COVER CROPS in South African Vineyards

CHAPTER 7 EFFECT OF COVER CROPS ON SOIL CARBON AND MINERALS

Organic carbon The trial site had medium-textured soils with approximately 27% clay. Soil organic carbon in 1994, one year after the start of the trial, averaged 0.57%, and was similar in the different treatment plots. Figure 7.1 shows soil organic carbon in 1994, 1996 and 2003. Levels of organic carbon in the 0–150-millimetre soil layer were significantly increased after two years in the triticale and grazing-vetch treatments, and in the triticale-vetch two-yearly rotation. After nine years, soil organic carbon had increased in both the 0–150-millimetre and the 150–300-millimetre soil layers in all the treatments with annual cover crops. For all annual cover crops, there was sufficient soil organic carbon that no fertiliser would be required for wine grapes. In table- and raisin-grape vineyards this increase in soil fertility could increase the harvest, thereby boosting profits. Soil organic carbon initially increased in the perennial fescue treatment, but its performance declined over the course of the trial, due to invasion by perennial weeds such as field bindweed. Poor long-term performance is one of the reasons why perennial cover crops are not recommended. The increase in soil organic carbon in the control without a cover crop is ascribed to the dominance of annual grasses, notably ripgut brome, on the relatively fertile soils at this site. ation ion

1,20

1,00

Triticale Rye|faba bean mix Triticale|vetch two-yearly rotation Triticale|vetch annual rotation

0,80

0,60

Grazing vetch Dwarf fescue No cover crop

0,40

0,20

Percentage soil organic carbon

0,00

1994

1996 Year

2003

FIGURE 7.1. Soil organic carbon measured in the 0–300-millimetre soil layer in cover-crop treatments compared to a control in the Breede River valley. Adapted from Fourie (2012).

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