The chemistry of the pedoderm - part 4: grasslands and savannas within Mokala National Park, South Africa

Treed savannas and treeless grasslands occur across Mokala National Park, South Africa. To identify soil properties potentially underpinning these marked differences in vegetation structure, we analysed the soil chemistry of 27 composite pedoderm samples in both savannas and grasslands. Savannas had a greater pH and greater mean concentrations of P, K, Ca, Mg, Mn, Cu, Zn, B, Na and C compared with the grasslands. Tree seedlings are likely to be more constrained in the grasslands than in the savannas because of Al toxicity and deficiency of P, Cu and B. Of the nutrients analysed, B is likely to have had the greatest effect on vegetation structure for two main reasons. Firstly, the mean concentration of B in the grassland (0.05 mg kg(-1)) was approximately ten times less than the concentration expected to constrain plant growth. Secondly, the extreme scarcity of B is likely to have reduced the vigour of tree seedlings more than that of the grass sward because grasses have a considerably lower requirement for B than eudicot trees. We suggest that abiotic soil forming processes such as colluvial action and aeolian deposition are the primary factors influencing pH and concentrations of B, both of which have a disproportionate effect on tree establishment.