SOIL PREPARATION

An abrupt transition from the overlying horizon to the clay will cause free water to accumulate above the clay. The effective depth of soils with clay in the subsoil is directly limited by the clay and also indirectly by the recurrent waterlogging in the overlying more sandy layer. 2.5 Hardpans In South Africa three different types of hardpans that restrict root penetration, occur as described below. 2.5.1 Hard Plinthite Hard plinthite consists of iron (and sometimes also manganese) oxide. It normally forms under the influence of a fluctuating water table, but due to its resistance against erosion it is also found as relic material in old landscapes with no active water table. Genetically, hard plinthite is similar to soft plinthite, but the process of localisation and accumulation of iron-oxide have, however, continued much further to a stage where the material has hardened to such an extent that it cannot be cut, even when wet. Internally the structure of hard plinthite can vary from porous to massive and in most cases can be broken up to allow root penetration. In the South African classification system, hard plinthite is accepted as a B horizon, e.g. in Dresden, Wasbank (Figure 2.6) and Glencoe soil forms.

FIGURE 2.6: Hard plinthite in the subsoil of a Glencoe soil. Without deep loosening, this soil will be marginal for grape production (Photo: B. Oberholzer, Nulandis).

26 | SOIL IMPEDIMENTS TO ROOT PENETRATION