Soil physical properties in relation to soil degradation rates in the Usambara Mountains, northeast Tanzania

Abstract

Soil erosion is one of the world’s most serious environmental problems (Jankauskas et al, 2008). It can cause extensive losses of crop yields and cultivated and potentially productive soils (Fullen & Catt, 2004; Morgan, 1995; Skoien, 1995). Soils which are highly eroded are more easily subjected to a redundancy of productivity and are poor environments for root growth, due to a degraded structure and lower organic matter contents (Frye et al., 1982; Lindstrom et al., 1994). The mountains and highlands of East Africa are facing the problems of land degradation as well, mainly due to the human impact on the natural ecosystems (Ezaza, 1992). This is enhanced by the high population density. Also deforestation of new land even on steep slopes and over-exploitation of mountain resources, due to the high population density has led to high soil erosion rates (Ezaza, 1988). Because this area is characterized by a steep and rugged topography and most people rely on agriculture due to the fertile soils and favorable agroclimate, many farms suffer from serious soil erosion. The aim of this research was to determine the relation of soil physical properties (i.e. aggregate stability, soil texture, bulk density, porosity, infiltration rate and soil cohesion) and soil erosion rates of several hill slopes with sparse vegetation in the Lushoto district situated in the West Usambara Mountains, Tanzania. To achieve this objective the soil physical properties of four highly erosive and four less erosive soils of hill slopes has been determined to find out which of the soil physical properties are contributing most to soil erosion. These hill slopes were approximately equal in amount of vegetation and slope angle. The MMF erosion model has been used to quantify the soil erosion for each hill slope where no erosion measurements are performed. The model has been validated on six other fields for which erosion measurements are performed by Wickama (2010). Aggregate stability was significantly higher for the non-erodible fields for both wet- and dryaggregates and is the most important soil physical property for erodibility of these fields. Difference between erodible and non-erodible fields for wet-aggregate stability is even more pronounced than the difference between these for dry-aggregate stability and shows that water is more important in eroding the soils than wind. Infiltration capacity values were equal higher for the erodible than non-erodible fields. This is contradicting with the aggregate stability values. Due to the installation of the double ring infiltrometer in the crust, which is formed by the disruption of aggregates, the crust is disturbed and infiltration can occur more easily than normally. Variation in profile depth and the occurrence of rubbers and plastics in the soils could have had a minor role in effecting the infiltration rates. Differences between soil texture for erodible and non-erodible fields are only apparent for the fine sand when both depth layer were taken together. For all eight research fields silt and clay content were such that the soils were not vulnerable to erosion with respect to soil texture. The eight fields of investigation, when compared for the erodible fields and non-erodible fields, were quite similar with respect to soil profiles. The erodible soils were somewhat more red and less yellow or brown. From this it can be concluded that on the erodible fields leaching of minerals has occurred leaving only iron and/or aluminium oxides in the soil. No differences occur with respect to soil cohesion and shear strength on the erodible and nonerodible fields. The same applies for bulk density and porosity. The MMF erosion model shows that the erodible and non-erodible fields are approximately equal in erosion rates in case of average rainfall, though when the ratio between rainfall and amount of rain days increases (rain intensity), which is predominantly the case when the amount of rainfall is above average, differences in the amount of erosion become clear: the erodible fields experience more erosion than the non-erodible fields. Protection of the fields to erosion is predominantly due to a high aggregate stability.