Physico-chemical findings related to the resilience of different soils in the semi-arid parts of Tanzania and their implications on sustainable agriculture

Abstract

As part of an interdisciplinary study on Sustainable Agriculture in semiarid Areas (SASA) in south-central Tanzania, soils from five different locations were investigated for their mineralogical composition and physico-chemical characteristics in order to determine their resilience in relation to land use. The results of the investigation show that soils developed in such areas on sediments of Holocene age to a certain extent are capable of retaining their chemical fertility as long as low intensity agriculture is being practised. The content of weatherable minerals in the quite silt-rich soils is so large that the weathering taking place, seems sufficient under the prevailing semiarid conditions to ensure adequate supply of plant nutrients for low intensity crop production with the exception of N, P and S. The sand and silt fractions contained substantial amounts of mica and plagioclase and smaller amounts of hornblende/pyroxene. In addition to kaolinite and iron oxides, the clay fraction contains illite and smaller amounts of other 2:1 layer silicate clay minerals. The extractable amount of P is low, although the P-retention is expected to be limited, as the content of P-fixing components is limited. In contrast, the soils in the area formed in situs on metamorphic rocks contain only quartz and very small amounts of microcline and muscovite in the sand fraction. The silt fraction is high in quartz too besides having some kaolinite and gibbsite, The clay fraction of these soils is composed mainly of kaolinite with smaller amounts of Fe-oxides and illite, Chemically, the capacity of these soils is so low that they will not to any degree release any plant nutrients by weathering. The extractable P is also low in these soils, as the P present may be considered to be strongly retained due to the mineralogical composition of these soils. Due to their content of stable micropeds the strongly weathered soils developed in situs are more porous than the less strongly weathered soils developed on the Holocene sediments. However, their available water holding capacity may be expected to be lower than that of the soils developed on the sediments, as such soils normally have a limited number of pores in which plant available water is stored.

Keywords: different soils, semi arid puts

Tanzania J. Agrlc. Sc. (1999) Vol. 2 No.1, 37-52