Sodicity – a dirty word in Australia
Glossary
Brownian motion. The constant, erratic motion of small particles when they are suspended in water or floating in air. The motion is the result of the particles being knocked about by the movement of molecules in the water or air. The phenomenon is named after Robert Brown (1773-1858), the Scotsman who first described it.
electrical conductivity. When a voltage is applied across a substance, an electric current will only flow if the substance conducts electricity. When salts dissolve in water, ions are formed and the solution (the electrolyte) will conduct electricity. As a general rule, the higher the concentration of ions in solution (ie, the higher the salt concentration) the better the solution conducts electricity; in other words, its electrical conductivity increases. Electrical conductivity is often expressed in units such as deciSeimens per metre (dS/m). Rain water, for example, has a conductivity of 0.02-0.05 dS/m, while sea water has a conductivity of 50-60 dS/m.
electrolyte. A substance that produces ions (particles with an electric charge) when dissolved in water. The resulting solution (which can also be referred to as an electrolyte) conducts electricity.
exchangeable sodium percentage (ESP). Calculated by dividing the concentration of sodium ions clay particles by the concentration of divalent ions on clay particles, then multiplying by 100. A high ESP is an indication of a sodic soil.
ion, anion, cation, and divalent ion. An ion is an electrically charged atom or group of atoms. The charge is the result of the loss (positive ion) or gain (negative ion) of one or more electrons.
The gain of one or more electrons produces an ion with a negative charge (anion). The loss of one or more electrons produces an ion with a positive charge (cation). Ions that have gained or lost two electrons are called divalent ions.
sodium adsorption ratio (SAR). The ratio of the concentration of the sodium ion concentration in the soil solution to the square root of the total divalent ion concentration. A high SAR indicates a sodic soil.
soil salinity. This characteristic of soils relates to their salt content. These salts usually involve sodium chloride, but other salts occur in some soils. Soil salinity can be measured by determining the electrical conductivity of a solution, obtained by saturating a soil sample with water (a soil 'saturation extract'). A soil is said to be slightly saline when the electrical conductivity of a soil 'saturation extract' is 4-8 deciSeimens per metre (dS/metre), moderately saline at 8-16 dS/metre, and strongly saline at more than 16 dS/m.
soil structure. Refers to the arrangement of soil particles and the pore spaces between them.
The mineral or inorganic part of soil consists of particles of different sizes. Between the soil particles are spaces, called pores. Their number and size strongly influence soil structure. Pores are important in allowing air and water to penetrate the soil. Small pores enable the soil to hold greater amounts of water; larger ones allow for faster drainage. Good soil needs many pores, varying in size with some holding air and some water. Loss of pores changes soil structure for the worse as far as plant growth is concerned.
turbidity. A measure of the amount of suspended solids (usually fine clay or silt particles) in water. Increased turbidity reduces the penetration of light through water, reducing the growth of aquatic plants. For more information see Turbidity (Waterwatch Australia).
Page updated June 2010.