Science - September 23, 2004

Tracking salt from the air

Measuring electromagnetic fields from an aeroplane is one way of tracking down accumulations of salt in the soil, and thereby mapping risks for agriculture.

The Australian ministry of agriculture is working together with the International Soil Reference and Information Centre (ISRIC) in Wageningen on the problem. The accumulation of salt brought in by sea winds is becoming an acute problem in Australia. In the Murray Darling river basin alone the amount of salt in the soil results in annual crop yield losses worth 250 million Australian dollars. For this reason the researchers carried out test flights in the area. A special electromagnetic pulse transmitter and receiver measures the conductivity of soil layers, giving an indication of the amount of salt present. The data collected show that a lot of salt accumulates in old riverbeds consisting of sand and gravel.

Dr Richard Cresswell, who works at the Australian ministry of agriculture, and Dr David Dent of ISRIC used the data to construct a 3-D map of salt occurrence. They traced the salt deposits to a depth of 150 metres. To drill this deep would be very expensive and time-consuming. Nevertheless some field measurements of salt accumulations are necessary for calibration. Without these it is impossible to determine reliable absolute values for measuring salt quantities.

By combining these values and the 3-D map made from aerial measurements, the researchers say that a detailed salt-risk map can be drawn up for agriculture. This will help the government to implement more efficient measures. / HB