COVER CROPS in South African Vineyards

CHAPTER 8 EFFECT OF COVER CROPS ON GRAPEVINE NUTRITION AND PERFORMANCE

nitrogen can help to complete the development of the permanent structure of all types of young non-bearing grapevines. From 1999/2000 onward, the cover crops that had previously been sown every two years were sown annually, and chemical control started in the first week of October. Earlier control combined with increased organic carbon in the soil helped to improve the nitrogen status of the grapevines in these treatments. Soil organic carbon levels are discussed in more detail in Effect of Cover Crops on Soil Carbon and Minerals in Chapter 7 (p124). Nitrogen levels in the grape juice from vines grown in the legume cover- crop treatments are shown in Figure 8.23. In general, nitrogen levels were higher in the treatments than in the control during the 1994/95 and 1997/98 seasons.This was also true for grasses in the 1994/95 season. trol rol ic medic

1400 1200 1000

800 600 400

1994/95 1997/98 1998/99 2002/03

200

0

Nitrogen in milligram per litre

Sown annually

Sown annually

Sown annually

Sown annually

Sown two-yearly

Sown two-yearly

Sown two-yearly

Sown two-yearly

Chemical control

Mechanical control

Paraggio bur medic

Parabinga bur medic

Pink serradella

Grazing vetch

No cover crop

FIGURE 8.23. Nitrogen measured in the grape juice from vines in legume cover-crop treat - ments compared to a control in the Olifants River valley. Adapted from Fourie et al (2007).

High nitrogen levels in 1997/98 occurred in spite of the reduction in nitrogen fertiliser applied at grapevine bud-break to the legume cover-crop treatments. These results suggest that vines can be oversupplied with nitrogen both in the early part of the growing season, and from flowering to harvest, and that nitrogen may have to be omitted from the fertiliser program at full bloom to avoid an oversupply of nitrogen on these sandy soils.

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