Podzols in the Chimanimani Mountains (Zimbabwe)

C. Samimi, H. Wagenseil
Institute for Geography
University of Erlangen-Nuremberg
Kochstr. 4/4, 91054 Erlangen, Germany



1. Introduction

Podzols are soils found in all climatic zones, but dominantly in the temperate and boreal regions of the Northern Hemisphere (STÜTZER 1998). They are defined by a spodic B-horizon (FAO-UNESCO 1997). In typical podzols (Haplic Podzols) the spodic horizon underlies a pale grey, strongly leached albic E-horizon. The spodic horizon is characterized by an accumulation of organic material and/or iron and aluminium. The intensity of soil processes is governed by climatic, lithological and biotic factors. These factors also determine the development of the different soil units within the podzol soil group.

Geology Eastern Highlands Although podzols are found in the tropics, they seem to be limited to two specific ecological environments (VAN WAMBEKE 1992, YOUNG 1976). The podzols of the tropic lowlands typically develop over thick quartz deposits under mainly udic to perudic moisture regimes (KLINGE 1966, 1968). The second group is limited to cool isomesic temperature regimes at high elevations, such as those described by VAN RANST et al. (1997) in Rwanda.

In Zimbabwe climatic conditions similar to those in Rwanda can be found in parts of the Eastern Highlands which stretch for about 300 km along the border to Mozambique (Figure 1). The climate of the Chimanimani Mountains, a part of the Eastern Highlands, is characterized by an fluctuation between dry winters and wet summers as is typical for tropical southern Africa. No records are available for the study area. However the data of stations in the Eastern Highlands show that between 70 and 80 % of the rain falls from November to March (PHIPPS and GOODIER 1962), but even in the dry season rainfall is observed. The mean annual rainfall therefore reaches 1000 to 2000 mm, in the highest parts of the Eastern Highlands up to 3000 mm (PHIPPS and GOODIER 1962, DEPARTMENT OF METEOROLOGICAL SERVICES 1984). According to the altitude the temperatures are cool, ranging from 12 to 15 °C during the cold season and 18 to 26 °C during summer. Frost is common from May to October (PHIPPS and GOODIER 1962). The high precipitation combined with low temperatures cause a typical tropical mountain climate (Photo 1).

The geology of the Eastern Highlands and the surrounding regions is shown in Figure 2. The dominating rocks are dolorite, shale, schist, siltstone, sandstone, quartzite and gneiss. The examined soil profiles are limited to schists and quartzites as parent materials (Figure 3). These were studied along a transect between 1560 and 1760 m east of the main gate of the Chimanimani National Park (Figure 4)

.

2. Profile characteristics

Profile 1 is situated on a little slope-flattening under Uapaca kirkiana and Julbernardia globiflora woodland with a depth of 74 cm. The parent material are sandy, micaceous and chloritic schits. According to FAO-UNESCO (1997) it is classified as a ferralic cambisol. The classification is based on macromorphological, physical and chemical properties. Under an Ah-horizon two Bw-horizons developed. The color varies from 7.5YR 4/2 in the Ah-horizon to 7.5YR 5/8 in the Bw2-horizon.

Based on macromorphological properties Profile 2 is classified as a gleyic podzol (FAO-UNESCO 1997). The chemical properties partly support this classification matching the FAO-UNESCO (1997) criteria for spodic horizons (Table 1).

Table 1: Chemical criteria for spodic horizons (FAO-UNESCO 1997)
Feox in % (Feox+Alox)/clay > 0.2
if Feox > 0.1% 		(Alox+C)/clay < 0.2
if Feox < 0.1% 		(Feox+Alox)/
Fedi+Aldi) > 0.5
Profile 2: Gleyic Podzol
Bhr 0.14 0.14 0.4 0.36
Bsm 0.16 0.15 0.29 0.51
Profil 5: Haplic Podzol
Bh 0.03 0.04 0.3 0.12
Bsm 0.22 2.88 3.01 0.96

The cementation in the Bsm-horizon by iron causes stagnic properties visible in the pinkish grey color (5 YR 6/2) and the mottling of Fe3+ ions. The soil developed on quartz under grassland with ericaceous scrubs (Photo 2).

Profile 3 and profile 4, also ferralic camibsols, are less mature than Profile 1. They developed under Protea scrub with Festuca grassland on sandy, micaceous and chloritic schits.

Profile 5 is a haplic podzol. Like profile 2 its parent material is quartz. The pedon is situated on a plane under grassland with ericaceous scrubs (Photo 3). Under a thin Ah-horizon transition horizons are found to a depth of 29 cm. They are followed by a bleached, almost white (7,5 YR 8/1) E-horizon. It is almost without any iron, organic material or clay. The translocated material is accumulated in a Bh-horizon and a cemented Bsm-horizon. Both horizons matched the FAO-UNESCO (1997) criteria for spodic horizons (Table 1).


3. Conclusions

Along the studied toposequence in the Chimanimani Mountains two different soil types have developed which are strongly related to the parent material. The climatic change from an altitude of 1560 m to 1760 m seems to have no major influence on the soil forming processes; the climatic gradient is accordingly very small.

The soils on the schists can be classified as ferralic cambisols (FAO-UNESCO 1997). Beneath an Ah-horizon the soils show a well developed Bw-horizon, which can be divided in profile 1 and 3. None of the profiles show any sign of podzolisation. The Fe-ratios and the mineralogical properties of the clay fraction indicate tropical soil forming conditions.

On quartzites podzols which have developed, profile 2 is classified as gleyic, profile 5 as haplic (FAO-UNESCO 1997). In both soils the mobility of sequioxides qualify for spodic horizon according to FAO-UNESCO (1997) and organic material is translocated. The soil forming conditions derived from the mineralogical properties are comparable to the cambisols.

Therefore the differences in the parent material control the development of the soils along the toposequence. The mineralogical differences of profile 1 and 3 compared to profile 2 and 5 pertain mainly to the concentration of Al2O3 and Fe2O3 of the parent material and to a minor extent to the size fraction with slightly higher contents of the silt and the clay fraction. The parent material of profile 4 has a very similar mineralogical composition to profile 2 and 5 but the size fraction and therefore the permeability are very different.

Compared to the podzols described in Rwanda under similar conditions, the authors have determined that the chemical indications for the process of podzolization are comparable (VAN RANST et al. 1997). As in Rwanda, the soils are podzols using the macromorphological properties even when the chemical indications are not very clear.


4. Acknowledgements

We thank A. Stützer for numerous discussions. R. Kalchschmid and E. Ebenhöch helped during the field campaigns under harsh weather conditions. DaimlerChrysler partly funded our projects in Southern Africa.


5. References

FAO-UNESCO (1997): Soil Map of the World. Revised Legend, with corrections and updates. World Soil Resources Report 60, FAO, Rome. Reprinted with updates as Technical Paper 20, ISRIC, Wageningen.

KLINGE, H.C. (1966): Verbreitung tropischer Tieflandpodsole. Naturwissenschaften, 53: 442-443.

KLINGE, H.C. (1968): Report on Tropical Podzols. FAO. Rome.

PHIPPS, J.B.; GOODIER, R. (1962): A Preliminary Account of the Plant Ecology of the Chimanimani Mountains. Journal of Ecology, 50: 291-319.

VAN RANST, A.; STOOPS, G.; GALLEZ, A.; VANDENBERGHE, R.E. (1997): Properties, some criteria of classification and genesis of upland forest Podzols in Rwanda. Geoderma, 76: 263-283.

VAN WAMBEKE, A. (1992): Soils of the Tropics: Properties and Appraisal. New York.

WATSON, R.L.A. (1969): The Geology of the Cashel, Melsetter and Chipinga Areas. Rhodesia Geological Survey , Bulletin No. 60. Salisbury (today Harare).

YOUNG, A. (1976): Tropical Soils and Soil Survey. Cambridge Geographical Studies 9. Cambridge, London.

ZIMBABWE GEOLOGICAL SURVEY (1994): Geological Map of Zimbabwe, 1:1000000. Harare.