Soil salinity

-------------------------------------------------------------------------------- Box 1. Salinisation – causes and prevention Surprisingly in such a dry continent as Australia, salinity occurs when there is too much water. To understand why salinisation occurs, you need to know about ground water. Ground water rising Ground water is, as the name implies, water in the ground. Usually, somewhere below the surface of the soil, the soil is saturated with water. It is not quite like an underground lake – the water is most commonly held within the soil profile rather than in some vast underground cavern. The top surface of the ground water layer is called the water table. Ground water recharge is the amount of water being added to the ground water. If this is higher than discharge, which is the amount of water lost from the ground water, then the water table rises. As it does, the water dissolves salt held in the soil profile, and the salt becomes more and more concentrated as the water moves upwards. If the salty water keeps rising, it eventually reaches the surface and subsurface layers of the soil. The water evaporates, leaving the salt behind. Why is the ground water rising? In our quest to prepare Australian soils for agriculture, we cleared trees by the billion. Yet trees played a crucial role in maintaining the water balance in our ancient soil profiles. It was our success in clearing trees that has led to the development of dryland salinity. (Irrigated-land salinity is caused by a similar effect – the application of excess water to land causes the water table to rise. The problem is made worse if the irrigation water itself is also saline.) Trees help control ground water levels in two ways: by decreasing recharge and by increasing discharge. Decreasing recharge. Most ground water recharge is supplied by rainfall (except in the case of irrigated-land salinity), and more of it reaches the ground water when trees are cleared. This is because trees develop extensive root systems to trap the water, which is then used for tree growth or returned to the atmosphere by evaporation and transpiration. Scientists estimate that the amount of water that percolates below the root zone of crops and pastures can be 10 to 100 times the amount percolating below trees. Increasing discharge. Trees also play a role in discharging ground water. The roots of many Australian tree species reach down deep into the ground, often making contact with the water table. During drought, such trees use this water in order to survive and keep growing. In contrast, annual crops that farmers plant on cleared land usually don't have deep roots. Moreover, many crops only grow during winter, spring and early summer while most Australian trees require water all year round. Trees: weapons against salt? If salinisation is caused by the removal of trees from the landscape, it seems logical that putting them back will solve the problem. Farmers throughout the country, including those in zones most affected by salinisation, have embarked on a massive tree-planting campaign, giving hope that the rural landscape will recover from its many ailments, including salinisation. The ability of trees to reduce salinisation is still not fully known, although they have been shown to lower water tables in some areas. Australian scientists continue to investigate the potential of trees to reclaim saline areas and to prevent currently unaffected land from becoming salty. In the meantime, programs such as the Joint Venture Agroforestry Program have produced guidelines to ensure that any trees planted have the maximum positive benefit. For example, healthy, highly productive trees will be more effective than less productive trees in lowering water tables because they will use more water. Some farmers may be tempted to plant trees on the salty areas, but unless these are specially adapted to saline soils they may not grow well and therefore not play much of a role in solving the salt problem. In some situations, trees planted higher up in the catchment, in areas of high recharge, may be more effective. And, in general, the more trees planted, the more impact they will have on water table levels. Other weapons against salt Tree-planting is just one of many strategies that show promise in the fight against salinity. For example, deep-rooted perennial crops such as lucerne lower water tables and may often be a viable alternative to trees. In places where soils are likely to remain saline for some time, salt-tolerant species such as saltbush – which can be eaten by sheep – have had some success. And scientists and agriculturalists are working to enhance the salt tolerance of other plant species through breeding programs. Innovative farmers are experimenting with other possible solutions. For example, driven by the knowledge that salt is a potentially valuable product, some farmers are pumping their saline ground water into evaporation ponds. The salt harvested from these ponds can be sold as a raw material in the production of important chemicals such as sodium carbonate and sodium hydroxide, or as table salt. Soil salinity-Box 1