SOIL PREPARATION

CHAPTER 6

EC e < 200 ohms, respectively. Electrical resistance of the soil paste is not considered a very accurate indicator of soil salinity, and consequently the classification of brack soil is not based on this measurement. 6.5.1.2 SODIUM HAZARD Soil with a high ESP contains too much Na relative to Ca and Mg and will have a poor physical condition. The clay particles in such soils disperse and the soil swells when in a wet condition. When clay is dispersed, the clay particles wash down the profile, block the soil pores, and consequently drainage is impeded. In dry condition, soil with a high Na percentage becomes hard and dense and forms a crust on the surface. Furthermore, when high Na in the soil causes high pH (above 8.5), it reduces the availability of many elements needed for nutrition. The ESP of a soil can be determined in the laboratory, but it is a tedious process. Instead, the cation composition of the soil solution can be used to calculate the Sodium Adsorption Ratio (SAR) which is approximately equal to the ESP of the soil. > 400 mS/m and R s

Na

SAR =

Ca+Mg

where Na, Ca and Mg are in mmol/L.

There is no uniform threshold ESP value to indicate when there is too much adsorbed Na in the soil or in terms of which a soil can be classified as saline or sodic. The accepted norm used in this book is ESP = 15 % as proposed by the United States Salinity Laboratory (Richards, 1954). Other threshold ESP values are also proposed in the literature. Before attempting to reclaim a brack soil, it is necessary to determine whether the soil can be drained, what the origin of the salt is, and what type of salinity is present. According to Richards (1954), brack soils can be classified into three classes, namely: i) Saline soil (white brack) defined as having an EC e > 400 mS/m, an ESP < 15 % and pH H 2 O < 8.5. Excessive salt content is the problem with this kind of brack soil. Free lime or gypsum is often present in white brack soils. ii) Sodic soil (black brack), i.e. soil having an EC e < 400 mS/m and ESP > 15 %, contains a high percentage of Na on the clay particles, but not a high salt concentration. Sodic soils often have a pH H 2 O > 8.5 and no free lime and/ or gypsum is present in the soil.

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