SOIL PREPARATION

6.5 Reclamation of saline soil Saline (brack) soil is a term referred to when crop performance is reduced due to excessive salts and/or high Na content. Such soils need to be reclaimed before vineyards are planted. The origin of salinity can be rocks or irrigation water that are rich in salt or brackish soil water that drains from higher lying land to lower lying areas. In most cases, however, the accumulation of salt is associated with high water tables and poor drainage. Therefore improved drainage to lower the water table is key to the reclamation of brack soils (Van Woerkom & Streutker, 1971). 6.5.1 Characterisation of brack soils Saline soils are characterised by determining a few chemical properties, namely the amount of soluble salts and the exchangeable sodium percentage (ESP). 6.5.1.1 TOTAL SOLUBLE SALTS An excessive amount of salt in the soil reduces plant growth and crop yield due to the effect of the salt on the plant available water. The plant has to use more energy to take up water, and this is done at the expense of its other functions. In addition, Na and chloride (Cl – ), normally associated with a high salt concentration in the soil, become toxic and cause further harm to the plant. Grapevines are generally considered as medium sensitive to salinity and sensitive to high Cl – in the soil (Dept. of Water Affairs and Forestry, 1996). An easy and convenient way of determining the amount of soluble salts is to measure the electrical conductivity of a solution extracted from a saturated soil paste. This value is called electrical conductivity (EC e ) and is presented as milli-Siemens per meter (mS/m) or deci-Siemens per meter (dS/m) at 25°C. The higher the total concentration of soluble salts, the higher the EC e will be. Electrical resistance (R) – the reciprocal of electrical conductivity – measured on a saturated soil paste in a standard cell with specific dimensions, is another test still being used in South Africa to determine the concentration of soluble salts in the soil. Currently there is, however, no reliable formula to convert electrical resistance to electrical conductivity or vice versa for South African soils. This includes the often-quoted formula below: EC e ≈ 250/R s where 250 is the cell constant of a standard USDA soil cup containing the saturated soil paste on which R s (in ohms) was measured; EC e is in dS/m. The generally accepted threshold values for too much soluble salt in South African vineyards soils in terms of electrical donductivity and resistance are

104 | SOIL DRAINAGE