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SOLUTION 10

Legumes for saltland

 

10.4 Level of confidence in legumes for saltland

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How reliable is the information?

Until recently there has been relatively little research into the salt-tolerant legumes and their use in saline conditions in Australia. The primary focus for saltland research has been the salt tolerant grasses and shrubs because they are so much more salt-tolerant than the legumes. However, in more recent years, the effort into legumes has increased because of their outstanding value for animal forage, and for their ability to fix nitrogen, which is almost invariably deficient in saline soils.

From this research and farmer experience we can state reliably:

  • There are no currently available legumes suitable for moderate to highly saline situations and none available for moderately saline areas subject to waterlogging. The burr medics and possible lucerne extend the salinity range, but only in the absence of waterlogging.
     
  • Subterranean clover, the most common annual legume in southern Australia is highly susceptible to salinity and is one of the first species to disappear from a pasture when salinity encroaches.
     
  • The burr medics, particularly Scimitar, perform well on sites with low to moderate salinity where waterlogging is only transient. They have much greater salinity tolerance at the seedling stage than balansa clover and also as adult plants. There is some evidence that barrel medics are able to tolerate mild salinity on well drained, alkaline soils.
     
  • Lotus corniculatus has good waterlogging tolerance, but lower salinity tolerance than strawberry clover and should not be sown on saltland.
     
  • Balansa clover may thrive in very wet conditions on saline sites as the salinity levels in the shallow root zone can be relatively low during the growing season for annuals, giving the impression that it is salt tolerant. However the real test for annual legumes is whether they persist and both research and farmer experiences suggest that while balansa clover can be very productive in the year of sowing, it usually fails to re-establish in saline situations. Research suggests that balansa owes its success in discharge areas to salt avoidance rather than to salt tolerance.
     
  • The relevant agencies in WA, SA, Victoria and NSW have produced basic information sheets that cover well-tested establishment and management techniques for balansa clover, and in some instances for balansa as a companion legume for salt-tolerant grasses. There is much less practical information available for other salt-tolerant legumes, particularly burr medic which appears to be significantly more salt-tolerant than balansa.
     
  • Persian clover has performed similarly to balansa clover and appears to also have high waterlogging tolerance but low levels of salinity tolerance.
     
  • White clover is also susceptible to salinity and should not be sown on saltland.
     
  • Strawberry clover is a much better perennial legume option than white clover on saline land subject to waterlogging, but it is not at all drought tolerant. The most commonly used cultivar is Palestine, and while considerable genetic variation has been found within populations there has been little work done to improve the production and persistence of this species in saline conditions;
     
  • Lucerne has some salinity tolerance but is very susceptible to waterlogging, which is often associated with saline sites. Salado lucerne was introduced as a commercial salt-tolerant cultivar,  but results on farms have generally been disappointing and it has not shown any advantage over other lucernes on saline soils.
     
  • Legumes from non-saline sites provide excellent nutrition for grazing animals – the same nutritional excellence can be expected from legumes growing on saline land because the legumes do not accumulate salt.

There are other aspects associated with legumes for saltland that are not so clear:

  • There is considerable uncertainty associated with our knowledge of the potential production from legumes as salinity increases from near zero to the low/moderate levels that the various legumes can tolerate.
     
  • There is considerable uncertainty about which legumes might perform best on and around the margins of a particular saline site, so a mixture of locally proven legumes is often sown.
     
  • Establishing legumes in the inherently hostile environment of a saline site (often characterised by both salinity and waterlogging) is a significant challenge and the ‘rules’ for establishing legumes are not well formed because the conditions vary greatly from region to region, and even from site to site.
     
  • Legumes are invariably sown with grasses or shrubs that are intended to form the long-term basis for the saltland pasture. This can complicate sowing operations and more than one sowing operation may be required to establish the desired mixture, depending on the pasture components.
     
  • Realistic expectations should be made about pasture legume performance on even low salinity sites. Identify areas (using soil tests, examining indicator species) that are likely to be too saline for pasture legumes and don’t waste money by sowing these areas to them.
     
  • There is evidence from some saline sites that the surface soil conditions under a saltbush or tall wheatgrass pasture may improve over time, making the site more amenable to mildly salt-tolerant legumes. Research is yet to clarify just where this might occur, and when attempts to introduce legumes into these sites.
     
  • For legumes to thrive in saltland, they need symbiotic rhizobia that can also survive in such conditions, but development of these for potentially more salt-tolerant legumes such as Melilotus species is in the early stages.

The Pastures Australia website contains information about all the commercially available pasture legumes, though these are primarily for non-saline sites.

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Farmer experiences

With the benefit of 40 years of experience in the upper south-east of SA, Trevor Egel has developed a successful approach to pasture establishment on the expansive salt-affected flats. This involves sowing a mix of pasture species, typically comprising of puccinellia, clovers, lucerne and barley. Pasture species then compete in the variable soil conditions, which change with seasons and surface elevation. Provided watertables are sufficiently low, the higher productivity lucerne and clovers will grow on the higher ground. On the lower ground (only tens of centimetres height difference) the salt-tolerant puccinellia dominates, providing useful feed where nothing else of value would grow.

Trevor Egel, SA

"The saltier land tends to be too wet for lucerne in most years. Across a paddock there might be variations in surface elevation of only 10 or so centimetres, but this will be enough to strongly favour one pasture species over another.

After years of trying to establish either just lucerne with barley (for forage) or just pucci with burr medic, we have learned that it is far more effective to sow them all together and let each find its niche.

This also helps us deal better with the seasonal variations, so that if the season is particularly wet and wipes out the lucerne, we still have the pucci to fall back on.

Productive pastures do not last long here in these harsh and unpredictable conditions, so we look to resow on average every five years. This might seem a bit extravagant, but we contain some of our costs by harvesting our own seed for both lucerne and pucci.

In the case of lucerne we have benefited from some of SARDI’s trials that have pointed us towards the most suitable varieties for this country. Then when you harvest your own seed grown under extreme conditions, you effectively screen out the weakest performers. So we find that we have customised a lucerne ‘variety’ that we nick-name ‘Survivor’ that seems to suit us better every year.

The pucci is usually still performing well when the lucerne needs resowing, and we find that we can generally sow the lucerne straight back into the pucci.” 


Eyre Peninsula (SA) farmers Leon and Robyn Modra planted about 14,000 trees around the edge of a 100 ha piece of salt-affected land. The trees so improved the overall appearance of what had been thought of as wasteland that they trialled 7 ha of Genesis lucerne with puccinellia broadcast over the top.

Leo and Robyn Modra, Eyre Peninsula, SA

“From bare scald or a thin cover of sea barleygrass we now have a highly productive pasture, with lucerne thriving where the land does not get too wet and the pucci dominating the wetter ground.

The result was better than we expected, so this year (2004) we will sow another 40 ha, but be a bit more selective about where we sow the lucerne. It remains to be seen how long the lucerne and the pucci survive and whether they eventually become salted out, but at this stage it looks as if we have turned unsightly wasteland into something highly productive.”

Unfortunately by 2008 the lucerne had all vanished and little puccinellia remained. 


David and Belinda MacLure, Tarcutta, southern NSW, sowed three different lucerne varieties (Salado, Pioneer L55 and Genesis) around the edges of salt affected land, with a mix of puccinellia, tall wheatgrass, balansa and strawberry clovers on the scalded country. They sowed the less salty areas to phalaris, sub-clovers, strawberry clover and fescue.

David and Belinda MacLure, Tarcutta, NSW

“I think this salt area will eventually be as good as or even more productive than our other country. We’ve got the salt-resistant species in the ground and we’ll always have the water. I can see it being green in February without rain which is really good.

We’ve learned a lot from this exercise and I’d probably do a few things differently a second time. If I had the chance I would have the area mapped electro-magnetically so I knew how close I could sow the lucerne to the salty areas. In our case, some small areas of lucerne have died out and we’re resowing those bits with strawberry clover.

The potential of the place is just amazing. We had 1000 lambs on 25 ha of balansa clover, but within three weeks we had to take the lambs off because of problems with grass tetany and bloat. I just couldn’t get enough sheep on it to keep the feed down.” 


In the Eastern Mount Lofty Ranges (SA) salinity tends to be confined to areas of only a few hectares, usually along drainage lines, and as a result farmers have been reluctant to make the capital investment to manage these areas for production. However Maurice Collins, Tungkillo, saw unrealised potential in these areas which tend to stay green long after annual pastures have dried off.

Maurice Collins,Tungkillo, SA

“An interesting feature of these mildly saline areas is that they often support quite extensive stands of strawberry clover and have done so for as long as most of us can recall. It is always fairly stunted and interspersed with sea barleygrass, but we have never really done anything to improve it, partly because the areas are generally small and it is difficult to justify fencing it off and looking after it.

This year the Landcare Group has commenced an SGSL project on one of my sites to see if we can improve the productivity of the strawberry clover, either by itself or in combination with other salt-tolerant pasture species.”


An SGSL trial at Wagga Wagga also showed excellent results with salada lucerne but this was during a drier than normal time.

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Risks and challenges

The most significant challenge for ‘salt-tolerant’ legumes is that they are not very salt-tolerant, and the combination of salinity and waterlogging tolerance (the conditions common in many saline sites) is rare.

Establishment. Many of the difficulties in establishing legumes are similar to those associated with other pasture species on saline sites. Because many saline sites are waterlogged for significant periods, there is often only a small window of opportunity to control weeds and sow the pasture.

If possible seeding should be done following good opening rains that are sufficient to flush salts from the soil surface. The seedling and early establishment stages are very susceptible to salinity. Dry sowing should be avoided, particularly in unreliable rainfall areas, as the opening rains may be sufficient to germinate seeds, but insufficient to flush salts from the surface, causing major seedling losses. In many areas, this may mean a very narrow sowing window, as areas prone to waterlogging may get too wet for sowing very quickly.

In higher rainfall, and more northerly (summer rainfall) areas spring sowing is often preferred. However this poses additional challenges for legumes, which are usually sown with other, more salt-tolerant pasture species. While spring sowing might be suitable for the temperate perennial legumes and grasses and is nearly essential for the sub-tropical grasses and saltbush, it will not suit the annual legumes, as they will have insufficient time to flower and set seed before the onset of the summer drought. Sowing annual legumes at this time would therefore be a waste of time and money. To incorporate them into the system, autumn-winter sowing is the preferred option, even if it requires a separate seeding in the following autumn.

Agronomic/grazing management. As with establishment, the management of established mixed pastures always present a challenge, with grasses needing heavy grazing in spring to maintain vegetative growth, whilst annual legumes should be allowed to set seed. Tall wheatgrass in particular becomes rank, unpalatable and can render a paddock untrafficable if not grazed hard during spring and early summer. Where legumes are a minor component of saltland pastures, grazing management decisions should be made to advantage the other desirable pasture grass or shrub components that represent the long term stability of the pasture.

Weediness. None of the currently available legumes with some salinity and/or waterlogging tolerance have any significant weed potential – quite the opposite, getting them to persist on even mildly saline sites can be a major challenge.

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Prospects

Current research

A review by researchers involved in the CRC Salinity confirmed that few pasture legumes were suited to Australia’s discharge environments and none were capable of performing across the full range of saltland conditions. It also reinforced the variable and complex nature of saline environments, and the necessity to ensure that species are adapted to the prevailing conditions.

Melilotus siculus and burr medic, appear to offer the best prospects for introducing legumes into southern environments. In contrast to balansa clover, the present industry standard for waterlogged, saline environments, M. siculus possesses true salt tolerance, in combination with waterlogging tolerance, and offers exciting prospects for producers. It also lacks high levels of coumarins found in some other Melilotus species. These studies emphasise the importance of understanding the mechanisms of salt and waterlogging tolerance and how they impact on individual pasture species.

Further research by the CRC Salinity quantified the production and persistence of many legumes on saltland, but it is important to recognise that this information represents snapshots at specific places at a particular time. Notwithstanding that, research at five saline sites in WA and SA showed encouraging results for production and persistence of burr medic cv. Scimitar and Melilotus siculus compared with other annual legumes (including balansa clover).

Priority species were grouped by the CRC Salinity into categories according to their readiness to be introduced into farming systems and the level of further breeding and development required. The three categories are:

  1. immediate application: species and cultivars that have good adaptation and that can be immediately introduced into farming systems;
     
  2. breeding program in place or high priority for breeding: species that proved well adapted and have a clear role to play in the farming system but require further breeding to increase their usefulness;
     
  3. species of interest: species showing promising adaptation but requiring further evaluation to better understand their potential and their likely role in the farming system.


Melilotus siculus
was one of the most salt-tolerant legumes tested and was identified as a priority for further development. It has been the focus of intensive selection efforts, with the intention of releasing the first cultivar of this species. M. siculus possesses higher levels of salt tolerance than all other commercial legumes tested. This, in conjunction with its waterlogging tolerance and low coumarin levels, suggests that it can make a significant contribution to Australian agriculture. However, the selection work is currently on hold while the Future Farm Industries CRC research focusses on finding a suitable rhizobium that is able to persist over summer on saline soils and nodulate regenerating seedlings of M. siculus. Unless this problem can be solved, the species cannot be commercialised. A persistent annual pasture legume adapted to moderate–highly saline, waterlogged environments would complement the contribution made by grasses such as puccinellia and tall wheatgrass.

The same research suggests that burr medic (Medicago polymorpha) is under-utilised in discharge environments. Increasing the waterlogging tolerance of this moderately salt tolerant species would further enhance its potential. These observations suggest that future plant selection efforts within burr medic may be better directed towards selecting for increased waterlogging tolerance, combined with salt tolerance. However, it is unlikely that burr medics will ever be truly waterlogging-tolerant plants.

Table 10.1 summarises CRC Salinity and Future Farm Industries CRC research, searching for new or existing legumes with potential for improvement, with respect to discharge sites.

Table 10.1 Status and priority of legumes identified in the field discharge nurseries
1, cultivars available for immediate use; 2, breeding in progress; 3, further breeding a high priority; 4, performance warrants further investigation

Plant species and priority rating  

Target landscape  

Agro-ecological niche  

Hedysarum coronarium (sulla) 1, 4  

Well drained, mildly saline soils

New, short-lived perennial legume. Thought to possess moderate salt tolerance but lacks waterlogging tolerance 

Lotus tenuis (lotus) 4

Low salinity, waterlogged environments 

Requires adequate summer moisture to persist  

Medicago polymorpha (burr medic) 1, 2, 3 

Well drained, low–moderately saline environments 

Potential to exploit salt and waterlogging tolerance through further breeding  

Medicago sativa (Lucerne) 1, 4

Only suited to well drained environments

Performed well in the summer dominant rainfall zone. Potential to select for increased waterlogging tolerance 

Melilotus siculus 2  

Low lying moderately saline environments 

New annual legume with moderate levels of salt and waterlogging tolerance 

Trifolium fragiferum (strawberry clover) 1

Suited to low-lying, waterlogged situations that remain moist over summer  

Requires soil moisture over summer for good persistence but productivity often low 

Trifolium michelianum (balansa clover) 1

Non-saline or mildly saline areas with high levels of waterlogging and/or inundation

Low salinity tolerance. Early maturity assists in salt avoidance in some environments

Trifolium tomentosum (wooly clover) 4  

Low salinity waterlogged environments 

Small seeded annual legume capable of good seed yields and regeneration. Tolerates waterlogging. 


Future prospects

Legumes for saltland have been described in this Saltland Solution as the ‘holy grail’. If legumes were available for a wide range of saline (including waterlogging) situations, then the economics of saltland pastures would receive a considerable boost. This boost would come from a combination of the direct contribution that legumes can make to improved performance from grazing animals, and the indirect contribution that legumes make (via fixed nitrogen) to the growth and feed value of the grasses in the pasture mix.

For these reasons, legumes for saltland continue to receive significant research effort and farmer interest which must improve their potential to play a greater role in saltland pasture systems.

However, the gap between current reality and the ‘exciting future prospects’ for legumes in saltland is considerable. Firstly, legumes as a group tend to be much less salt-tolerant than grasses, apparently due to their relative inability to exclude the toxic salts that disturb enzyme activity once taken up into the plant. Secondly, 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 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.

In other words, three quite significant challenges need to be overcome simultaneously, and research in progress aims to achieve this.

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