COVER CROPS in South African Vineyards

CHAPTER 7 EFFECT OF COVER CROPS ON SOIL CARBON AND MINERALS

These results show that a reduction in the amount of nitrogen applied at grapevine full bloom is possible after growing pink serradella and grazing vetch as cover crops for three seasons. The elevated nitrogen concentrations in the soil of the cover-crop treatments under minimum tillage probably came from the cover-crop roots. In a table- or raisin-grape vineyard, the slow release of additional nitrogen can contribute to increased production of quality grapes. Figure 7.12 shows the soil nitrogen levels after harvest for treatments where cover crops were sown every two years, and where full-surface chemical control was applied from the end of November to harvest. The levels of soil nitrogen were significantly higher in the grazing vetch and pink serradella treatments than in the other treatments and the control.

No cover crop Grazing vetch Pink serradella

Parabinga bur medic Paraggio bur medic

Saia black oats Overberg oats Henog rye

0,00 1,00 2,00 3,00 4,00 5,00 6,00 7,00 8,00 Nitrogen in milligram per kilogram soil

FIGURE 7.12. Soil nitrogen measured after harvest 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). Although soil nitrogen levels were also higher in the grazing vetch and pink serradella treatments when cover crops were sown annually, the difference was not significant. Therefore, there is more nitrogen available during the grapevine post-harvest period when the legumes are controlled chemically at grapevine berry set than when they are controlled at grapevine bud-break. These results suggest that the timing of cover-crop control affects the levels of nitrogen at grapevine full bloom and harvest.This creates an opportunity for the producer to manipulate nitrogen availability through cover-crop management during the grapevine-growing season on sandy soils.

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