Sodicity – a dirty word in Australia
Box 3 | Rehabilitating sodic soils
Examples of the use of the electrolyte effect in rehabilitating sodic soils are given below:
Australia
During development of the Riverina District of New South Wales for irrigation, farmers encountered immense difficulties with impermeable, sodic soils. The clay fraction dispersed at the soil surface and, when dried, formed hard crusts that prevented the emergence of pasture and crop plants.
Investigations revealed that the Riverina clay contained 23 per cent exchangeable sodium (ESP), for which the threshold concentration solution was calculated to require an electrical conductivity of 1 deciSiemens per metre (dS/m). The problem for irrigators was that the water they were using had a conductivity of only 0.1 dS/m.
Once this was known, the remedy was simple: gypsum was dissolved in the irrigation water to raise conductivity above the threshold concentration. One dose of this treated irrigation water was sufficient for the successful establishment of the pasture plants. The same principles and technology for the dissolution of gypsum is currently being used to irrigate sodium-affected soils for the production of sugar cane in the Burdekin Valley of northern Queensland.
Large areas of the cropping lands of southeastern Australia have either sodic topsoils or sodic subsoils or a combination of both. Many land managers are using about 5 tonnes per hectare of gypsum every 10 years or so to ameliorate the sodic conditions. Spectacular improvements have occurred in soil structure and subsequent crop and pasture growth.
As well as improving agricultural profitability, reducing erosion and improving water quality, the application of gypsum has widened land use options and crop types. It has also reduced damage to infrastructure such as roads and buildings.
USA
Similar ingenuity might have rehabilitated a degraded farming region in Arizona, USA. For many years, it had been successfully irrigated with Colorado River water, which contained an electrolyte concentration sufficient to sustain soil permeability (that is, it was above the threshold concentration). However, during a shortage of river water, groundwater was used for 3 years, which increased the ESP to 25. When river water was re-introduced, the soil ‘froze up’ (became impermeable). The reason? The threshold concentration for this amount of exchangeable sodium had increased above the electrolyte concentration in the Colorado River water. This experience pre-dated notions of the threshold concentration concept. If this knowledge had been available, it would have been appropriate to mix the groundwater with the river water to obtain the threshold concentration. Over time, the proportion of river water could have been increased as the excess exchangeable sodium was gradually washed out of the system, until the original condition of the soil in equilibrium with the river water was attained.
Sounds good in theory, and in fact it has been done in practice. Water from the Salton Sea was mixed with Colorado River water to reclaim a saline soil with an ESP of 37 per in the Coachelle Valley, California. By ensuring that the manufactured water had an electrolyte level always above the threshold concentration, soil permeability was maintained, thereby reducing the period required for reclamation. During the reclamation process, the sea water was, in stages, increasingly diluted with river water; this favoured the exchange of calcium ions onto the surface of particles in the clay fraction.
Posted June 1999.