What you have to know about soil classification of Zimbabwe
Soil taxonomy
Orders
Groups
Family
Series
The concept of catena
Importance of soil classification
Distribution and use of soils in Zimbabwe
Reference
Nyamapfene, K.W., 1985. Distribution and management of some problem soils of Zimbabwe. FAO
World Resource Bulletin No 58.
Soils of Zimbabwe
Figure Error! No text of specified style in document..1: Distribution of major soil groups in Zimbabwe
(Source: Department of Surveyor General 1979)
The soils of Zimbabwe can be classified into eight groups. These are the regosols, lithosols,
vertisols, siallitic, fersialitic, paraferralitic, orthoferralitic and the sodic (Nyamapfene, 1991). The
fersialitic soils are the most extensive soils in Zimbabwe. These are mainly formed from granite
rocks. The soils are predominant in the middleveld and the central watershed (Figure 2.2). The
fertility of the fersialitic soil is moderate hence a wide range of crops can be grown on them under
rainfed and irrigated agriculture.
The siallitic which are rated number two in terms of area covered are mainly found in low rainfall
areas and hence have a high base status, and clay contents. The soils can however be very shallow
(less than 25 cm) and this tends to limit the rooting depth. The other notable limitation of these
soils is the aridity of the environments in which they occur.
The lithosols which are rated third in terms of area covered occur in flat areas in the low rainfall
areas especially in areas where the rocks are resistant to weathering, hence limited profile
development (Nyamapfene, 1991). In the high rainfall areas, they occur in areas where erosion
rates are high. These soils are not arable but are used for ranching purposes.
The regosol group is rated fourth in terms of area covered. The soils are mainly found in the
Nyamandlovu area north of Bulawayo and also in the Gonarezhou area. There are also some
sporadic occurrence in Mvuma and Chivu. The limitations of theses soils include low water
holding capacity, low permeability and low nutrient reserves. They are therefore used for
commercial forestry and also for production of crops like millet and /or sorghum (Nyamapfene,
1991).
The paraferralitic, orthoferralitic and sodic soils have the same rating in term of area covered. The
paraferralitic are found in association with granite rocks only. The paraferralitic soils are found in
upland areas in Harare, Marondera and Headlands. The soils are also found just outside Harare
eastwards through Epworth. There is a long belt that extends southwards through Wedza to the
Range - Chivu and then Buhera (Nyamapfene, 1991). The soils are rich in potassium since they
are derived from granite rocks. These soils are the best for tobacco and maize production.
Groundnuts can also be grown on the soils but in all cases, there is need to apply nitrogen and
phosphorus. Liming is also essential (Nyamapfene, 1991).
The orthoferralitic soils are mainly found in areas where rainfall is greater than 900 mm per
annum especially in the Eastern Highlands, Bikita - Ndanga, Mvurwi, Marondera and Headlands.
Those orthoferralitic soils in the Eastern Highlands are more strongly leached and have
considerable organic matter. These soils generally have high clay content and are potentially
productive (Nyamapfene, 1991).
Sodic soils occur in the low rainfall areas, which include the Save and Zambezi Valleys. In
moderate rainfall areas, interaction of parent material and relief results in significant soils for
example Gokwe, Chivu and Chegutu. The soils have undesirable physical and chemical
properties for example, their high pH which inhibits nutrient availability (Nyamapfene, 1991). In
the low rainfall areas farmers do grow millet and sorghum on them since these can tolerate both
high pH levels and sodium (Nyamapfene, 1991). This practice is common in areas where surface
horizon depth is 25-30 cm overlying dense impermeable subsoil. In the latter case the surface soil
that is light to medium textured acts as the rooting medium while the dense subsoil caters for
water storage. In most cases however, the soils are shunned for agriculture and better left for
grazing.
2.2.1 Soil classification
Soil classification is an artificial means by which similar soils are grouped together. The
classification of soils is very important since it forms the basis for scientific exchange of
information about soils. The soil classification currently in use in Zimbabwe was developed in the
middle of the twentieth century (Thompson, 1965) and was largely based on the principles used
by the Inter - African Pedological services (S.P.I) for the soil map of Africa. A cardinal principle
is that the classification is based on the degree of weathering and leaching to which the soils
derived from a given parent material have undergone. The classification system by Thompson
and Purves (1978) has some agronomic bias as it includes those characteristics considered
important in agricultural production which include depth, water holding capacity, nutrient status
and structure.
The approach used in Zimbabwe has been from the upper taxonomic level going downwards.
Most of the important soil characteristics and properties are defined at higher taxonomic levels.
The highest taxonomic level is the order followed by the group then the family and lastly the soil
series level (Nyamapfene, 1991). In agroecological zoning, the most important taxonomic level is
the group level since at this level, important soil characteristics such as depth, texture, nutrient
status and water holding capacity can be inferred and hence soil productivity.
2.2.2 Soils and agroecological zoning
Soils equally play a pivotal role in the possible land utilization types but the climate usually
dominates the other factors limiting agricultural production. The degree of land utilization
depends on the soil type (Vincent and Thomas, 1960). The soil type is an indicator of the inherent
fertility of the soil, governs nutrient availability to crops, determines how much water is stored in
the root zone, the time during which the water is stored for plant use and therefore how much
water is available for plant use (Brady, 1999). Therefore, mean annual rainfall amount cannot be
used as a single indicator in land evaluation.
According to Cook et al. (2000) land suitability can be done on assumptions that certain
economically feasible improvements have been made on the land. In the case of the soils of
Zimbabwe, the orthoferralitic soils, paraferralitic soils and fersialitic soils which have lower
fertility than the vertisols and siallitic soils can be made highly productivity when certain
nutrients are added to the soil. The same cannot be said about the lithosols whose limitation to
cropping is depth and also the regosols which have a very poor structure. As for the sodic soils
the major problem is that of high bulk density and poor structure (Nyamapfene 1991).