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SOLUTION 10

Legumes for saltland

 

10.1 Legumes for saltland in a nutshell

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A quick summary

A robust salt-tolerant legume is something of a ‘holy grail’ for graziers with saltland and for researchers. During the community consultations that led to the design of the Sustainable Grazing on Saline Land (SGSL) Producer Network project, this was identified as ‘the dream outcome’ if it could be achieved. It is still some distance away but there are options for areas of mild salinity.

Clovers and medics underpin most Australian improved pastures on non-saline land, not only for their significant contribution to animal nutrition, but also on account of the nitogen they are able to fix from the atmosphere which later becomes available to grasses. Unfortunately, subterranean clover, the most commonly sown pasture legume in southern Australia, has very low salt tolerance, and is among the first pasture species to disappear with the encroachment of salinity. Likewise, white clover, the most important clover species worldwide and important in the high rainfall zone and for irrigated pastures is very susceptible to salinity.

Legumes are usually much less salt-tolerant than grasses, apparently due to their relative inability to exclude the toxic salts (ions) that disturb enzyme activity once taken up into the plant.

Furthermore, for legumes to achieve their potential they must fix nitrogen, which means that not only does the legume need to be able to tolerate salinity, but that a salt-tolerant rhizobia is also needed, Finally, a ‘salt-tolerant’ symbiotic relationship between the legume and the rhizobia must be able to form in the hostile environment of a saline soil. These three quite significant challenges need to be overcome simultaneously and in most cases this has not happened.

As a general guide, the most common legume species for saltland are (ranked from most to least salt- tolerant) burr medic, strawberry clover, lucerne and balansa clover. However this ranking is only meaningful when set alongside waterlogging tolerance, which will often eliminate lucerne and burr medic from the choices. Melilotus species, some of which have high levels of salt- and waterlogging-tolerance, are currently limited by the lack of salt-tolerant symbiotic rhizobia. 

Balansa is highly waterlogging-tolerant so if established after flushing autumn rains it can experience quite low surface soil salinity and early flowering cultivars can set seed and avoid the high salinity levels over summer. Lucerne is widely used in salinity management across southern Australia to reduce recharge, and is often sown around saltland to reduce local upward pressure from the watertable and thereby reduce discharge onto the saline site. However it is not well suited to saltland itself, having only a moderate tolerance for salinity and low tolerance for waterlogging. 

Because legumes have only limited tolerance to salinity, in most saltland pasture situations, the critical component is the salt-tolerant grass or shrub. For more information, see Saltland Solution 5 – tall wheatgrassSaltland Solution 6 – puccinelliaSaltland Solution 8 – Temperate perennial grasses with limited salinity toleranceSaltland Solution 9 – sub-tropical grasses with limited salinity tolerance and Saltland Solution 4 – saltbush and understorey. The key decisions relate to which grass, shrub or mixture is the most appropriate for the particular saltland site. Legumes are included in the seeding mix to take advantage of less saline areas where they can establish and make a significant contribution to pasture quality and/or soil nitrogen.

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History

Subterranean clover, the most commonly sown pasture legume in southern Australia, has long been recognised as having very low salt-tolerance, and is among the first pasture species to disappear with the encroachment of salinity. This is one of the key reasons behind the search for other, more salt-tolerant options.

South Australian, Western Australian and Victorian researchers over the past 15 years have investigated the performance of a wide array of pasture legumes under both field and glasshouse conditions. They found that of the commercially available pasture legumes, strawberry clover (Trifolium fragiferum), balansa clover (T. michelianum) and Persian clover (T. resupinatum) were the best options for salt-affected land prone to waterlogging, while burr medic (Medicago polymorpha) and to a lesser extent barrel medic (M. truncatula) and lucerne were best suited to saltland not subject to waterlogging. However even these species were still relatively salt-sensitive and persisted in only low to moderate levels of soil salinity. 

Balansa clover (Trifolium michelianum), an annual self-regenerating clover is well adapted to most soils where subterranean clover grows well, particularly those prone to waterlogging. Paradana was the first cultivar, introduced from Turkey and released commercially in the mid 1980s. It was also observed to grow well on slightly salt-affected soils, particularly in the year of sowing. The waterlogging tolerance of balansa suggested to some landholders that it might also be salt tolerant, as the two conditions often co-exist and balansa would sometimes thrive when sown onto saltland that became wet in winter. However regeneration of balansa the following year was invariably disappointing. Subsequent research has shown that this was partly due to rapidly rising soil salinity in spring either inhibiting seed set, or producing very small seeds. This is compounded by its relatively low salinity tolerance at germination. As well, balansa suffers from rapid hard seed breakdown in early summer, making it very vulnerable to false seasonal breaks.

The National Annual Pasture Legume Improvement Program (NAPLIP) and related state agency research programs led to the development of an early maturing cultivar of balansa clover, cv. Frontier, suited to 350-500 mm rainfall areas. Frontier is moderately salt-tolerant for a legume, but like Paradana shows excellent waterlogging tolerance. The early maturing characteristic enabled adequate seed set in suitable situations, but this alone did not guarantee regeneration the following year. Subsequent research indicated that high soil salinity levels at the break of season, before opening rains have had an opportunity to flush salt from the upper soil profile, seriously inhibiting germination. For more information, see Salt Tolerance of Balansa or FRONTIERÞ - an early maturing balansa clover for the wheatbelt.

Scimitar burr medic has been an excellent performer in trials in Western Australia, South Australia, and northern NSW particularly in better drained areas where waterlogging is limited. Cavalier burr medic also performed well in WA. While burr medic possesses far greater salinity tolerance at germination than balansa clover, its waterlogging tolerance is not as good. Part of the reason for the better performance of these new burr medic cultivars is their lower level of hard seeds than other cultivars, allowing better regeneration. Caliph barrel medic also performed well on a low rainfall, moderately saline soil in WA, and may have promise for alkaline soils in such environments.

Strawberry clover is a perennial with high waterlogging tolerance but only moderate salt tolerance. It is derived from the Mediterranean region, and needs some summer moisture if it is to maintain productivity over this period and survive the summer drought – it is shallow rooted and therefore has poor drought tolerance. It is generally suited to higher rainfall (>500 mm) zones.

Grasslands Onward is a New Zealand cultivar of strawberry clover that is described as a small leafed compact clover tolerant of wet, waterlogged and/or saline soils where other clovers may not persist. It is claimed to perform and persist better than white clover in environments that have waterlogged and/or saline soils.

Since the mid-1990s there has been considerable research interest in the suitability of Melilotus species to saline conditions in southern Australia., particularly M. albus and more recently M. siculus (sometimes referred to as M. messanensis). The Melilotus species have generally shown superior levels of salt tolerance combined with good dry matter production compared with both balansa and strawberry clover, apparently due to their ability to exclude sodium and chloride from their roots.

Jota, the first cultivar of annual Melilotus albus commercially released in Australia by the National Annual Pasture Legume Improvement Program, is intended for neutral to alkaline soils where it can be used as a companion legume for tall wheatgrass. It has higher salinity tolerance than other commercial legumes, but has poor waterlogging tolerance. In Western Victoria it has performed very well on raised beds for 2 years before disappearing and not regenerating. A potential concern with this species is its relatively high coumarin levels, which could lead to a haemorrhagic condition in livestock under some conditions. The target area has saline soils receiving more than 500mm of annual rainfall and a soil pH of 6 or higher - at this stage it is not recommended in WA because of its high coumarin levels. 

Lucerne is usually thought of in terms of recharge reduction rather than discharge management, but it is one of the more salt-tolerant of the perennial legumes. However, its susceptibility to waterlogging precludes its use on many saline environments.

In the late 1990s a commercial cultivar, Salado, was released with a reputation of being a more salt- tolerant lucerne. It was hoped that Salado could be used as a salinity management tool, resulting in reclamation of saltland and improved farm profitability and sustainability. Salado was the result of more than 15 years of selections, not only for germination, but in combination with establishment, vigour, and forage yield under saline conditions. Although quite widely trialled by farmers in Australia, it has not shown any advantage over other lucernes on saline soils.

Some interest has been shown in the waterlogging-tolerant birdsfoot trefoils (Lotus corniculatus and L. tenuis). However, L. corniculatus has low salinity tolerance, while L. tenuis has better salinity tolerance than strawberry clover. No cultivars of L. tenuis are available in Australia, but future research could develop a perennial legume for high rainfall mildly saline waterlogged areas where strawberry clover fails to persist. This species has shown some promise in trials in WA but persistence was disappointing in the summer rainfall region of northern NSW.

Sulla (Hedysarum coronarium) is a relatively new perennial legume species to Australia with a reputation of having some salt tolerance. Trials on low salinity land to date have been inconclusive, with encouraging results in Victoria but poor performance in northern NSW. Further testing is required with sulla before recommendations can be made about its potential use on saltland.

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Identifying legumes

SALTdeck cards were developed by the Sustainable Grazing on Saline Land(SGSL) initiative to provide visual identification clues and supporting notes for the 50 most common/desirable species that occur on saltland. SALTdeck includes the main legume species that exhibit some degree of salt tolerance: balansa, gland, woolly, strawberry and Persian clovers, plus barrel and burr medics (Figures 10.1 to 10.7). These can be viewed on this website .

Melilotus species were not included in SALTdeck because none were commercially available in Australia when SALTdeck was developed. Images of Melilotus alba are available. 

Another good identifying kit is the GRDC Ute Guide – “Pasture legumes for temperate farming systems” which has a much broader list of legume cultivars. 

 Figure 10.1 – SALTdeck card to assist with the identification of balansa clover

Figure 10.2 – SALTdeck card to assist with the identification of gland clover 

Figure 10.3 – SALTdeck card to assist with the identification of woolly clover 

Figure 10.4 – SALTdeck card to assist with the identification of strawberry clover 

Figure 10.5 – SALTdeck card to assist with the identification of Persian clover 

Figure 10.6 – SALTdeck card to assist with the identification barrel medic 

Figure 10.7 – SALTdeck card to assist with the identification of burr medic

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