Improvement of the nitrogen content of grape must with fertilisation

English: Stuck fermentation is currently a serious production associated problem in the South-African wine industry. This can mainly be attributed to insufficient levels of nitrogen in grape must, thus are not being able to supply in yeast demands. This study was undertaken to investigate whether the nitrogen content of grape must can be improved by fertilisation. In order to achieve this three nitrogen application treatments (N1C = 20 kg N/ha-¹ post-harvest; N2F = 20 kg N/ha-¹ post-harvest, followed with 20 kg N/ha-¹ after budbreak and fruitset respectively; and N2V = 20 kg N/ha-¹ post-harvest, followed with 20 kg N/ha-¹ after budbreak and veraison respectively), were applied to six grape cultivars (Cabernet Sauvignon, Chenin blanc, Pinot noir, Weisser Riesling, Chardonnayand Pinotage) for two seasons (1999/2000 = 1st season and 2000/2001 = 2nd season). The vineyard is situated on a high potential loam soil, classified as a red-brown Oakleaf, with a mean organic matter content of 1.4%. The effect of these nitrogen application treatments on the growth characteristics, grape must composition, leaf nitrogen content, as well as the soil nitrate content, were measured. Nitrogen applications during the vegetative phase had a positive effect on the shoot length and shoot elongation during both seasons, although more so during the 1st season. The pruning mass was significantly affected by cultivar during both seasons. Treatments receiving no nitrogen fertilisation during the vegetative phase (N1C) resulted in higher free amino nitrogen (FAN) and FAN/oB ratio in grape must, indicating a negative reaction to nitrogen fertilisation for soils having a high organic matter content. No nitrogen fertilisation during the vegetative season resulted in significantly lower bunch and berry mass values than those receiving fertilisation. This might indicate a negative relationship between bunch and berry mass and the FAN content of grape must. The nitrate content of the soil differed for the different soil depths and was affected by sampling date and N treatments. Results indicated that 70% of the nitrate was available in the top 30 cm of soil. Nitrogen fertilisation during the vegetative season on soils with an organic matter content of 1.4% should be strongly discouraged. This study therefore indicates that although high organic matter content and inorganic fertiliser applications may increase the nitrate content of the soil, this might not have the same effect on the nitrogen status of the vine.