SOIL PREPARATION

CHAPTER 8

SUMMARY AMELIORANT APPLICATION

• A large percentage of Western Cape soils have a pH KCl < 5.0 and must be limed. Lime is poorly soluble in water and its neutralising effect does not move downwards in the soil. Consequently lime must be mixed with the soil during soil preparation. Calcitic lime must be applied when Ca:Mg ratio is < 6 while dolomitic lime is needed at a Ca:Mg ratio > 6. Attempts to blow lime into the soil behind the plough share have been discarded, and currently lime is worked into the topsoil followed by deep tillage that mixes it throughout the soil profile. • No proof exists in South Africa that the incorporation of organic matter during soil preparation has any beneficial effect on grapevine performance. More research re- garding this is, however, needed. The growing of a legume as cover crop before soil preparation is recommended to ensure a good reserve of organic matter and

nitrogen during establishment of young grapevines. • The phosphorous requirement, based on soil analyses to a depth of 60 cm, should be applied during soil preparation. Superphosphate can be applied on the soil surface and incorporated during soil preparation. In high pH soils (pH KCl > 7) the quantity of P applied at soil preparation can be reduced while annual applications can be increased. Band placement of P is preferable when the quantity of P in clayey soils becomes too great to be economical. • Gypsum is applied when soils are saline i.e. when the exchangeable sodium percentage > 15 and the soil has more than 6 % clay. The amount of gypsum applied during soil preparation should be calculated for 90 cm depth. Fifty percent of this quantity can be incorporated into the soil during preparation and the rest applied on the soil surface to leach down through rainfall and irrigation.

SOIL PREPARATION | 141