SOIL PREPARATION

In South Africa, gypsum is normally recommended when soils have more than 6 % clay and the exchangeable sodium percentage (ESP) is > 10. Indicators for the application of gypsum were summarised for Australian conditions as follows (Cass, 2002): • Sodic soils having an ESP > 6 % or a sodium absorption ratio (SAR) > 6 in a saturation extract or a SAR > 3 in 1:5 water extract. • Soils that can be classified as type 1 and 3 in a dispersion test (Emerson, 1967). • Subsoils that have strong blocky, angular blocky, prismatic or columnar structure that has strong consistency. • Topsoils that crust, readily causing water to run off before wetting the soil properly. • Cracking clays that have poor surface properties and remain excessively wet and un-trafficable after irrigation or rain. • Soils containing aluminium levels close to or above 100 mg/kg. The Australian indicators listed above are not necessarily directly applicable to South African soils and should be researched to establish their applicability to local conditions. Gypsum, especially products that have a high grade of purity and fineness, are water soluble and can be applied on the soil surface in established vineyards. The Ca ++ will move down into the soil and replace Na + that is adsorbed onto the soil particles. Soil preparation is however, an ideal time for gypsum applications. According to Oberholzer (2016), the amount of gypsum applied during soil preparation should be calculated for a maximum of 90 cm soil depth. Fifty per cent of this amount must then be incorporated into the soil during soil preparation while the rest must be spread on the soil surface to leach down through rainfall and irrigation. Good soil drainage is essential to leach the sodium and other toxic ions, not only downwards, but also out of the vineyard.

140 | APPLICATION OF AMELIORANTS DURING SOIL PREPARATION