complexes and the grade and spatial distribution of denudation of the land surface are important characteristics for evaluating the vulnerability to erosion and the site properties of eroded areas in land use planning. The saprolites were formed by intensive chemical weathering under warm and humid climatic conditions dating from Lower Cretaceous times. The saprolitisation process can be considered polygenetic, subdividing the saprolites into a near-surface oxidation zone and an underlying reduction zone caused by the geochemical environment. Assuming isovolumetric weathering, the mass losses of the upper saprolite zones, obtained as pore volume, amount to between 41.1 and 54.0%, depending on the mineral composition of the parent rock. Even in the lower saprolite zones plagioclases were transformed into white pseudomorphs of kaolinite. The weathering of feldspars, micas and amphiboles and the amount of Fe-oxides increase towards the surface. The clay fraction of the saprolites is dominated by kaolinite, with small amounts of illite and smectite. The latter reaches a maximum in the middle saprolite zones which coincidentally show a distinctly lower amphibole content. Therefore, smectite is formed as an intermediate phase and is not stable at higher weathering intensities in the upper saprolite zones which are characterized by desilification and removal of bases. Typical soils of the Middleveld are Ferralsols associated with Cambisols and Acrisols. Most of the recent soils have developed from colluvial sediments characterized by a multi-layer structure often marked by embedded stone lines. A strict genetic dependence between the parent rocks and the overlying soils has not been established. The different sediments which form the allochthonous part lead to compound horizons. The micromorphological investigations indicate a former Luvisol-Acrisol phase with deep-penetrating illuviation during the polygenetic soil formation. The mineral composition of the soils is dominated by quartz and kaolinite accompanied by small amounts of illite. Gibbsite is present in the uppermost soil horizons and can be interpreted as indicative of the recent phase of ferrallitization.

The problem of soil erosion in Swaziland is reviewed in the context of land management of communal grazing lands. Aspects of farm operations, cropping patterns and range management are considered in a typical Swazi traditional land management scenario. The location of farm plots, existing land tenure arrangements, farmers' awareness of the erosion problem and available expert advice on land management are analysed in the context of results of a questionnaire survey. It is concluded that land tenure conditions constrain land management, while the social stigma on livestock ownership and limited advice on soil conservation constrain positive measures toward erosion control. Most constraints are beyond the control of individual land users.

Rainfall simulation studies on rangeland in the Ntondozi area of Swaziland showed that soil loss decreased exponentially with increasing vegetation cover. Vegetation exerted an important hydrological control by increasing the infiltration capacity of the soil which, in turn, influenced the time to and duration of runoff. The expected effects of vegetation on soil protection and soil strength were not demonstrated. Instead, the amount of soil loss occurring in an individual storm appeared to depend on the supply of loose material on the surface which could be transported by the runoff.

Soil erosion has worsened considerably in the Middleveld of Swaziland over the last 20 years. Most of the erosion has occurred on deep colluvial and saprolitic materials and is associated with convex-slope breaks and rejuvenation shoulders on valley sides. Although the risk of erosion seems to reflect natural instability in the landscape, the way in which the land is used determines the ultimate severity of the problem. Land systems can serve as functional units as far as erosion assessment is concerned. The most severely eroded land occurs within the Manzini, Jabuleni, Lobamba and Chibidze land systems. The universal soil loss equation (USLE) and the soil loss estimator for southern Africa (SLEMSA) give vastly different estimates of the rate of soil loss but no information is available to validate or refute the predictions.

The main conclusions of the research project on soil erosion and sedimentation in Swaziland are as follows. (1) Soil erosion has worsened over the last 20 years. The proportion of an 1800 km² study area in the Middleveld classified as ‘high erosion class’ has increased from 6.7 to 13.6% between 1972 and 1990. (2) Gully erosion is the main process and is concentrated within the Manzini, Jabuleni, Lobamba and Chibidze land systems, all characterised by soil-saprolite complexes. (3) Overgrazing and compaction along paths and tracks lower the infiltration rate of the clay-rich ferralitic soils, promoting surface runoff and the formation of rills. (4) Once the rill deepens and cuts through the soil on to the underlying saprolite, gullies develop rapidly because of the low shear strength of the material. (5) Soil erosion problems are compounded by the existing system of land tenure and increasing pressure of livestock and population on the land. (6) In some instances, population pressure can lead to improvements in land management and better soil protection. Recommendations for controlling soil erosion and for further research are presented.

At sample locations in the Swaziland Middleveld comparison of erosion severity class at two dates (1972 and 1990) shows where the incidence of erosion has decreased (stabilising sites) or increased (destabilising sites). The changes in erosion status are analysed in relation to on-site changes in vegetation, land use, path density, settlement density and land management practice (including soil conservation measures). The results show that it is impossible to identify simple cause and effect relationships. Criticisms of the methodology are presented. More research is recommended on the processes and mechanisms which influence the observed trends in erosion severity.