FERTILISER GUIDELINES FOR THE WINE INDUSTRY

APPLICATION OF GYPSUM

THE OBJECTIVE OF AGRICULTURAL GYPSUM APPLICATION The objective with the application of agricultural gypsum is primarily to recover saline soils. Dissolved salts from salt-rich rocks like Malmesbury shales that accumulate in soils create problems in vineyard soils, especially because the soil becomes too compact, decreasing water infiltration. High salt levels in irrigation water can also compress the soil over time and this can be corrected with gypsum. In saline soils the exchangeable sodium as percentage of the total cation exchange capacity (CEC) is too high and the sodium adsorbtion ratio (SAR) of the saturated extract is also too high. Another use of gypsum is also for the improvement of the soil structure. If the infiltration ability of a soil is low, gypsum improves the water relations and air permeability. The electrolyte concentration of irrigation water is low – irrigation of fresh water together with the leaching in of gypsum makes it possible to irrigate without the soil being compacted. Application of agricultural gypsum does not increase the pH of the soil and therefore should not be used for this purpose. It can however increase the exchangeable calcium levels. HOW MUCH GYPSUM IS REQUIRED? To determine the required amount of gypsum that should be applied per hectare to saline soil, the Na (expressed in cmol/kg) may be multiplied by 3,4 to determine how many tons of gypsum per hectare are required for each 300 mm of soil depth. Some analysis reports indicate sodium in mg/kg or parts per million (ppm). In such cases the Na (mg/kg or ppm) should be divided by 230, to obtain the amount of Na in cmol/kg. Since watersoluble Na can simply be washed out through effective drainage, gypsum is only required to displace the exchangeable Na, i.e. that which has been adsorbed onto the soil particles. Laboratory analyses usually indicate total Na, i.e. soluble as well as exchangeable Na, which means that in practice too much gypsum is often recommended. It is therefore advisable to apply no more than 10 tons of gypsum at any one time. After a year, soil analyses should in stead be performed again to determine whether additional gypsum is required.

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