Tanzania Journal of Agricultural Sciences 1999, Vol. 2(1) : 37-52
As part of an interdisciplinary study on Sustainable Agriculture in Semiarid Areq.s (SASA) in
south-central Tanzania, soils from five different locations were investigatedfor their mineralogical composition
and physico-chemical characteristics in order to determine their resilience in relation to land
use. The results of the inves~igation show tha,t soils developed in such areas on sediments of Holocene
age to a certain extent are capable of retaining their chemicalfertiliry as long as low intensity agriculture
is being practised. The content ofweatherable: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 onlyquar:tz andyery small amounts of
microcline and muscovite in the s{mdfraction. The siltfraction 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-ox ides 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 ,stronglyretained 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