Pasture legumes and grasses for saline land

Page last updated: Friday, 12 April 2019 - 11:53am

Saline lands in Western Australia (WA) often suffer winter waterlogging, with the levels of salinity and depth to watertable varying markedly both spatially and between seasons. These areas have traditionally had poor pasture production, but productivity can be markedly increased through use of adapted salt tolerant legumes and grasses, particularly when used in mixtures. The recent release of Neptune messina means that salt tolerant legumes are now available for all but extremely saline land.

Saltland principles

Approximately one million hectares in the south-west 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 watertable. Waterlogging causes soils to become oxygen deficient, resulting in increased salt uptake and decreased potassium 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 traditional pasture 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 saltbush-based pastures when used as an understorey. 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 centimetre) salinities vary throughout the season (see Figure 1). 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 centimetre (cm), soil salinity tends to stay more constant.

Salinity is at the max (over 30dS/m) from Jan to March, and drops significantly post-break in May/June, being under 5dS/m from July to Sept
Figure 1 Seasonal salinity changes in the top 10cm of a saline sandy loam at Darkan (the units of salinity are discussed below). Note this level of fluctuation may not apply to heavier textured soils, which have less leaching of salt down the profile

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. On the other hand, saltbushes have shallow roots that extend many metres which seek less saline water.

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.

Table 1 Terms for describing the severity of soil salinity, in terms of electrical conductivity units of EC1:5 or ECe, for different soil textures. Note acronym: deciSiemens per metre (dS/m)
Salinity class EC1:5 range - sands (dS/m) EC1:5 range - loams (dS/m) EC1:5 range - clays (dS/m) ECe range (dS/m)
Non-saline 0–0.14 0–0.18 0–0.25 0–2
Slightly saline 0.15–0.28 0.19–0.36 0.26–0.50 2–4
Moderately saline 0.29–0.57 0.37–0.72 0.51–1.00 4–8
Highly saline 0.58–1.14 0.73–1.45 1.01–2.00 8–16
Severely saline 1.15–2.28 1.46–2.90 2.01–4.00 16–32
Extremely saline >2.28 >2.90 >4.00 >32

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.

Assessing waterlogging

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.

Pasture establishment

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. Soil cultivation will enhance salt leaching.  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. The aim in the year of sowing should be to maximise the seed set of annuals and the ground cover of perennials to set up a long-term pasture.

Table 2 Annual legume options (registered cultivars) for different categories of salinity, waterlogging, soil texture, soil pH and rainfall. Key: 1Other proprietary cultivars may be available that have not been tested. Note acronym: millimetres (mm)
Annual rainfall (mm) Salinity (see Table 1) Waterlogging risk Texture pH (CaCl2) Suitable species - common name Suitable species - scientific name Suitable species - cultivars1 Comments
275–325 Moderate–high Low Sandy loams–clay loams 5.2–8.5 Burr medic Medicago polymorpha Serena Less salt tolerant than Scimitar
275–350 Moderate Low Loams–clays 5.8–9.0 Barrel medic Medicago truncatula Caliph
300–425 Moderate–high Low Sandy loams–clay loams 5.2–8.5 Burr medic Medicago polymorpha Scimitar, Santiago Scimitar is the best performing burr medic on saltland
325–500 Moderate Low Loams–clays 5.8–9.0 Snail medic Medicago scutellata Sava, Silver, Essex
>350 Moderate-severe High Sandy loams-clays 5.5-9.0 Messina Melilotus siculus Neptune Needs salt-tolerant rhizobium
350–550 Moderate–high Low Sandy loams–clay loams 5.2–8.5 Burr medic Medicago polymorpha Cavalier
350–550 Moderate Low Sandy loams–clays 4.8–8.0 Sphere medic Medicago sphaerocarpos Orion
350–550 Low–moderate High Sandy loams–loams 4.5–8.0 Balansa clover Trifolium michelianum Frontier
425–650 Low–moderate High Loams–clays 5.0–8.0 Persian clover Trifolium resupinatum Prolific, Nitro Plus
450–700 Low–moderate High Sandy loams–loams 4.5–8.0 Balansa clover Trifolium michelianum Paradana
>550 Moderate–high Low Loams–clays 4.5–8.0

Bokhara clover/

White sweetclover
Melilotus albus Jota High coumarin levels can cause haemorrhaging of stock if fed mouldy hay
>600 Low–moderate High Sandy loams–loams 4.5–8.0 Balansa clover Trifolium michelianum Bolta
>650 Low–moderate High Loams–clays 5.0–8.0 Persian clover Trifolium resupinatum Kyambro
>700 Low–moderate High Loams–clays 5.0–8.0

Persian clover/

Shaftal clover

Trifolium resupinatum Many cultivars One year fodder types
Table 3 Temperate annual grass options for different categories of salinity, waterlogging, soil texture, soil pH and rainfall. Note: 1Other proprietary cultivars may be available that have not been tested
Annual rainfall (mm) Salinity (see Table 1) Waterlogging risk Texture pH (CaCl2) Suitable species - common name Suitable species - scientific name Suitable species - cultivars1 Comments
275–650 Moderate Low Sands–clays 5.0–8.0 Annual ryegrass Lolium rigidum Wimmera, Safeguard Safeguard has resistance to annual ryegrass toxicity
>550 Low–moderate Low Loams–clays 5.0–8.0 Italian ryegrass Lolium multiflorum Many cultivars
Table 4 Perennial legumes for different categories of salinity, waterlogging, soil texture, soil pH and rainfall. Key: 1Other proprietary cultivars may be available that have not been tested
Annual rainfall (mm) Salinity (see Table 1) Waterlogging risk Texture pH (CaCl2) Suitable species - common name Suitable species - scientific name Suitable species - cultivars1 Comments
>400 Moderate Low Sands–clay loams 5.6–9.0 Lucerne Medicago sativa Many cultivars Winter active and highly winter-active cultivars perform best in WA
>550 Low–moderate High Sandy loams–clays 5.6–9.0 Strawberry clover Trifolium fragiferum O’Connors, Palestine
Table 5 Temperate perennial grasses for different categories of salinity, waterlogging, soil texture, soil pH and rainfall. Key: 1Other proprietary cultivars may be available that have not been tested
Annual rainfall (mm) Salinity (see Table 1) Waterlogging risk Texture pH (CaCl2) Suitable species - common name Suitable species - scientific name Suitable species - cultivars1 Comments
>400 High–severe High Sands–clays 5.5–9.0 Puccinellia Pucinellia ciliata Menemen Best suited to waterlogged, saline soils, dormant over summer
>400 High Moderate Sandy loams–clays 4.5–9.0 Tall wheat grass Thinopyrum ponticum Dundas, Tyrell Tolerates ECe levels >16 dS/m if it can access fresh water at depth, has weed potential in Victoria
>500 Low–moderate Moderate Sands–clays 4.3–8.5 Tall fescue Festuca arundinaceae Many cultivars Summer dormant cultivars perform best in WA
>550 Low–moderate Moderate Loams–clays 4.5–8.0 Phalaris Phalaris tuberosa Many cultivars Winter active cultivars perform best in WA
Table 6 Sub-tropical perennial grasses for different categories of salinity, waterlogging, soil texture, soil pH and rainfall. Key: 1Other proprietary cultivars may be available that have not been tested
Annual rainfall (mm) Salinity (see Table 1) Waterlogging risk Texture pH (CaCl2) Suitable species - common name Suitable species - scientific name Suitable species - cultivars1 Comments
>350 High-severe High Sands–clays 5.5–8.0 Distichilis Distichilis spicata NyPa forage Only propagated by cuttings
>400 Low–moderate Low Sands–loams 5.5–8.0 Rhodes grass Chloris gayana Many cultivars Low frost tolerance, suited to northern and south coastal districts
>400 Moderate Moderate–high Sands–clays 3.7–7.0 Kikuyu Pennisetum clandestinum Whittet Performs poorly north of Perth

 

Contact information

Ed Barrett-Lennard
+61 (0)8 9368 3798