Soil salinity is the term used to describe a soil in which the soluble salt content has reached a level harmful to crops. Soil salinity reduces the osmotic potential of the soil solution making water extraction impossible for plants in some cases and therefore the productivity of agricultural land is degraded.

Human induced soil salinity is caused by a rise in the water table through reduced evapotranspiration and an increase in water percolation to the subsoil. Irrigation contributes significantly to the subsoil water table and may bring saline water tables to the surface.

Vast areas of land used for today's agricultural purposes rely on irrigation for their productivity. Consequently there are extensive areas of land that are under threat of developing salinity problems. The detection, mapping and controlling of such problems is vital to sustainable agriculture.

Soil salinity can be measured using inductive electromagnetic techniques that determine the conductivity of soils. Electrical conductivity devices produce an electric field which in a salty soil will produce a second electric field. The electrical conductivity of a soil is influenced by clay content, mineralogy, salts and moisture content. This is measured to convert to a salinity reading. The depth to which these meters measure to, is controlled by the spacing between the transmitter and receiver.

Instruments using conductive electromagnetic techniques are now widely used to map terrain conductivity. These devices which electromagnetically induce small currents in the ground, measure the magnetic field strength generated by these currents to determine conductivity. They are well suited to assessing soil salinity as they respond to more conductive (and thus more saline) soils and, furthermore, do not require electrical contact with the ground. Exploration depth of these devices is determined by the spacing between the transmitter and receiver coils; commercially available instruments measure from a depth of approximately 1m to several tens of meters. There chief advantage is that large areas can be surveyed quickly (McNeill, 1992), and thus in considerable detail. Because of the various factors other than soil salinity which affect the bulk soil conductivity, saturated paste extract samples must still be taken, but at much reduced intervals and at locations now known to be statistically significant. The electrical conductivity of the in situ bulk soil is a function of electrical conductivity of the soil water, which is directly proportional to the soil salinity. However, the in situ conductivity is also a function of the degree of saturation of the soil (which controls the continuity of the current paths), the porosity of the soil (which effectively controls the tortuosity of the paths through which the current flows), the temperature of the soil (i.e., the temperature of the electrolyte, the conductivity of which increases at about 2% per degree C), and finally the clay content the soil (since clays with high ion-exchange capacity such as smectite can supply additional ions to enhance electrical conductivity).

At Narabri, ground conductivity meters were attached to a small tractor which was also fitted with a GPS which was specifically used for mapping soil conductivity over large areas of land. An EM31 and an EM38 were suspended respectively across the front and rear of a tractor. Both were separated sufficiently to ensure there was no interference between them. The EM38 and EM31 can take measurements in the vertical or horizontal plane. When in a vertical position the instrument measures deeper into the profile than in the horizontal position.
 
 

Advantages of this technique are that measurements can be made quickly, above the ground and in situ which allows for more accurate soil maps. These devices produce an extremely accurate value for the bulk conductivity of the soil, as it automatically averages the measurement over a lateral area which is approximately equal to the depth of exploration (McNeill, 1992). Disadvantages of this technique are that the cost is very high.

The general trend of both the EM 38 and EM 31, in both the vertical and horizontal positions, clearly show that the electrical conductivity is higher in the tilled sites than in the pasture. As it can be assumed that the clay content, salts and minerals are the same throughout the entire surveyed site, the difference in soil conductivity can be attributed to the moisture content. Many of the other experiments performed also support this evidence that the tilled is more moist than the pasture.