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UNIT 4

Plant & animal performance

 

4.1  Understanding salinity & waterlogging

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The techniques and tools for measuring soil salinity and waterlogging are presented in Unit 7 of the general saltland information – The saltland toolbox. The information presented here is about understanding salinity and waterlogging, and the impacts on plants growing on saltland.

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Soil salinity measurement

The salinity of water is determined by measuring its ability to conduct electrical current. This works because, in water, the salt separates into sodium (Na+) and chloride (Cl-) ions that have an electrical charge and therefore can carry an electrical current.

There are two ways to measure soil salinity:

  1. ECe is the electrical conductivity of a saturated soil paste extract and can only be measured through a laboratory.
     
  2. EC1:5 (EC 1 to 5) is the electrical conductivity of a 1:5 soil/ water mix. EC1:5 is cheap and easy requiring only access to a hand-held EC meter and a vessel in which to mix 1 part soil and 5 parts rainwater by volume.

Researchers prefer to use ECe because it is directly related to the salinity of the soil solution – ie, the salinity that plants growing in the soil actually experience.

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Soil salinity classification

The severity of salinity in soils can be classified on the basis of ECe on a simple logarithmic scale. Table 4.1 below defines ECe ranges for soils that are non-saline or that are of low-, moderate-, high-, severe- or extreme-salinity. This table also provides conversions between the various ECe ranges and EC1:5 ranges for soils with sandy, loamy or clay textures.

Table 4.1. Australian classification system for classification of soil salinity.

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 

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Changes in soil salinity

Soil salinity measurements can help predict which plants will grow on a piece of saltland and which will not. Unfortunately, the salinity of the soil varies dramatically with the depth interval over which it is sampled and the time of year. This point is well illustrated using the data collected by Stan Smith in 1956 on a completely bare piece of saltland near Quairading in Western Australia (Figure 4.1). This is quite a typical pattern in southern Australia as the winter rains wash the surface salt down into the soil, while evaporation over summer draws salts to the surface.

This kind of variation can potentially have major consequences for site diagnosis. For example, if we had diagnosed the capability of this soil based on the surface soil readings in winter, we might have concluded that the site is suited to the growth of saltbushes and small leaf bluebush. However the relatively non-variable subsoil salinity measurements clearly show that this is not the case; with ECe values of 45 dS/m, this site would have only been suited to the growth of samphire.

These data illustrate an important point – surface soil salinity measurements can give quite misleading information about the capability of saltland sites. We therefore routinely base our assessments of saltland capability on the salinity of the subsoil (depth 25–50 cm) and recommend this for others. Because these values don’t change very much seasonally, we can take the readings at any time of year to predict a site’s capacity.

Alternatively, a series of surface samples (0-10 cm) can be taken throughout the year (autumn, winter and spring) to make an assessment of the site for the potential of establishing grasses and legumes.

figure 4.1

Figure 4.1. Seasonal changes in soil salinity down an uncultivated soil profile near Quairading WA (after Smith, 1962).

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Spatial variation

Many saline soils show strong patchiness, with mosaics of plants actively growing intermingled with bare patches over distances of a few metres. What causes these effects and should they influence where we take our soil samples to measure salinity? Figure 4.2 shows the variation in surface (0-0.4m) soil salinity at the SGSL research site near Hamilton (Vic).

Figure 4.2

Figure 4.2 Variations in soil salinity at the SGSL site near Hamilton – measured with an EM38 metre in the vertical mode. The stronger the red colour, the higher the salinity reading.

How should we measure soil salinity across a site given these effects? There is no easy answer! If there are bare and grassy areas we suggest that separate samples be taken of subsoil salinity beneath grasses and beneath the bare areas. This will give a good indication of the extent of variation and recommendations for which species to use can be based on the two readings.

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Waterlogging

Dryland salinity is caused by the presence of shallow watertables in the landscape. Watertables typically rise and fall seasonally in response to rainfall, internal drainage and evapotranspiration. If watertables become shallower than about 30–40 cm, the soils may become “waterlogged”.

Waterlogging causes soils to become devoid of oxygen, often within a few days. In addition, waterlogging causes an accumulation in the soil of compounds that can be toxic to plant roots. Measuring these things at paddock scale is nearly impossible so waterlogging is classified on the basis of the depth to watertable in winter as tabulated below.

Severity of waterlogging

Average depth to watertable
(m) in winter

Suitable plants

Non-waterlogged

Deeper than 0.5

Old man saltbush, small leaf bluebush

Low waterlogging 

0.3 – 0.5

River saltbush, rhodes grass, kikuyu  

Moderate waterlogging

0.1–0.3

Tall wheatgrass

High waterlogging 

0 – 0.1 

Puccinellia, saltwater couch, samphire  

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Saltland capability

Soil salinity is seldom the sole limiting factor to plant growth on saltland. Aside from waterlogging and inundation mentioned above, other factors might include soil sodicity, acidity or alkalinity, soil type and rainfall.

It is therefore more useful to talk about saltland capability when assessing the opportunities for productive use of salt-affected land.

Other things being equal, high capability saltland can support productive plants such as balansa clover, lucerne, tall fescue and possibly even barley, and presents opportunities for improvement through good management. At the other extreme, low capability saltland will support little other than samphire.

Between these extremes there is potential for saltbush, puccinellia, tall wheatgrass and other plants with significant grazing potential.

The prospects for managing saltland across Australia vary from region to region, although there will also be significant differences within regions because of the heterogeneous nature of saltland.

The CRC Salinity publication Prospects for Saline Land contains a comprehensive review of the recommended plant-based systems for managing saltland across 16 regions in Australia. For more information, see Saltland Prospects Part A for an overview, or Saltland Prospects Part B and Saltland Prospects Part C for more detailed, region by region analysis. 

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