SOIL PREPARATION

CHAPTER 7

soil surface. Such slaked clays will cause poor water infiltration and enhance runoff. The direction of deep tillage should be perpendicular to the contour in order to improve lateral drainage. The wing plough will be a suitable implement for duplex soils.

FIGURE 7.19: Clay in the subsoil brought to the surface by using the wrong implement during soil preparation (Photo: J. de Jager, Vinpro).

Duplex soils with a strong structure directly underneath an Orthic A horizon, namely Swartland, Sterkspruit, Valsrivier, Sepane and Klapmuts forms, are challenging when it comes to soil preparation. The structural units swell and shrink in reaction to wetting and drying of the soil and inhibit root penetration. If the clayey B horizon is thin and overlies better material such as saprolite, a wing plough or shift-delve plough can be used for preparation. When the topsoil is relatively deep, e.g. > 50 cm, these soils can still be loosened using a plough that penetrates the clay 20-30 cm. It will be best to prepare such soils while in a dry condition. The close packing of the structural units in its natural state will be destroyed and space will be created for root growth. A basic principle in preparing these soils is prevention of the structural units to be ploughed to the surface. On clay layers which are shallow (< 40 cm) and thick, ridging should be done for vineyards to be sustainable.

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