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Helgren (1982) and Graz (unpublished data) found that the sand component in these soils between 95% and 99%.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Graz (unpublished data) found a significantly higher pH in the upper soil layers than lower down, in frequently burnt areas.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.
 
See also:
Fire regime