Soil salinity-Key text

The term 'dryland salinity' strikes fear into the hearts of many Australian farmers. Some call it the white death because it conjures up images of lifeless, shining deserts studded with dead trees. Fears of the 'white death' seem justified. Dryland salinity currently affects more than 5 million hectares of land, mostly in southern Australia and causes damage totalling $270 million each year. What is dryland salinity? There are two kinds of soil salinity: dryland salinity (occurring on land not subject to irrigation) and irrigated land salinity. Both describe areas where soils contain high levels of salt. Usually, plants and soil organisms are killed or their productivity is severely limited on affected lands. Much of Australia's landscape is naturally saline – think of the great salt lakes in our interior. Many of our agricultural lands also contain vast reservoirs of salt, but normally these are held deep within the soil profile where they don't affect plant growth. The problem occurs when this salt is brought to the soil surface by rising water tables (Box 1). Where does the salt come from? The salt that sits deep in the soil profile may have several sources. In Western Australia, the main source is believed to be the ocean – salt is carried inland by the prevailing winds and deposited on the land in rainfall and dust. Over a time scale of millions of years, this process has deposited large amounts of salt in what is now the West Australian wheatbelt. Some salt in the soil profile may date back even further, to when the parent rocks themselves were formed. These rocks release salts as they weather. Other possible sources of salt are ancient drainage basins or inland seas that evaporated during arid periods, leaving behind salt deposits that still remain today. Monitoring the problem In the past, farmers estimated the extent of salinisation on their properties in response to questionnaires issued by the Australian Bureau of Statistics. This method is thought to have underestimated the extent of salinisation, partly because the definition and recognition of salinisation varies between farmers. Nor do such methods provide maps of where the salinity is, where it is spreading to, or the rate at which it is spreading. In recent years, scientists have developed new techniques for monitoring salinity. Most involve what is known as remote sensing. This is the collection of data using devices fitted to an aeroplane, satellite or some other craft located above the Earth's surface. Such technology can be used to gather a range of information related to salinity. Often remote sensing involves cameras that can record electromagnetic radiation – particularly visible light and infrared light – reflected from the Earth's surface. Electromagnetic reflections – sensing the differences When the sun's rays – made up of electromagnetic radiation of many different wavelengths – strike plants, water bodies, soils and other features on the Earth's surface, some wavelengths are absorbed by molecules in these features and some are reflected. Different features on the Earth's surface will absorb and reflect different parts of the electromagnetic spectrum depending on their chemical make-up. In this way, different parts of the electromagnetic spectrum provide information about the Earth's surface that may be useful for the detection of salinisation. Monitoring by aeroplane Australian scientists have tested a number of techniques to collect and analyse electromagnetic information. For example, colour infrared film can be used to take photographs from aeroplanes. Different colours (corresponding to different wavelengths within the infrared band) will show vegetation under varying levels of stress, which can then be related to the degree of salinity. Dark-green vegetation produces a bright red image, light-green foliage a pink image, barren saline soil a white image, salt-stressed vegetation a reddish-brown image. If such photographs are taken of the same area over different years, changes in the pattern of salinisation can be monitored. Similarly, video cameras can be used from aeroplanes to collect information in the visible band of the spectrum. The videos show salinity patterns and the way these change over time. Another airborne electromagnetic technique makes use of the fact that electrical conductivity increases with increasing salinity. It involves an aeroplane flying low over the ground. Mounted on board is an electromagnetic transmitter and trailing behind on a cable is a receiver. The transmitter sends out pulses of electromagnetic radiation. When these hit the ground, they induce electrical currents to flow in conductive areas. The decay of these currents produces a magnetic field which is recorded by the receiver trailing behind the aircraft. The recording is then analysed to determine the conductivity of the ground. Monitoring by satellite Increasingly, scientists are also using satellite images to analyse salinity patterns across large areas. Most images are supplied by a series of scientific satellites known as Landsat. These orbit the Earth, recording information about the electromagnetic radiation reflected by the Earth's surface. In Landsat satellites, an instrument called a Thematic Mapper makes regular observations in bands ranging from the visible to the thermal on each area of the Earth's surface, sending the information back to Earth. Many scientists consider that data produced in this way can be used effectively for the detection and monitoring of salinity, and experimental results support this view (Box 2). Predicting and preventing the advance of salinity The knowledge gained from the new monitoring techniques, along with that generated by decades of painstaking field research, is offering many insights to the causes of salinisation. Importantly, this is aiding scientists in the development of methods to predict sites most at risk of salinisation so that preventative measures such as tree-planting can be taken (Box 1). Armed with the information such methods will provide, a coordinated community response could succeed in combatting the white death, before it eats out our agricultural heart. Soil salinity-Key text