FERTILISER GUIDELINES FOR THE WINE INDUSTRY

For soil preparation the gypsum requirement should be determined to a soil depth of 900 mm. Only 50% of the gypsum should then be worked into the soil. The rest must be broadcasted across the surface to assist infiltration and leaching. The calcium ion content of the rain/irrigation water is increased so that it can displace the sodium and prevent the soil from dispersing. Note that a maximum of no more than 10 tons/ha should be applied in total. As far as existing vineyards are concerned, the gypsum cannot be placed in the subsoil, so it serves no purpose to calculate the gypsum requirement deeper than 300 mm. Once again a maximum of 10 tons per hectare per annum may be applied, but only 5 tons at a time to prevent too much K and Mg from being displaced. Reconsider the situation after application of the first 5 tons. Preferably gypsum should be applied before the rainy season or before irri gation, to ensure that the gypsum is washed into the soil and the Na leached. The amount of agricultural gypsum that needs to be applied to correct nutrient imbalances, usually latent S deficiencies, is approximately 1 ton per hectare. As gypsum has a very low solubility, large amounts will never be harmful. FINE AND COARSE GYPSUM The reactivity of agricultural gypsum is determined by the purity and fineness thereof – the finer the gypsum, the quicker it will dissolve. In soil with low permeability, fine gypsum that can dissolve quickly is required. In soil with a high permeability (sandy soil), on the other hand, fine gypsum may leach out before soil correction and nutrition is effectively achieved. A coarser gypsum is therefore recommended for such soils. Agricultural gypsum is a 90% to 95% calcium sulphate product and contains on average about 22% calcium and 17.5% sulphur on a dry weight basis. The water content varies from 23% to 30%, while in the case of gypsum as by-product of double superphosphate preparation it contains from 0,4% to 1% phosphorus, e.g. phosphogypsum. WHERE DOES AGRICULTURAL GYPSUM ORIGINATE FROM? Natural gypsum comes from Yzerfontein, Kolkies River and Vanrhynsdorp, and phosphogypsum from Chloorkop, Potchefstroom, Phalaborwa and Pho keng. Both types are available at SA Lime and Gypsum. Phosphogypsum is the by-product of the wet-acid production of phosphoric acid from phosphate rock. It is primarily hydrated calcium sulphate (CaSO42H 2 O) with impurities of phosphorus (P), fluorine, (F) silicon (Si) and aluminium (AI), as well as traces of various trace elements and heavy metals.

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