FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

CHAPTER 6

Fertilisation on sands should preferably be done on a regular basis in smaller instalments. This is on account of sandy soils not having the capability to retain nutrients, and applied nutrients will leach easily out of the system. On clayey soils which are buffered better, the applications can be larger, because the risk of leaching is much less. The fact that fertiliser materials are salts and that large instalments will elevate the salt load in the root zone, should also be kept in consideration. This may have a temporary negative affect on root growth and nutrient uptake. Over fertilisation is not only a waste of money, but has other negative effects as well: • Leaching of, e.g. N and K, which pollute ground water and thus the environment. • Build-up of certain nutrients, e.g. P and K in the root zone, causing imbalances which may impede the uptake of other nutrients. • An excess of K in the root zone may have the same negative effect as high Na concentrations and may affect root growth negatively. • An excess of N stimulates unnecessary vegetative growth that cause bad grape quality and require additional labour actions to control the vigour. Correct application placement – the placement of the product within the root zone for assimilation by the grapevines, ensures effective utilisation of the administered products. Here the irrigation system and scheduling play a major role. The effectiveness of the fertiliser application is directly dependable on the effectiveness of the irrigation. Other factors to be taken into consideration are the presence and type of cover crop, water infiltration/run off, form of ridging walls and variations in weather patterns. A healthy root system is the primary requirement for healthy growth. If the root zone does not offer favourable growing conditions for the roots, the grapevine cannot function optimally. The following components of the root zone must be managed: • Soil chemistry – this includes maintaining the correct pH, maintaining the correct balance between nutrient ratios, and management of the salt content in this zone. • Water balance – oxygen in the root zone is as important as water, because air and water compete for the same pores in the soil. Irrigation management, therefore, not only determines the amount of water required in the root zone, but also the amount of air/oxygen that must remain. Over irrigation not only wastes water and leach nutrients, but also deprives the root zone of oxygen. • Soil physics – physical restraints on account of poor soil preparation, compaction or incorrect preparation of planting holes impedes root growth and water distribution. These problems often manifest as bad patches within the block, large variation in the growth of the grapevines or secondary deficiency symptoms. Alleviation of these restraints is often problematic. • Biological activity – a healthy, diverse microbiological population is essential for a healthy root system. The micro-organisms play an important role in the complex biochemical processes which occur in the soil, and in the contact zones between soil and plant roots, to ensure nutrient uptake and healthy roots.

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