Sub-tropical grasses with limited salinity tolerance

Landscape niche

All plants have landscape niches or zones (combinations of climatic and soil conditions, and management) where they are most competitive or where they will perform best. Saltland plants are the same, each tending to have a particular set of climatic (rainfall, temperature etc) and soil (salinity, texture, waterlogging etc) factors which determine where they will be able to survive, and where they are likely to thrive. For sub-tropical grasses with limited salinity tolerance, these factors are summarised in Figure 9.3. 

Figure 9.3 Most likely situations for sub-tropical grasses with limited salinity tolerance.

Key to symbols
	This is the zone most preferred by the temperate grasses with limited salinity tolerance and where they are highly recommended;
	The temperate grasses with limited salinity tolerance are one of the possible options for this zone but it is outside their preferred conditions

Common indicator species

The ‘classical’ indicator species for saltland (such as sea barleygrass, marine or saltwater couch, water buttons etc) all tend to indicate levels of salinity and/or waterlogging that will exceed the tolerance of the sub-tropical grasses such as Rhodes grass and kikuyu. The classical indicator species mentioned above are likely to be towards the centre of a saline site where the salinity levels are typically highest, while the land suitable for sub-tropical grasses will tend to be towards the edges of the saline site and in the surrounding non-saline areas.

The more likely indicator species for where the sub-tropical grasses might perform well will tend to be those species that are similarly tolerant of moderate salinity and waterlogging – barleygrass, annual ryegrass or windmill grass (a native, warm season grass) – as shown in Figures 9.4, 9.5 and 9.6. These ‘indicator’ species are quite widespread in mildly saline and non-saline environments, and should be considered as ‘weak’ indicators unless they are specifically located around the margins of more saline land.

SALTdeck cards can be viewed on this website or they ordered from the Land Water and Wool website. 

 Figure 9.4 - SALTdeck card showing the photo and text sides for barley grass. 

Figure 9.5 - SALTdeck card showing the photo and text sides for annual ryegrass. 

Figure 9.6 - SALTdeck card showing the photo and text sides for windmill grass.

Salinity and waterlogging requirements

Data on salinity tolerance of these species under production conditions is limited, however comparative research with marine and saltwater couch and Distichlis irrigated with saline water, showed that shoot dry weight by was reduced by 50% for the couch and Distichlis species over the range 15-40 dS/m.

In contrast, kikuyu had a 50% decrease in shoot dry weight at a mere 4–5 dS/m. It is generally considered that Rhodes grass and kikuyu have similar salinity tolerances, although NSW information has indicated that 6.6 dS/m in irrigation water was required to reduce Rhodes grass growth by 10%, while only 4.1 dS/m was required to reduce kikuyu growth by 10% - note – salinity levels measured in the soil are not directly comparable with measuring the salinity of irrigation water that may be applied.

In summary, both kikuyu and Rhodes grass will grow well in soils with low salinity (2-4 dS/m) and to a lesser extent in soils with moderate salinity (4-8 dS/m).

Waterlogging is damaging to the growth of sub-tropical grasses by reducing the oxygen supply to the roots. Plants that are highly tolerant of waterlogging usually have special ‘pipes’ in their roots to bring air from the soil surface. For the sub-tropical grasses, and others that are not highly salt-tolerant, any increase in waterlogging reduces the tolerance for salinity.

For the sub-tropical grasses (ie kikuyu and Rhodes grass), that have only limited salinity tolerance, surface water management can be critical. If surface water can be diverted away from the site at low cost, then it is likely that either these grasses will be more productive, or a larger proportion of the saline area will be suitable for them. There may be legal restrictions associated with diverting non-saline surface water away from a saline site in order to reduce inundation and waterlogging.

On the other hand, drainage options that include drawing water from a saline water table can create disposal problems and are therefore heavily regulated in most jurisdictions – local advice is essential before planning any such drainage.

Soil & Climatic requirements

Soils

Rhodes grass is widely adapted across many soil types. It performs best on light to medium textured soils as some establishment difficulties have been reported on heavy soils. Soils need to be reasonably well drained as Rhodes grass has poor tolerance of waterlogging. Rhodes grass does not tolerate extreme soil acidity and high exchangeable aluminium levels. High levels of soil fertility are required to support high yielding Rhodes grass pastures.

Kikuyu is widely adapted across many soil types, similar to Rhodes grass, and is naturalised on many of the higher fertility, coastal soils. It is more waterlogging-tolerant than Rhodes grass and performs well on medium and well-drained soils with high fertility. Kikuyu is highly tolerant of acid soils. In SA and WA kikuyu grows on sandy soils with low fertility and withstands summer drought.

Climatic requirements

Kikuyu and Rhodes grass are C4 species that require temperatures in excess of 15^oC for their photosynthetic pathway to operate. From a salinity management perspective, this means these species are only suited to coastal regions, particularly to the northern agricultural region in WA and to central and northern NSW.

It has been generally considered that Kikuyu is more highly drought-tolerant than Rhodes grass although there is some evidence that Rhodes grass might be more drought-tolerant than kikuyu. In any case, both are reasonably hardy and will respond well to summer/autumn rain. For saline sites where salinity levels often build up over summer, this summer rainfall is probably essential for leaching salt out of the topsoil and keeping the salinity levels within the range that can be tolerated by the sub-tropical grasses. If the saline site has a relatively shallow watertable that is only mildly or moderately saline, these sub-tropical grasses are deep rooted and will be able to access this groundwater.

Both species prefer frost free climates, although both will survive in frost prone areas that have summer rainfall such as the northern slopes of NSW. 

Kikuyu has a much wider range of climate tolerance than Rhodes grass and is widely grown in nonsaline areas of southern Australia, including the Southern Coast of WA, Kangaroo Island and Upper South East of SA and the South West  and  Gippsland Districts in Victoria.