Received for publication 14 July 1981. Adapted from part of the author's PhD dissertation in agriculture. Appreciation is extended to my promoter, H.W. Weber, of the Department of Soil and Water Science, Univ. of Stellenbosch, for initiative and guidance. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.