Measurement of salinity
Table 1 shows the Australian soil salinity classification system and its relationship to soil texture. Soil salinity is generally estimated by its electrical conductivity (EC). Most commercial laboratories measure the EC1:5, which is the electrical conductivity of a mixture of 1 part soil to 5 parts of water. However, to compare salinities of different soil textures, scientists generally measure the ECe, which is the electrical conductivity of the water in saturated soil. The terms used to define the severity of salinity range from non-saline to extremely saline, and the ECe and EC1:5 readings associated with these terms for different soils are given in the table.
|Salinity class||EC1:5 range - sands (dS/m)||EC1:5 range - loams (dS/m)||EC1:5 range - clays (dS/m)||ECe range (dS/m)|
Measuring soil salinity for annual pastures
The greatest stress self-regenerating annuals encounter on saltland is high salinity on the soil surface during germination in the second and subsequent seasons. Therefore, the suitability of soils for annual pastures should be assessed by measuring the EC1:5 of the top 10cm over the summer-autumn period, prior to the autumn break. This may give an over-estimate of the salinity levels experienced by germinating seeds, but is a much better estimate than measurements taken in winter or early spring.
Measuring soil salinity for perennial pastures
The greatest salinity stress perennial plants encounter on saltland occurs in the subsoil during summer. Therefore soil samples should be taken 25-50cm below the soil surface. Although this should ideally be done in summer, the timing is less critical, as salinities at and below this depth remain relatively constant throughout the year.
Three terms are used here to define the risk of waterlogging. Soils of high risk will be sodden for much of the winter, often with prolonged periods of surface water. Soils of moderate risk will remain sodden for up to two weeks after heavy rain, often with surface water visible. Soils of low risk will be free draining.
Pasture options for saline land
Tables 2-6 list current commercially available grass and legume options for different classes of saltland. To select the best species for a particular paddock, you will need to know your rainfall zone, the EC1:5 measured at the relevant time and soil depth (see above), the potential for winter waterlogging, the soil pH and soil texture.
It can be seen there are few options for low rainfall regions. This reflects the limited range of pasture options for low rainfall regions in general. In contrast, higher rainfall regions have a wider range of options.
For soils classed as severely saline messina is the only commercially available legume while puccinellia and NyPa Forage are the only available grasses. Saltland classed as extremely saline should be fenced off and allowed to revegetate naturally.
Use of mixtures
Because salinity and waterlogging vary over short distances within paddocks, mixtures of appropriate species are recommended. The best adapted species will colonise those parts of the landscape to which they are suited.