COVER CROPS in South African Vineyards

COVER CROPS IN SOUTH AFRICAN VINEYARDS

FIGURE 1.3. Soil erosion in a vineyard with no protective mulch or winter-growing cover crop. (Photo: ARC Infruitec-Nietvoorbij).

Besides improving infiltration, cover crops and mulches also reduce water loss by creating a barrier to evaporation and by maintaining cooler soil temperatures.Water savings associated with cover crops are described in Effect of Cover Crops onWater Use in Chapter 6 (p118). Researchers have also found that microbial biomass and activity increase under surface residues, facilitating the release of available forms of nutrients from inorganic and organic sources. Grapevine nutrition Nitrogen contribution by cover crops depends on the species and cultivar, length of growing season, climate, and soil conditions. Legumes are well- known for their ability to fix atmospheric nitrogen. Researchers have shown that the amount of nitrogen fixed by annual medics is related to their total dry-matter production, and nitrogen fixation by legumes is greatest during flowering and pod fill.Temperate legumes retain 10%–29% of the fixed nitrogen in their roots. Higher levels of organic matter promote nutrient cycling by microbes in the soil and thereby the availability of nutrients to vine roots.The slow release of nitrogen by microbial activity supplies vines with additional nitrogen throughout the growing season. For wine-grape vines, Conradie (1994) states that soil with a clay content of 6% or more may provide wine-grape vines with sufficient nitrogen when

23