FERTILISER GUIDELINES FOR THE WINE INDUSTRY

ACIDIFYING EFFECT OF N-CONTAINING FERTILISERS All ammonium containing fertilisers contribute to soil acidification, even LAN which contains 20% lime. The theoretical maximum acidifying effect of N fertilisers and the corresponding amounts of liming material required to coun teract it, appear in Table 11. TABLE 11: Theoretical maximum acidification by Nfertilisers and amounts of liming material required to neutralise it.

N- content (%)

Amount (kg) liming material/100 kg fertiliser Dolomite Calcite Slaked lime Shell lime

N-fertiliser

Ammonium sulphate 21

227 248 121

221 241 118

111 121

200 219 107

Urea LAN

46 28

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According to this it can be calculated that about 3.4 tons calcitic lime will be required for neutralisation in the case of a vineyard with a production of 10 ton ha -1 where about 2.8 tons ha -1 LAN were applied, over a 20 year period. Lime does not move in the soil however, and in practice the effectiveness of neutralisation of acidification is therefore questionable. Phosphorus (P) Although an essential macro-element, P is only required by the grapevine in small quantities. The vine is furthermore able to utilise less accessible sources of P due to specific root properties and mycorrhiza associations, as well as the long growing season over which it is possible to do so. Reaction to P-fertilisa tion is consequently seldom observed in grapevines, despite the generally low P-contents of South African vineyard soils. Phosphorus is easily mobilised in the vine and is an ingredient of phospholipids and nuclein acids. It plays an essential role in energy transfer through adenos ine diphosphate (ADP) and adenosine triphosphate (ATP), and is required for cell division and meristematic growth. Soluble P in soil is derived from erosion of minerals, mineralisation of organic material or from fertilisation. It is fixed by clay, therefore Pnorms for soils usu ally increase with an increase in clay content. Most soils fix added soluble P so quickly that in practice little P-movement occurs. Where it does occur, however, is in bleached sands and peat, where there is an absence of sesquioxides. The P-concentration in the soil solution is pH-dependent, with the highest

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