SOIL PREPARATION

Conventional delve ploughs (Figure 7.5) are normally recommended for mixing texturally layered Dundee and Oakleaf soils, and sometimes also on red and yellow-brown apedal soils (Hutton and Clovelly) (ARC Infruitec-Nietvoorbij, 2002). Due to the shape of its mouldboard, this plough causes turnover of the soil and consequently its use will bring subsoil with unfavourable physical and chemical properties to the surface. The topsoil will be buried which is a further undesired effect. Clayey material also tends to break into large clods.

FIGURE 7.5: Delve plough (left) (Photo: ARC Infruitec-Nietvoorbij) used for soil mixing when there is no danger of bringing poor subsoil to the surface. Sketch with dotted lines (right) shows flow path of soil (Sketch: J.E. Hoffman, Stellenbosch University).

The shift-delve plough differs from a conventional delve plough with regard to the size and shape of the mouldboard (Figure 7.6) (ARC Infruitec-Nietvoorbij, 2002). The mould board can be modified in several ways. The upper part can be shortened to create a mouldboard with sides parallel to each other from top to bottom. Furthermore, by cutting the top in such a way that the top edge slope slightly downwards, a considerable amount of topsoil will flow over the board and mix behind it with the subsoil. It can also be made less concave; in fact most mouldboards are almost flat currently. The slope from bottom to top can be decreased as well as the angle from front to back. The angle of the point of the shear can also be changed from a normal 45° to 75° (compared to the horizontal). In some cases the middle third of the mouldboard is cut away to create an opening in the board. The larger the opening and the more vertical and narrow the mouldboard, the less subsoil will be brought to the surface and the different soil layers will mainly be displaced sideways. Such variations of the standard delve plough are used on soils having a high density in the subsoil,

114 | CHOICE OF IMPLEMENT FOR SOIL PREPARATION