SOIL PREPARATION

CHAPTER 10

A comparison of the three methods of soil preparation at Robertson showed no effect on pruning mass, but the 120 cm delve treatment yielded a significant cumulative gain of 30 tonnes of grapes per ha compared to shallow ploughing over a period of nine years. Grape yield of ripper plots was intermediate to those of delving and shallow ploughing, indicating that ripping might be an inferior method compared to delve ploughing on this soil type. The lack of response in pruning mass was probably due to the practice of regular topping of shoots during the growing season. A higher coefficient of variation for pruning mass compared to grape yield may also have been part of the reason. At Stellenbosch, similar to Robertson, plots that were delve-ploughed to depths of 75 cm and 120 cm respectively, gave similar pruning masses and grape yields. However, a comparison of delve-ploughed, ripper and shallow- ploughed plots (control), showed that the delve treatment grapevines were significantly more vigorous than their shallow-ploughed counterparts during the first six years after planting and also grew better than vines on ripper plots during three seasons. Furthermore, grape yield on delve-ploughed plots was significantly higher than on shallow-ploughed plots (control) and during two seasons outperformed vines on ripper plots (Figure 10.1). In the same field trial, foliar and soil analysis data indicated sub-optimal nutrition in respect of P, Ca and Mg. No relationship occurred, however, between either foliar analyses or soil analyses data with grapevine performance. This confirms the predominance of physical soil properties over soil chemical properties in determining grapevine performance. A difficulty experienced in some of the early soil preparation experiments was the inability of implements to penetrate to the soil depth specified in experiments. In a follow-up study, a penetrometer was used to assess the accuracy of the treatments with regard to effective soil depth in the Stellenbosch and Robertson experiments mentioned above. Effective soil depth determined by the penetrometer showed a highly significant correlation with grapevine performance (Table 10.1). This linear relationship however, did not indicate an optimum soil depth for vine performance.

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