COVER CROPS in South African Vineyards

CHAPTER 1 INTRODUCTION

Soil resistance in kPa x 10 2

0 2 4 6 8 10 12 14 16 *

10

*

*

20

* = soil surface

30

40

Soil after preparation After first pass After second pass

Soil depth in centimetres

50

60

FIGURE 1.5. Recompaction after two passes with a wheeled tractor on a prepared soil. Adapted from Van Huyssteen (1981).

Cultivation itself, also contributes to compaction.The ploughshares or discs from soil-cultivation implements that repeatedly pass at the same working depth will gradually seal the soil pores at that depth. Mechanical cultivation executed directly after soil preparation, to level the soil surface or establish cash crops before a vineyard is established, will cause the soil to compact at the working depth. Furthermore, rain and irrigation wash silt and clay particles from the loosened soil to the deeper soil layers, clogging the soil pores. This results in a compacted soil layer through which the grapevine roots penetrate with difficulty.The highest degree of compaction takes place when the soil water content is near field water capacity, which is unfortunately when soils are mostly cultivated (Figure 1.6).

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