SOIL PREPARATION

will give an indication of the cause of the water table and also of the drainage system to be installed. Profile pits for drainage purposes should be at least two meters deep or should be dug until they reach an impenetrable layer, whichever comes first. The type of information collected in these profile pits are as follows (Western Cape Department of Agriculture, 1998): • Relative height of the land using topographic measurement at the profile pit. • Description (thickness, depth, colour and texture) of each soil layer. • Height of the water table if possible. • Position of mottling that could indicate a fluctuating water table. • Depth of the impenetrable soil layer. • Depth of the profile pit. Drainage systems must take into account its two main components to be effective, namely land surface drainage and subsoil drainage. 6.3 Surface drainage Surface drainage refers to the removal of water that collects and ponds on the soil surface (Reinders et al ., 2016). Ridging, levelling of vineyard soils, the construction of surface inlets to subsurface drains or shallow ditches (waterways) are typical techniques to remove surface water and prevent prolonged periods of water ponding. Surface water must be removed at a flow rate that will prevent flooding and soil erosion and ensure good growing conditions. The outlet from surface drainage structures should ideally flow into a natural waterway that can follow the natural stream line (natural waterways). If the natural water courses are too far apart, the higher parts of a land have to be drained or, for a variety of other reasons, artificial waterways have to be constructed. The outlet from surface drainage structures should flow into a permanently grassed waterway, into an underground outlet or onto soil that has an adequate infiltration capacity to handle the runoff. Grassed waterways should preferably be shallow and wide. The design of such waterways should leave some topsoil (minimum 100-150 mm) after construction of the waterway for the establishment of grass. Several grass types can be used as a permanent vegetation cover such as Kikuyu, indigenous Kweek grasses ( Cynodon dactylon ), Rhodes grass ( Cloris gayana ) and Oulandgras (Eragrostis curvula). Grasses adapted for a specific climatic area and soil should however, be selected, e.g. Rhodes grass is well suited to saline soil, while Eragrostis curvula does not grow well in dry regions. For the winter rainfall region a local winter grass, Eragrostis trichoides is recommended. Grassed waterways have to be managed properly. The grass should be cut, but not too short (approximately 15 cm above the ground). It should be fertilised annually. Furthermore, the waterways should neither be used as access roads nor for grazing since they may lead to footpaths, and subsequently, to erosion.

88 | SOIL DRAINAGE