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Pasture legumes and grasses for saltland
Text only version of Farmnote 432 - PDF Available
Figure 1: Annual pasture between saltbush alleys at Pingaring, WA
Figure 2: Seasonal salinity changes in the top 10 cm of a saline sandy loam at Darkan
Approximately 1 million hectares in the southwest agricultural region are severely affected by salinity. A further 1.8 – 2.7 million hectares have been identified as being at risk from shallow watertables and/or soil salinity. Depending on future climate and watertable trends, cropping could become risky in these areas.
Saltland soils are often associated with waterlogging in winter, due to the presence of a shallow water-table. Waterlogging causes soils to become oxygen deficient, resulting in increased salt uptake by plants. The combination of both stresses makes saltland soils particularly difficult for plant growth. Salinity and waterlogging can be highly variable over short distances.
Most legumes are sensitive to salinity, and as a result saline soils are often nitrogen deficient. Subterranean clover is particularly sensitive to salinity and is one of the first species to disappear from pastures when land becomes salt-affected.
The growth of salt and waterlogging tolerant legumes and grasses can complement saltbushbased pastures when used as an understorey (see Figure 1). Research has shown this can increase pasture profitability up to $60 per hectare, depending on the severity of the site, input costs, livestock prices and pasture management.
Surface (0–10 cm) salinities vary throughout the season (see Figure 2). They are generally highest over the summer-early autumn period, prior to the break of season, and lowest in mid-winter. Rainfall flushes salt from the surface down the soil profile, while evaporation in spring and summer causes salt to rise again to the surface. Below a depth of about 25 cm soil salinity tends to stay more constant.
Consequences for annual plants
Annual plants on saline soils have a shorter growing season than those on adjacent non-saline soils. High salinities at the start of the growing season delay germination and can kill germinating seedlings of susceptible species. Plants adapted to saltland must, therefore, either have a high tolerance to salinity during germination or ways to defer germination until later, when surface soil salinities are lower. In spring, high salinities kill plants prematurely and reduce the time available for seed set.
Consequences for perennial plants
To survive on saltland, perennial plants need to cope with the additional stress of summer drought. Even under non-saline conditions, few perennial pasture species can cope with the long summer drought in low and medium rainfall zones. However, on saltland perennial plants must also either have mechanisms to cope with high salinity or have some form of salinity avoidance mechanism, such as summer dormancy (e.g. puccinellia).
In general plants use the freshest water available to them. Perennials, with their deeper root systems, can extract water from more of the soil profile than annuals. Tall wheatgrass, for example, avoids high surface salinity in summer by accessing less saline water from depth and then uses relatively fresh water closer to the surface during winter.
Measurement of salinity
Table 1 Terms for describing the severity of soil salinity...
Table 2: Annual legume and grass options...
Table 3: Perennial legume and grass options...
Table 1 shows a new 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.
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 after sowing. Therefore, the suitability of soils for annual pastures should be assessed by measuring the EC1:5 of the top 10 cm over the summer-autumn period, prior to the autumn break.
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 25-50 cm below the soil surface. Although this should ideally be done in summer, the timing is less critical, as salinities at 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 2 weeks after heavy rain, often with surface water visible. Soils of low risk will be free draining.
Pasture options for saline land
Tables 2 and 3 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 the 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.
There are no commercially available grasses or legumes for soils classed as severely or extremely saline. Saltland with these levels of salinity 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.
Germinating seedlings are very sensitive to salinity. Therefore, all species, apart from the sub-tropical grasses, should be sown soon after early season rains have flushed salt from the soil surface. Sub-tropical grasses should be sown in late-winter or early spring, while the soil surface is still moist and temperatures are beginning to rise.
Pasture legumes should be inoculated prior to sowing with the strain of rhizobium appropriate for the species. This is particularly important on saltland, as background rhizobia levels are likely to be very low (or nil).
As with any pasture sowing, good weed control is important to optimise pasture establishment.
Ideally, weeds should be controlled in the year prior to sowing to prevent seed set. Two knockdown herbicide applications are recommended, the first following an initial weed germination and the second just prior to sowing. It is important to note that many weeds (e.g. sea barley grass and ice plant) will have a delayed germination on saline sites (the delay will be longer on more highly affected sites).
Sowing and fertiliser rates, seeding depth, post-emergent weed and insect control and grazing management will be the same for these species as for establishment on non-saline land (see Further Information). The aim should be to maximise the seed set of annuals and the ground cover of perennials to set up a long-term pasture.
Frontier—an early maturing balansa clover for the wheatbelt, Farmnote no 3/2001
Scimitar and Cavalier annual burr medics, Farmnote no 83/2004
Perennial pastures for Western Australia, Bulletin 4690
Saltland prospects—prospects for profit and pride, Future Farm Industries CRC
Pasture Picker website: www.pasturepicker.com.au
Saltland genie website: www.saltlandgenie.org.au
This Farmnote was developed using information obtained from research funded by the Future Farm Industries CRC.
This is the Text only version of Farmnote 432 - full PDF Available
Page amended: October 2010