COVER CROPS in South African Vineyards

CHAPTER 1 INTRODUCTION

40,00

4,00

35,00

3,50

30,00

3,00

25,00

2,50

Water run-off Irrigation Water run-off Rainfall Erosion Rainfall

20,00

2,00

15,00

1,50

10,00

1,00

Soil erosion in tonnes per hectare

5,00 Water run-off as percentage of total

0,50

0,00

0,00

Bare soil

Straw mulch

Cover crop

Research conducted in vineyards by Louw and Bennie (1992) showed that rainwater runoff was approximately 27% on bare soils compared to about 1% on soils covered by either mulch or a cover crop. Irrigation runoff was approximately 38% on bare soils compared to about 0.6% on soils covered by either a mulch or a cover crop (Figure 1.2). Bare soil is vulnerable to erosion by wind and water, with the most fertile topsoil the first to be lost (Figure 1.3). Soil loosened by preparation is especially vulnerable to erosion, so it is wise to lay out the vineyard and establish a cover crop immediately after preparation. The mulch created by this cover crop can also help to suppress weed emergence in newly established vineyards. In the work done by Louw and Bennie (1992), approximately 3.8 tonnes of soil was lost from bare soil per hectare after seven rain events, compared to losses of 0.5 tonnes per hectare from soils that had cover crops, and 0.03 tonnes per hectare from mulched soils. Louw and Bennie (1992) found that a mulch of 6–8 tonnes per hectare should eliminate runoff from and erosion of most soils. Other studies have found that mulches of 5 tonnes per hectare prevented crust formation, water runoff, and soil erosion. Loch and Donnollan (1988) indicated that as little as 0.1 tonnes per hectare significantly reduced erosion. FIGURE 1.2. Water run-off during a 46 millimetre irrigation event, and measured over 34 rainfall events, and soil erosion during seven rainfall events, on a Glenrosa soil. Adapted from Louw (1991).

22