COVER CROPS in South African Vineyards

CHAPTER 7 EFFECT OF COVER CROPS ON SOIL CARBON AND MINERALS

Cover crops were sown annually in autumn. Full-surface chemical control was applied from just before grapevine bud-break to harvest in the treatments and the control. Figure 7.10 shows soil organic carbon in 1993, 1998 and 2003. In general, levels increased in the cover-crop treatments over the course of the trial, whereas levels decreased in the control. Organic carbon levels were higher in all the cover-crop treatments than in the control after ten years.

0,25

0,20

Henog rye Overberg oats Saia black oats

0,15

Paraggio bur medic Parabinga bur medic

0,10

Pink serradella Grazing vetch No cover crop

0,05

Percentage soil organic carbon

0,00

1993

1998 Year

2003

FIGURE 7.10. Soil organic carbon measured in the 0–300-millimetre soil layer in cover- crop treatments compared to a control in the Olifants River valley. Adapted from Fourie et al (2007). The decrease in soil organic carbon in the control is most likely due to the proliferation of soft herbaceous broad-leaved weeds that contribute little organic matter. These results demonstrate that rye, black oats, pink serradella, and grazing vetch can significantly increase organic carbon in the 0–300-millimetre soil layer of a sandy soil. Organic carbon in this soil layer increased more when the cover crops were left on the soil surface as a mulch, compared to when cover crops were mechanically incorporated into the soil at grapevine bud-break. Nitrogen in the soil Nitrogen was applied at 14 kilogram per hectare during the second week of April, when cover crops were sown, and at 28 kilogram per hectare when

136