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UNIT 2

Saltland Basics

 

2.2 Understanding salinity

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Signs & symptoms

Saltland areas are much easier to rehabilitate before the salinity and waterlogging become severe and reduce the options available, so early identification is a great benefit. There are a range of indicators (signs or symptoms) that may appear if dryland salinity is affecting a site – some of the most common, starting with the earliest, might include:

  • Changes in crop health, with patches of poor growth;
  • Changes in pastures, with a tendency for these to lose their legumes and become increasingly dominated by grasses
  • Tendency for a site to be wetter than the rest of the paddock, and possibly remain green further into summer when other annuals have died off;
  • Soil becoming darker;
  • Noticeable tree decline or patches of unthrifty grasses and shrubs;
  • Sheep starting to graze these areas preferentially because of the salt content;
  • Salt-tolerant plants (indicator species) starting to colonise;
  • Bare and 'scalded' patches appear, exacerbated by overgrazing;
  • A white salt crust appears on the soil surface in summer when the soils are dry.

Some of these indicators are not always easy to detect, and may take several years before being really noticeable. Some might be indicators of issues other than (or as well as) salinity - for example, bare areas may be caused by acidic soils, and tree dieback may be the result of other factors like pest and disease attacks.

There are specific measurements/investigations that can confirm the presence and severity of salinity and waterlogging, as explained in Unit 7 of the general saltland information – The saltland toolbox.

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Salinity or waterlogging?

Dryland salinity has two different impacts on plants: an osmotic (drought) impact as salt in the soil makes it increasingly difficult for plants to take up water, and a toxic impact where the salt, once taken up into the plant slows down plant functions.

Prolonged waterlogging results in less oxygen available to plant roots. Salinity is often found together with waterlogging, as both stresses are generally associated with elevated watertables.

Waterlogging increases the susceptibility of plants to salt damage by causing the plant roots to become more permeable to salt, so that salt uptake into the shoots is vastly increased. To successfully grow in many saline situations, plants have to be tolerant of both waterlogging and salinity.

The series of photos in Figure 2.1 show how the combined effect of salinity and waterlogging has a much harsher effect on plant growth, compared to salinity on its own. In the graph below, waterlogging with zero salt would have decreased plant yields by ~50%, however the combination of waterlogging with low level salinity (2 dS/m) would have decreased plant yields by 100%. Interestingly the plants grown under drained conditions with the highest level of salinity (12 dS/m – equivalent to about 20% of seawater) were still alive and would have produced some grain.

figure 2.1a

Figure 2.1a

figure 2.1b

Figure 2.1b

figure 2.1c

Figure 2.1c

Figure 2.1. In each photo, the 2 pots of wheat on the left are waterlogged while the 2 pots on the right are freely drained with (a) no salt, (b) salt equivalent to 4% sea water, and (c) salt equivalent to 20% sea water.

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Saltland indicator plants

Saltland indicator plants can be of great value for diagnosing land affected by dryland salinity, however perennial plants might be indicating a previous salinity/waterlogging problem rather than the current situation. Because plants grow in response to the combined impact of salinity, waterlogging and other site constraints, they can be the most useful ‘indicators’ of the site’s potential. At the very least, they can complement direct measurements of soil salinity and depth to groundwater.

Plants fall into three broad groupings with respect to salinity tolerance. Halophytes (such as marine couch, puccinellia and saltbush), salt-tolerant non-halophytes (such as barley and barley grass) and salt-sensitive non-halophytes (such as most legumes). Their typical responses to salinity are graphed below in Figure 2.2. The absolute values of salt in Figure 2.2 do not matter - more significant are the relative differences:

  • Halophytes actually grow better on sites with low salinity than on non-saline sites. They are therefore key components of saltland pastures. They will persist in soils of high to extreme salinity.
     
  • Non-halophytes vary in their tolerance to salt. The more tolerant species will survive soils with low and even moderate salinity and may therefore be components of saltland pastures, while the less tolerant species will be sensitive to soils of low salinity and will therefore almost certainly not be components of saltland pastures

figure 2.2

Figure 2.2. Response of 3 plant species to increasing levels of salinity. River saltbush (a typical halophyte) can grow at more than 100% sea-water, while barley (a tolerant non-halophyte) and beans (a sensitive non-halophyte) are much less able to grow as salinity increases.

To assist farmers and others to identify indicator plants and saltland pasture species, the SGSL (Sustainable Grazing on Saline Land) initiative developed SALTdeck – a series of plastic coated cards that provide photos and descriptive information for the 50 most common saltland species – both saltland pastures and indicator plants. Figure 2.3 shows the two sides of a SALTdeck card – in this case sea barleygrass, one of the most common indicator species found right across Australia. SALTdeck cards can be viewed individually on this website .

SALTdeck Card  example front

Figure 2.3. SALTdeck card for sea barley grass – one of 50 saltland species in the SALTdeck series and one of the most common indicator species for saline land.


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Saltland classification & capability

The potential benefits from saltland pastures vary between sites, between enterprises and from farm to farm because of the combined stresses of salinity and waterlogging. The term ‘saltland capability’ is used to describe the specific ability of a salt affected site to support pasture growth and therefore in all situations a site assessment is required before action such as saltland agronomy is taken.

The Future Farm Industries CRC has developed a classification system for saltland because the term ‘salinity’ has been used quite loosely, and definitions differ between states and regions. The new system is based (Table 7.1) and defines the terms for different ranges of ECe values. The system also suggests appropriate conversions between these suggested ECe ranges and EC1:5 ranges for soils of different textures.

Table 2.2. Future Farm Industries CRC classification system for Australian soil salinity.

Suggested Term

ECe range
(dS/m)

 EC1:5 range
 

 Typical plants
affected

For sands
(dS/m)

For loams
(dS/m)

For clays
(dS/m)

Non-saline

0–2

0–0.14

0–0.18

0–0.25

-

Low salinity

2-4

0.15–0.28

0.19–0.36

0.26–0.50

Beans

Moderate salinity

4-8

0.28–0.57

0.37–0.72

0.51–1.00

Barley

High salinity

8-16

0.58–1.14

0.73–1.45

1.01–2.00

River saltbush

Severe salinity

16-32

1.15–2.28

1.46–2.90

2.01–4.00

Puccinellia

Extreme salinity

>32

>2.29

>2.91

>4.01

Samphire

 

One of the key principles to emerge from the Sustainable Grazing on Saline Lands (SGSL) initiative is that the primary drivers of saltland capability are sub-soil (25-50cm) salinity and depth to watertable. This combination effectively defines where particular saltland plants will be able to survive, and where they are likely to thrive. This concept is illustrated in Figure 2.4, using saltbush as the example saltland plant. In this case, saltbush can grow across quite a wide range of salinity levels (into the extreme salinity range in Table 2.2) but will struggle if the watertable during winter is routinely within 0.3m of the surface, or in summer within 1.0m of the soil surface.

Figure 2.4 Defining saltland capability for saltbush - most likely situations for effective saltbush plantings.


Subsoil salinity/ depth to watertable matrix

 Winter

Summer 

 2.4 Winter

 2.4 Summer

Drivers of saltbush zonation 

  • A halophyte and so does best with some salinity;
  • Sensitive to waterlogging but can survive a higher watertable in winter than summer;
  • Growth in summer assisted if roots can access groundwater;
  • Summer grower, doesn’t like cold;
  • Rainfall 300-400 mm

 


Key to symbols

red dot

This is the zone most preferred by saltbush and where it is highly recommended;

Small Dot

Saltbush is one of the possible options for this zone but it is outside its preferred conditions;

Dash Ring

Saltbush will most likely survive in this zone, but its growth will be poor and therefore it is not recommended.

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