FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

CHAPTER 6

The use of water-soluble fertilisers is complicated when the soil remains wet in spring/ early summer, since irrigation is not required under these conditions. Irrigation, done for application of fertiliser only, may lead to leaching of nutrients from the root zone. Furthermore, roots will not be able to assimilate nutrients effectively, because soil temperatures are still low. Leaching of nutrients then occurs easily during irrigation. It is therefore better to hold back on the first fertiliser application until the soil temperature has risen (> 15 °C), after which it can be applied with a light irrigation. Liquid Fertilisers – the most used products are UAN(32) and AN(19). The popularity of these products relative to water soluble fertiliser has declined, since transport and storage thereof are often difficult and expensive. The risk or practical implications of using these products are like those of water-soluble products. Slow-release fertilisers – There are several products with different formulations and releasing periods on the market. In the last while the technologies in the slow- release products have improved appreciably and nutrient release from the pellets is more controlled. Even though the products are more expensive than normal fertiliser, it may potentially be applied with good effect when new blocks are established, poorly performing vines are replaced and as treatment for weak patches within existing blocks. In addition to the right choice of product, product placement is also important, especially in drip irrigated vineyards. Enriched organic fertilisers – chicken manure and fertiliser are mixed in varying ratios to form enriched organic fertilisers. There is a variety of formulations (12-3-3(18), 10-2-6(18), 9-2-9(20), etc.) and brand names for these products. The products usually have a lower nutrient concentration than pure fertilisers and are available as a pelleted product (4 mm pellets) or as a powder mixture. Pelleted products are easier to handle with mechanical spreaders, but the powdered products offer the advantage that they can be formulated for specific use/conditions. The use of enriched organic products has become popular, because it adds an organic component to the normal fertilisation programme, which is regarded as beneficial for soil and plant health, in several aspects. Although these products are usually applied in volumes that will not increase the organic material content of the soil significantly, it may have a stimulating effect on soil micro- organisms. Furthermore, nutrients are released slower from the pelleted product, thus preventing undue leaching of nutrients. IMPACT OF FERTI LI SERS ON SO I L Soil acidification Acidification under drippers, due to the application of fertilisers (especially N) through the irrigation system, is a real danger and must be monitored and managed. If this is not done, root growth and uptake of nutrients from the drip zone will be hampered. Soil type (texture) must be considered when decided the size and timing of nutrient applications. Take note that all ammonium containing fertilisers will contribute to soil acidification, even LAN that contains 20% lime.

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