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UNIT 7

Saltland Toolbox

 

7.2 Measuring salinity & waterlogging

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Impact on plants

Salinity refers to the presence of dissolved salts in soil and water, and the impact on plants is independent of whether the salinity is natural (‘primary’ salinity that was present prior to the development of land for agriculture) or human-induced (‘secondary’ salinity largely caused by land use change).

Salinity affects crops and pastures by effectively reducing the amount of water available to the plant – somewhat ironic, given that salinity often occurs in association with waterlogging. The more salt that is present in the soil, the harder it is for plants to extract water from the soil. There can also be impacts from the toxic effects of some salts, poor soil aeration, or other harmful soil properties such as sodicity.

Some plants are extremely sensitive to salinity, whilst others are moderately tolerant and yet others highly salt-tolerant. Figure 7.1 gives a broad overview of the relative tolerance of crops and pastures to soil salinity – the salinity figures in Figure 7.1 are ECe values.

Charts such as shown in Figure 7.1 should be used only as a general guide, as not all salts are the same and in a paddock situation soil salinity can never be considered in isolation from other factors such as waterlogging and soil type.


Figure 7.1 Approximate salinity ranges over which individual crops and pasture species will grow.

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Salinity indicators

Indicators are signs or symptoms that suggest salinity might be affecting a site. Some of the common indicators include:

  • Sites may be moist in summer. Sheep especially like to overgraze these and camp on them during summer, so they become bare and 'scalded';
  • Often soil colour will change, becoming darker as the site stays wet longer;
  • The sites may be especially waterlogged (boggy) in winter; water may  pool on the soil surface
  • Patches in cropping paddocks show poor health;
  • Salt-tolerant plants begin to appear;
  • Trees and shrubs decline or die; and
  • White crusts develop on the soil surface when it dries out.

However, indicators are not always easy to detect and some are similar to indicators of other problems, for example, tree dieback may be caused by non-saline waterlogging, insect attack or fungal diseases. Local knowledge is very useful so if you are unfamiliar with the particular symptoms associated with early stages of land salinisation in your district, then consulting someone with local knowledge is a good idea.

The actual salinity measurements associated with different classifications of soil salinity are presented in Table 7.8, but the indicators in Tables 7.3 to 7.7 can provide some ‘guidance’ as to the severity of salinity and waterlogging.

Table 7.3 Indicators for sites with low salinity.

Salinity Classification – Low (subsoil (25 - 50 cm) salinity 2 – 4 dS/m ECe)
Sensitive plants start to be visibly affected on the saltland site, there is reduced vigour of annual legumes and the most salt sensitive (eg sub-clover) disappear. The vigour and yield of some grain legumes are affected, but cereals are generally not visibly affected by low salinity levels.

Indicator species for low/moderate waterlogging:
capeweed, annual ryegrass, barley grass, woolly clover, smooth heliotrope, pigweed, ice plant, windmill grass, prairie grass, burr medic.

Sown pasture species such as Italian ryegrass, Rhodes grass, kikuyu, Persian clover, gland clover, balansa clover, lucerne, barrel medic, phalaris, or tall fescue may also be present.

Indicator species for high levels of waterlogging:
Yorkshire fog, spiny rush, toad rush, beard grass, barley grass, buck’s horn plantain, common couch.

Sown pasture species such as tall wheatgrass, puccinellia, kikuyu, strawberry clover, Persian clover, gland clover or balansa clover may also be present.


Table 7.4 Indicators for sites with moderate salinity.

Salinity Classification – Moderate (subsoil (25 - 50 cm) salinity 4 – 8 dS/m ECe)
Most agricultural plants are visibly affected. Annual legumes are struggling to survive or are absent from the site. Plant growth is often patchy. Grain legumes and cereals are strongly affected. Most perennial pasture grasses also show reduced growth and vigour.

Indicator species for low/moderate waterlogging:
Sea barleygrass, beard grass, black roly poly, woolly clover, ruby saltbush, common couch, Yorkshire fog, smooth heliotrope, pigweed, ice plant, bushy starwort, buck’s horn plantain, groundsel bush, rhagodia, orache, wavy-leaf saltbush, small leaf bluebush, ruby saltbush,

Sown saltland and pasture species such as river saltbush, old man saltbush, creeping saltbush, golden wreath wattle, Italian ryegrass, annual ryegrass, tall wheatgrass, Rhodes grass, burr medic, phalaris or tall fescue may also be present.

Indicator species for high levels of waterlogging:
Yorkshire fog, toad rush, spiny rush, Australian saltgrass, beard grass, orache, groundsel bush, river saltbush, marine couch, saltwater couch, common couch,

Sown saltland and pasture species such as puccinellia, kikuyu, tall wheatgrass, strawberry clover, Persian clover or gland clover may also be present.


Table 7.5 Indicators for sites with high salinity.

Salinity Classification – High (subsoil (25 - 50 cm) salinity 8 – 16 dS/m ECe)
Even salt tolerant plants are affected at this level of salinity. Annual legumes are completely absent and even the most salt tolerant cereals (barley and cereal rye) are highly restricted by the conditions. Sea barley grass often dominates these highly saline sites. Bare areas are likely to be present – these may be large if uncontrolled grazing by sheep is allowed. Trees may be dying on and around the site.

Indicator species for low/moderate waterlogging:
Sea barley grass, small leaf bluebush, buck’s horn plantain, ice plant, stonecrop, salt sand spurrey, bushy starwort, beard grass, orache, wavy-leaf saltbush.

Sown saltland species such as river saltbush, old man saltbush, golden wreath wattle or tall wheatgrass may also be present.

Indicator species for high levels of waterlogging:
Water buttons, streaked arrow grass, glasswort, creeping brookweed, curly ryegrass, Australian saltgrass, samphire, marine couch, saltwater couch.

Sown saltland species such as puccinellia or distichlis may also be present.


Table 7.6 Indicators for sites with severe salinity.

Salinity Classification – Severe (subsoil (25 - 50 cm) salinity 16 – 32 dS/m ECe)
Such sites are only suitable for highly salt tolerant plants. There will usually be significant areas of bare ground and it is likely that salt crystals will form on the soil surface over summer. Waterlogging is common, and trees will usually be dead or dying.

Indicator species for low/moderate waterlogging:
Samphire, curly ryegrass, marine and saltwater couch, ice plant, streaked arrow grass, salt sand spurrey, sea barley grass, old man saltbush, bare ground.

Indicator species for high levels of waterlogging:
Samphire, glasswort, curly ryegrass, marine couch, saltwater couch, curly ryegrass, streaked arrow grass, water buttons, creeping brookweed, puccinellia, distichlis, bare ground.


Table 7.7 Indicators for sites with extreme salinity.

Salinity Classification – Extreme (subsoil (25 - 50 cm) salinity 16 – 32 dS/m ECe)
Only the most salt and waterlogging tolerant species can survive at these salinity levels and saltland pastures are not an option.

Indicator species for low/moderate waterlogging:
Mostly bare ground and extensive salt crystallisation at the soil surface.

Indicator species for high levels of waterlogging:
Samphire, glasswort, water buttons, bare ground

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Measuring salinity of soil samples

Measuring soil salinity in the plant root zone can be important in confirming dryland salinity and in the selection of appropriate salt-tolerant species so as to avoid expensive mistakes. There are many publications that can assist in taking and interpreting salinity measures. Soil salinity can be measured in the same way as for soil nutrient testing. However, a few additional points should be considered:

  1. sample at the end of summer after an extended dry period to determine the salt concentrations at their greatest
  2. take comparative samples during the year to determine seasonal variation in soil salinity influenced by leaching by rainfall
  3. for deep rooted plants subsoil salinity measurements (25 - 50 cm) are required.

For more information, see Measuring Salinity or Testing for soil and water salinity

Salt increases the ability of water to conduct electricity – the more salt there is in a solution, the easier it is for electric current to flow. Therefore, measuring electrical conductivity (EC) is an indirect measure of the salt content, often expressed as desiSiemens per metre (dS/m).

There are two ways to measure soil salinity:

  1. ECe is the electrical conductivity of the saturated soil paste extract and can only be measured in a laboratory. The soil saturation extract is created by adding water to a dry soil until it just becomes saturated. This water is then separated from the soil, and the salinity of the water is then measured. ECe values are the standard way to display plant soil salinity tolerance data, but ECe cannot be measured by farmers in the paddock, and is more expensive.
     
  2. EC1:5 (EC 1 to 5) is the electrical conductivity of a 1:5 soil/ water mix. EC1:5 values provide a cheap and easy way to estimate ECe. This requires access to a hand-held EC meter and a vessel in which to mix 1 part dry soil and 5 parts rainwater by volume. The mixture is shaken vigorously and after allowing time for settling, the electrical conductivity of the clearer fluid at the top can be measured.

Researchers prefer to use ECe as the measure of soil salinity because it is directly related to the salinity of the soil solution – ie, the salinity that plants growing in the soil actually experience. In saturated (waterlogged) soil, roots experience a salinity equivalent to the ECe. As the soil dries however, the plants experience increasing salinity in the soil solution, until at wilting point, the salinity of the soil solution will be approximately four times the measured ECe.

Fortunately there are rules of thumb than can be used to convert EC1:5 readings to ECe readings, but they depend on soil texture, as shown with the following conversions:

  • For sands multiply the EC1:5 value by 15
  • For loams multiply the EC1:5 value by 9.5
  • For clays multiply the EC1:5 value by 6.5
  • Or use values in between for ‘intermediate’ soil types

Table 7.8 shows the Australian classification for soil salinity based on ECe values, with conversions for EC1:5 values in soils of different textures

Table 7.8
. Australian classification system for classification of soil salinity

Term

ECe range

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

>2.9

>4.01

Samphire


Many people relate to salinity through their experience with seawater. A useful rule of thumb to remember is that salinities can be converted to their equivalent as a percentage of seawater knowing that the electrical conductivity of seawater is 45 - 60 dS/m.

However, salinity is often measured by researchers using a wider range of units (eg. moles per litre, milligrams per litre, MegaPascales, etc.). Also many older farmers still refer to salinities in terms of “grains per gallon”. Values in any unit can be converted into their approximate equivalent using the conversion factors given in Table 7.9. The conversion factor varies depending on the salts in the water sample. If the salt is purely NaCl the conversion is 550 and conversely is 800 to 900 if the salts are gypsum or bicarbonates. In SA the factor 640 is widely used based on the analysis of a large number of water resources.

Table 7.9 Unit conversions for soil and groundwater salinity

Soil and Water
Salinity Units

dS/cm

mS/cm

mS/cm

mg/L
ppm

gr/gal

mol/m3

mmol/L

conversion factors

decisiemens per metre

dS/m

 

1

1000

670

40

12

12

millisiemens per centimetre

mS/cm

1

 

1000

670

40

12

12

microsiemens per centimetre

mS/cm

0.001

0.001

 

0.67

0.04

0.01

0.01

milligrams per litre;
parts per million

mg/L
ppm

0.0015

0.0015

1.5

 

0.06

0.02

0.02

grains per gallon

gr/gal

0.02

0.02

20

14

 

0.3

0.3

moles per cubic metre

mol/m3

0.085

0.085

85

55

3

 

1

millimoles per litre

mmol/L

0.085

0.085

85

55

3

1


Conversion:
select the unit to be converted down the left hand column and then move across to the column containing the desired unit and multiply by the corresponding factor.

Example:
to convert dS/m to mg/L, multiply by 670 (i.e. 7 dS/m is equivalent to 4690 mg/L)

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Measuring salinity in situ

The salinity of soil samples is determined by measuring the electrical conductivity of a soil-water solution because conductivity increases with salinity. The same principle can be used in the field with “electromagnetic induction” used to estimate the apparent electrical conductivity of the bulk of the soil. The tools that measure this electromagnetic induction are called “EM” meters – either EM38 (for shallow soil measurement – less than a metre), or EM31 (for measuring deeper in the soil – up to 6m). The EM38 is used in the explanation below because it ‘measures’ salinity in the root zone and is therefore a more appropriate tool for identifying the potential for saltbush and other deep rooted perennials establishment on suspected saline sites.

The soil depth over which the EM38 will read depends on the orientation of the instrument – it has two orientations – “vertical” and “horizontal”. In the horizontal position, 50% of the measurement is taken from the upper 0.4 m of the soil profile; in the vertical position, 50% of the measurement is taken from the upper 0.85 m of the soil profile. If one wanted to locate shallow-rooted plant species in a saline landscape, then the EM38 should be used in the horizontal orientation, but if one wanted to locate deeper rooted plants in the landscape then the EM38 should be used in the vertical setting. The EM maps (Figure 7.2) for the SGSL site near Hamilton (Victoria) suggest that for some areas (for example in the lower left quarter of the maps), the salinity was highest in the deeper soil, while in other areas (mostly on the upper half of the maps), the salinity was higher in the shallow soil.

Unfortunately, EM38 readings are not just affected by soil salinity: they are also affected by: (a) soil moisture – other things being equal dry soils will have lower readings than moist soils, and (b) soil texture – other things being equal clays have higher readings than loams and sands. In other words, there will not be strong universal relationships between EM38 readings and plant growth and survival. However, the EM38 can be used as a mapping tool to indicate general areas of higher conductivity compared with general areas of lower conductivity, and this can be a valuable tool in helping farmers identify differences in saltland capability at the paddock scale and develop paddock-scale plans.

Because the EM38 measures an “apparent” electrical conductivity in the soil, these readings are often abbreviated as ECa readings.

Figures 7.2a and 7.2b EM38 ‘maps’ of the SGSL site near Hamilton – the stronger the red colour, the higher the salinity reading. Figure 7.2a shows readings taken in the ‘horizontal’ or shallow mode, while Figure 7.2b shows readings taken in the ‘vertical’ or deeper mode.

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Variability of soil salinity measurements

A big problem with surface soil salinity is that it can vary greatly through the year. Figure 7.3 shows ECe data from the SGSL site in Western Victoria for the surface 10 cm of soil for 3 areas from the same site as shown in the EM38 maps in Figure 7.2a and 7.2b. As a general rule, salinity peaks in late summer or autumn when much of the soil moisture has evaporated, concentrating the salt in the soil surface. In Figure 7.3, the concentration effect over summer was strongest at the locations within the site that had the highest overall salinity.


Figure 7.3. Soil salinity (ECe) variation over time at the SGSL site in Western Victoria. The red line is the average across parts of the site that have high salinity, blue from sites that have moderate salinity and green from sites that have low salinity.

These data clearly show what a poor diagnostic the single measurement of shallow soil salinity can be – the values are highly variable in time. A programmed is needed to provide a consistent view of the “inherent” capability of the site. For example, depending on the date of sampling, the soils of the red line could have been classified as being anything between highly saline (12 dS/m) to extremely saline (48 dS/m), the soils of the blue line could have been classified as being anything from moderately saline (6 dS/m) to severely saline (26 dS/m), and the soils of the green line could have been classified as being anything from non-saline (2 dS/m) to highly saline (9 dS/m).

Another example of the extreme variation in surface soil salinity is presented in Figure 7.4, from data collected by Stan Smith in 1956 on a completely bare piece of saltland near Quairading in Western Australia.

Figure 7.4. Seasonal changes in soil salinity down an uncultivated soil profile near Quairading WA (after Smith, 1962).

These data illustrate an important point – surface soil salinity measurements are extremely variable so that single samples can give quite misleading information about the capability of saltland sites. In WA where saltbush is the preferred system, assessments of saltland capability are routinely based on the salinity of the subsoil (depth 25–50 cm). Because these values don’t change very much seasonally (see Figure 7.4), readings at any time of year can predict a site’s capacity. However, for the establishment of many shallow rooted plant based systems or the understorey for saltbush, a programmed soil sampling (0 -10 cm) in summer and in the winter/spring is recommended. Valuable information can also be gained on soil fertility (particularly phosphorus status of the site) from these soil samples. Fertility status is important to assess the potential and the inputs required for renovating the site with a range of salt tolerant plants.

The challenge associated with the fact that salinity also varies greatly across most saline sites is more difficult. The spatial variation at the SGSL site in western Victoria (shown in Figures 7.2a and 7.2b) is quite typical. Taking an ‘average’ over such variable sites gives little indication of what might profitably be sown in a location. If a site consists of a mixture of 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; the recommended planting may need to consist of a shotgun mixture of species to accommodate this variation.

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Measuring waterlogging

Most 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 carbon dioxide, a range of organic compounds such as ethanol and organic acids, and the plant hormone ethylene. With prolonged waterlogging to the soil surface, anaerobic bacteria in the soil can change soil nitrogen, manganese, iron and sulphur to forms that are either lost from the soil (soil nitrogen is transformed into atmospheric nitrogen) or that are toxic to plants.

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 to both waterlogging and slainity.

Although oxygen deficiencies are the major cause of waterlogging damage to plants, such deficiencies are nearly impossible to measure on a paddock-scale. We therefore classify waterlogging on the basis of the depth to watertable in winter as shown in Table 7.10.

Table 7.10 Classification of soil waterlogging status using depth to the watertable in winter

Severity of waterlogging

Average depth to watertable (m) in winter

Suitable plants for saline areas

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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