FERTILISER GUIDELINES FOR THE WINE INDUSTRY

SOIL ANALYSES As already mentioned, the organic N-content of a soil gives an indication of the “Nsupplying capacity” of the specific soil. There is normally a direct correlation between the organic N-content and carbon (C) content of a soil. In Table 9 the guidelines for the expected N-requirements of soils according to the carbon (C) content thereof, are indicated.

TABLE 9: Potential N-application according to C-content of soils.

Carbon(C)-content (%)

N-instalments

Light texture soils (0-10% clay)

Heavier texture soils (> 10% clay)

< 0.3

< 0.3

3 2 1 0

0.3-0.5 0.5-0.7

0.3-0.6 0.6-0.9

> 0.7

> 0.9

Under South African conditions, soils are normally still cold at the time of bud break, which means that the organic material is mineralised at a slow rate. During the first month after budding the vines are therefore largely dependent on reserves accumulated during the previous post-harvest period. Normally N-fertilisation on soils with a low C-content is required approximately four weeks after bud break, which means that three N-instalments might be required on such soils (Table 9). On soils with high C-contents (>0.7% for light textured soils and >0.9% for heavier textured soils), natural N-supplying should be sufficient to meet the vine’s needs. As in the case of fertilisation according to vigour (Table 8), the post-harvest instalment should be the last one of the three instalments (one month after bud break, after flowering or at pea berry stage, post-harvest) to be omitted. In South Africa the inorganic N-content (normally predominantly NO 3 -N) of soils is seldom determined. However, where this content is more than 10 mg kg -1 three to four weeks after bud break (end of September), it means that the plant-available N in the topsoil (0-300 mm) will amount to 45 kg/ha -1 . Nitrogen fertilisation will be unnecessary under these conditions. During the rest of the growing season the release of inorganic N will largely be affected by factors like leaching, soil temperature and the water status of the soil. In cases where the inorganic nitrogen content after flowering is >15 mg kg -1 , it will not be necessary to apply any N-fertilisation during the rest of the pre-harvest period.

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