SOIL PREPARATION

SUMMARY RECLAMATION OF BRACK SOIL

• Soils are saline (brack) when crop performance is reduced due to excessive salts and/or high Na content. Such soils need to be reclaimed before vineyards are planted. Improvement of drainage and the presence/addition of lime are key factors in restoring brack soils. • An easy and convenient way of determining the amount of solu- ble salts is to measure the elec- trical 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). Soil with too much Na relative to Ca and Mg and will have a poor physical condition. The clay par- ticles in such soils disperse and the soil swells when in a wet con- dition. When clay is dispersed, the clay particles wash down the profile, block the soil pores, and consequently drainage is imped- ed. In dry conditions, soil with a high Na percentage becomes hard and dense and forms a crust on the surface. • Depending on the chemical com- position of the saturated soil ex- tract, brack soils can be classi- fied as saline (white brack), sodic (black brack), saline-sodic, acid

sodic and acid saline-sodic. Recla- mation measures will depend on the type of brack soil present. • As a first step in the reclamation of brack soil it is recommended to stop the source of salts, e.g. by installing cut-off drains and ensuring that leaking dams and canals are repaired. If compact soil layers occur within a meter of the surface, they must first be bro- ken up by suitable deep tillage to allow water to penetrate properly. Subsequently, subsurface drains can be installed to remove salts that are leached. • White brack soils, particularly if they contain free lime or gypsum, are the easiest to reclaim. A few heavy irrigations would suffice to leach the excess salts into the drainage system. In the case of sodic soils and saline-sodic salts that contain no gypsum or lime, gypsum must be applied before attempting to leach the soil. The incorporation of organic material into the soil together with gypsum will also help to improve water infiltration and allow gypsum to leach into the subsoil. Irrigation water used for leaching of sodic and saline-sodic soils that con- tain no gypsum and lime must itself contain salts (electrolites), otherwise the clay in the soil will

108 | SOIL DRAINAGE