Soil characteristics affecting water availability
Plants require water for growth and development. The ability of a
plant to obtain water is therefore of great importance. Since plants
obtain their water from the soil, soil texture
and soil chemistry are discussed here primarily
in terms of its effect on water availabiltiy.
Soil Texture and Organic Matter
Soil Texture
The dry savannah woodlands are generally found on relatively coarse Kalahari
sand. The dunes and higher lying plains identified by Graz
(1999) are comprised of aeolian sands that have been stabilized by a vegetation
cover.
Helgren (1982) and Graz (unpublished
data) found that the sand component in these soils between 95% and 99%.
This results in a high infilltration rate, with little or no run-off even
in heavy showers. At the same time, however, the soils have a very
poor waterholding capacity.
Organic Matter
While organic matter in the soil generally improves a soil's water holding
capacity, the environmental conditions are not supportive of soil organic
matter build up. On the one hand the highly variable soil water content
particularly in the top soil, severly retard decay. On the other
hand, periodic fires reduce any accumulation (Malaisse
et
al. 1992).
Soil Chemistry
In addition to the direct effects of fire on the vegetation, the ash left
behind by the fires changes the soil chemistry. Minerals accumulate
especially in the upper layers of the soil (Frost
& Robertson 1987, Graz 1996),
changing its water potential (particularly osmotic potential). Mitlöhner
(1999) suggests that the changes in osmotic potential of the soil due to
an increased fire frequency have caused the die-back of Pterocarpus
angolensis and Terminalia sericea in areas that are frequently
burnt. Both species have a relatively low water potential.
In addition to the change in the availability of nutrients, Harrington
& Ross (1974) report a marked increase in the pH of the soil.
Trapnell
et
al (1976) found that late
season fires
caused greater changes in pH and P than did early fires.
The change in pH makes water uptake more difficult (Grobelaar et al.
1979).
Graz (unpublished data) found a significantly higher pH in the upper
soil layers than lower down, in frequently burnt areas. This change
in the soil solution, especially of the upper soil layer, is of particular
importance for seedling establishment.