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Salinity

Background

Soil salinity is a problem confronting many farms in sub-humid and semi-arid zones of Australia. The origins of the salt are natural, and very large reserves lie in many subsoils and aquifers in Australia. Soil salinity is a natural feature of the Australian landscape, but farming and clearing have increased the salt levels in many soils.

Salts are moved into and out of soil profiles by water. There are essentially two types of salinity:

Irrigation-induced salinity

Salinity is increased by two separate mechanisms: (1) Salt is added in irrigation water; the water evapotranspires, leaving the salt behind. (2) Irrigation water raises the water table, allowing capillary action to bring it to the surface; if the groundwater is saline, the salt accumulates on the surface as the water evapotranspires. Irrigation-induced salinity is worse in soils with low permeability and where water tables are shallow.

Dryland salinity

The processes here are more complex but are still caused by the interaction of soil with water. Some dryland salinity has always occurred in Australia, but it is worse now owing to changes in the hydrology of the land. Clearing results in increased amounts of runoff that accumulates in river valleys, raising the water table, which is nearly always saline in inland areas. Capillary rise causes salt deposition as above.

Measuring salinity

Water salinity

The total salt level in water, or total dissolved salts (TDS), is usually assessed by measuring the electrical conductivity (EC) of the water. The common units of electrical conductivity are millisiemens per centimetre (mS/cm = dS/m) and µS/cm (1 mS/cm = 1000 µS/cm). The principle is that salty water conducts electricity and pure water does not.

Another unit sometimes used is milligrams of salt per litre (mg/L = ppm). This can be converted to EC by using a factor of 640: 1 mS/cm = 640 mg/L of TDS. (This conversion factor is approximate and depends on the actual salts present.)

Once you have a value for EC (from either your preferred laboratory or by measuring it yourself), you can use Table 1 to determine the suitability of the water for irrigation. (For more information, see How salinity is measured.)

Table 1. Interpretation of EC values

Soil salinity

To measure the amount of salt in a soil, we generally mix 1 part of soil with 2 parts of water (1:2 soil–water extract) and then measure the EC in mS/cm. Because this is a diluted extract, the result must be converted back to what it would be if it had been undiluted. This is called the saturation extract (EC_se). To obtain this value, we multiply the EC (1:2) by a factor ranging from 5 for clays to 25 for very coarse sands.

Plants vary in their salt tolerance. Table 2 shows the salinities tolerated by various crops and the salinities that cause yield to drop to 90%, 75% and 50%.

Table 2. Salt tolerance of some crops

From K.I. Peverill, L.A. Sparrow & D.J. Reuter, 1999. Soil Analysis – An Interpretation Manual.

Soil type has a bearing on the maximum tolerable salinity. For a sandy soil, the upper limit of EC (1:2) is around 0.8 mS/cm. For a heavy clay soil, it’s around 3.3 mS/cm. But these upper limits will vary with the salt tolerance of the crop, the stage of growth, fertiliser application, water quality, soil drainage, and the position and concentration of the salinity in the soil.

Remember that fertilizers are salts, so a low EC does not always indicate a good soil. Note also that some salts are preferable to others; for example, calcium salts can improve soil structure and reduce soil sodium levels. So although EC is a useful measure, a test of individual salts is also important.