FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY

CHAPTER 5

Sulphur (S) Sulphur is absorbed from the soil as the sulphate ion, which is then reduced in the plant to serve as component of amino acids (and thus certain proteins), certain vitamins and enzymes. It is also an enzyme activator and involved with the conversion of light to chemical energy. It is furthermore a component of flavourings. Sulphur is found in a large variety of minerals, like sulphides and sulphates, of which iron pyrite (FeS 2 ) and gypsum (CaSO 4 ) are the most important relating to soil. The sulphides erode by means of oxidation to sulphates, which precipitate as gypsum in arid regions. The solubility of gypsum is low (0.24%), but still high enough to meet the S requirements of plants. Sulphur containing sea spray occurs near the coast, while S, originating from industrial discharges and air pollution, also ends up in the soil in industrial areas. These amounts of S can vary between from 1 to 100 kg ha –1 yr –1 and is often enough to meet the requirements of plants. A considerable amount of S is also found in the organic fraction of the soil, with N:S ratios that vary between 10:1.2 and 10:1.5. This S is also released through mineralisation. Sulphur deficiencies are generally not expected in grapevines, due to the S containing fungicides that were commonly used in the past, while fertilisers like superphosphate contains much S in the form of gypsum. The quantity of S is applied by means of fungicides is currently negligible. Fertilisers are now also manufactured at higher concentrations from S-free compounds or mixtures, resulting in little or no S being added to the soil in this way. The S requirements of grapevines are thus becoming increasingly more important. Sulphur deficiencies have indeed been identified in vineyards, distant from the coast and industrial pollution, in cases where S containing fungicides or fertilisers have not been used. In grapevines S deficiency symptoms are non-specific. It exhibits a light yellowing of leaf edges, bush-like growth, shoots that curl back, cracks in the bark of shoot bases, red petioles, tendrils and pedicels, while the shoots of certain cultivars may show red colouration (Figure 14). The angle between the petiole and main vein may also shrink. In rootstocks the leaves fold upwards, together with clear leaf edge chlorosis and necrotic specks, especially at the back of the leaves (Figure 15).

FERTILISATION GUIDELINES FOR THE TABLE GRAPE INDUSTRY | 63