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SITE ASSESSMENT

Prior to action or saltland renovation

 

Site Assessment prior to action or saltland renovation

 

A quick summary

In the process of planning what to do with saltland it is imperative that the site is assessed for its potential increase in productivity after any proposed treatment or renovation.

Drawing on local knowledge and experience is basic to success. Look for successful local examples that will indicate the potential of different plant systems. Many of these can be found in the “Genie’s Maps” section of this website.

Key points:

A range of site conditions (eg. salinity, fertility, waterlogging, soil types, pH, rainfall, topography, etc.) combine to influence the type of plants that can be grown, and the resulting levels of plant and animal production. These factors are taken into account when you use the “Genie’s Advice” tool.

  • It is important to know your country and work within its limitations.
  • On sites with fewer limitations (ie. high capability saltland) development can significantly improve production.
  • Sites with extremes of salinity or waterlogging, or other limitations, (ie. low capability saltland) have limited potential for development and may be better as ‘fence off and forget” areas.
  • Mitigation works (eg. drainage) may improve production potential.
  • Increasing salinity, along with other limitations, reduces the capability of land to grow productive pastures.

We can think about salt-affected ground as being somewhere in the range between:

‘High capability’ saltland

and

‘Low capability’ saltland

Having highly favourable site conditions.

Capable of supporting highly productive plants and high levels of animal production.

Having high potential for improvement.

  <-------------------->  

Having poor site conditions.

Capable of supporting poorly productive plants and low levels of animal production.

Having low potential for improvement.

Site Conditions

The type of plants that can be grown (and the resulting levels of animal production) not only depend on soil and groundwater salinity but also conditions such as  soil texture and fertility, the extent and duration of waterlogging and inundation, patterns and quantity of annual rainfall and topography.

Different plants (including weeds) can cope with different conditions. Unfavourable site conditions can cause reduced productivity or plant death. Conversely, appropriate species can be identified by comparing site conditions to known tolerance ranges for different species. The major species can be found on the Saltdeck Cards. When planted these species increase the productivity of the site.

A range of site conditions are discussed below. It is the combination of these conditions (rather than any single condition) that will determine if an environment is suitable for particular saltland plants.

Fertility

Residual soil phosphorus from previous farming practice and levels of potassium and the other essential nutrients are basic in assessing the potential production of a site. If the nutrient levels are adequate the potential production of the site is a lot higher with a lot less cost than a site with low nutrient status. Soil testing is a most valuable tool in making a decision as to how to proceed. Even the most salt tolerant species such as puccinellia will respond to phosphorus where soil Colwell P is < 10 mg/kg, and if P is not adequate then response to nitrogen applications is limited.

Soil testing will also indicate if the site has any toxicity problems such as high Boron levels or unsuitable pH.

Waterlogging

Waterlogging (saturation in the plant root zone) typically reduces plant vigour, exacerbating the impacts of salinity. It is influenced by a combination of rainfall, soil type and landscape drainage. As rainfall increases, salinity levels may decline but the likelihood of waterlogging impacts will increase.

Inundation (flooding)

Inundation restricts gas exchange in aerial plant tissues and even short periods can be highly damaging. There are few species that will cope with flooding for extended periods. Puccinellia is a major exception. In the Upper South East of SA where salinity declines markedly with winter rainfall, it actually appears to benefit from flooding, provided the plant is not fully submerged and surface water is managed to prevent stagnation.

Landscape drainage

If surface water and/or groundwater cannot get away, salinity and waterlogging problems are sure to follow. Low-lying, flat areas (often depositional areas that have accumulated finer/ clayey materials) are more prone to inundation with surface water, as well as adverse impacts from shallow saline groundwater.

Soil Type

Soils influence vertical and lateral drainage, and the degree to which moisture and salts are drawn up from underlying groundwater through capillary action. Heavier clay soils draw and retain moisture and salts, are more prone to waterlogging, and more resistant to flushing of salts with rainfall. In comparison to sands or loams, heavy clay soils can also:

  • impede rates of plant transpiration and growth,
  • delay germination at the beginning of the season, and
  • bring on an early finish to the season.

pH

pH levels may restrict pasture choice, or limit productivity in some species (eg. puccinellia and lucerne do not like acid soils). In some cases it may be cost effective to adjust pH levels (eg. incorporating lime to ameliorate acidity).

In some areas rising saline watertables have altered the pH of soils. In the Upper South East of SA it has been reported that areas which were typically low pH (acid) sandy soils, have after years of saline discharge, become very high pH (alkaline) soils due to sodium carbonate in the groundwater.

Salinity

Salinity refers to the presence of dissolved salts in soil and water. There is some salinity in every soil, as there is in rainfall. Salinity can be natural (‘primary’ salinity) or due to human-induced changes (‘secondary’ salinity).Primary salinity dominated sites are characterised by naturally very high to extreme salinity, salt-tolerant vegetation and/or bare scalds. The salinity in these sites is historical, being present prior to the development of the land for agricultural purposes. These areas will have poor productive potential and should be left in their natural state to be managed for environmental values. It should be noted that native vegetation (including samphire), on either primary or secondary salinity sites may be protected as in South Australia under the Native Vegetation Act 1991.

Secondary salinity dominated sites occur where soil salinity has been largely caused by land use change. The term ‘dryland salinity’ is commonly equated with secondary (human-induced) salinity associated with shallow watertables, in non-irrigated areas. These areas are the focus for establishing saltland pastures (to help reclaim significant areas of a farm that otherwise appear lost to salinity), and hence are the focus of Saltland Genie.

Salinity is variable: across the paddock; through the year; and between years. Appreciating this variability is an important step in selecting the appropriate pasture mix to maximise production.

Figure 1 displays variability in salinity across a paddock, as measured by a ground electromagnetic (EM) survey. Noticing various indicator plants is another way to identify different salinity zones.

Salinity also varies with the seasonal cycles as shown in Figure 2. Salts are flushed with rainfall and build up with capillary action and evaporation. By covering the ground, saltland pastures help to reduce peak salinity levels in summer.

Figure 1: Varying salinity across a paddock, indicated by a ground EM survey.

Figure 2: Example of seasonal trends in surface soil salinity. (Large seasonal fluctuations, especially in surface soil, such as this are often found in parts of the Upper South East of SA.)

Dryland salinity also varies between years, as groundwater levels are heavily influenced by rainfall trends. Figure 3 shows groundwater levels in discharge zones of three adjacent sub-catchments in the Mt Lofty Ranges of South Australia, each with a different land use. A period of lower than average rainfall and higher water use in the sub-catchments under trees and lucerne, have caused major reductions in salinity by lowering the watertable. [The rainfall trend (blue) in Figure 3 is the cumulative deviation from mean monthly rainfall. Upward trends indicate consecutive months when the monthly rainfall is above average, while downward trends indicate consecutive months of below average falls.]

Figure 3: Groundwater trends are influenced by rainfall and differences in catchment water use (eg. annual crops, lucerne and trees).

Salinity Mapping

Areas dominated by primary (natural) and secondary (clearing-induced) salinity have been mapped across many agricultural regions. Generalised soil salinity maps are available from the relevant state government agencies.

Defining and measuring salinity

There is no easy way to define different levels of salinity. It is usually done using a combination of:

  • Plant and landscape indicators,
  • Soil salinity measurements (surface and subsoil), and
  • Depth to groundwater measurements (if available).

For example, in South Australia, 7 classes of salinity have been defined (as shown in Table 1). Areas of land can be assigned to the different salinity classes using each of the indicators as a guide. However, no single indicator should be used on its own. Figure 4 demonstrates this by providing a visual guide to how the depth to a saline watertable can impact on salinity levels and the persistence of different plant species.

Measuring peak soil salinity (between mid summer to mid-autumn) provides a standard measure of soil salinity. However it is worth bearing in mind that salinity is dynamic, changing through the year and between years. Salt can be very mobile, with water providing the transport mechanism. Soils, geology, drainage, rainfall and the type of groundwater flow system will influence the range of salinities possible and how quickly changes can occur.

Assessing the levels of salinity (and also seasonal changes) on your saltland will assist in identifying suitable saltland pasture options

Table1: Indicative salinity levels and capability (productive potential) of saltland.

      Other classification criteria
*Saltland Capability Salinity Level Vegetation & landscape indicators #Depth to saline watertable ##Indicative ECe (dS/m)
    No evidence of effects of salt No influence < 2 (surface)
< 4 (subsoil)
High Moderately Low Subsoil salinity – deep rooted horticultural species & pasture legumes affected. Usually deeper than 2 m. < 4 (surface)
4-8 (subsoil)
  Moderate Many field crops & lucerne affected. Salt tolerant species such as sea barley grass are usually evident. Yield losses in wheat. Shallower than 2 m, capillary effect reaches into root zone. 4-8 (surface)
8-16 (subsoil)
  Moderately High Too salty for most field crops & lucerne. Salt tolerant species are common (e.g. barley grass, curly rye grass and salt water couch). Strawberry clover productivity is diminished. Unsuitable for wheat. Yield losses in barley. Seasonally within 1 m of the root zone. 8-16 (surface)
16-32 (subsoil)
    Land dominated by salt tolerant species with bare areas. Samphire & ice plant evident. This land will support productive species such as puccinellia & tall wheat grass, etc. Seasonally near the surface. 16-32 (surface) > 32 (subsoil)
  Very High Land is too salty for any productive plants & supports only samphire, swamp tea tree or similar halophytes. Near the surface most of the year. > 32 (surface)
Low Extreme Bare salt encrusted surface. Near or at the surface most of the year. > 32 (surface)

Notes:

* These salinity classification criteria were developed by the South Australian Department of Water, Land & Biodiversity Conservation (DWLBC).

Sometimes indicators can be misleading:

# Depth to watertable may be an unreliable indicator of salinity if groundwater is fresh and provides a good source of sub-surface irrigation for perennial plants.

## Soil salinity measurements can be unreliable following rainfall events. Values shown in the table are considered to be the peak soil salinities found during summer-autumn.

Indicator plants may be impacted by other factors, aside from salinity (eg. waterlogging, pH, etc.). Actual species present will vary between regions. Different varieties of a species may vary in their salinity tolerance.

EM38 measurements can also be used to gauge salinity levels, however conductivity readings are averaged over the measurement depth (0-60cm or 60-120cm) rather than just in the plant root zone.

A combination of the indicators shown above help to define the salinity level.


Figure 4: How depth to a saline watertable can influence salinity levels and the persistence of different plant species (also see Table 1 and accompanying notes).

Rainfall

Seasonal patterns and total annual rainfall are important factors in determining salinity levels and appropriate species for a site.

In wetter areas, salts tend to be diluted and flushed from the landscape (subject to landscape drainage), and plants often require greater waterlogging tolerance.

In drier areas, evaporation can have a larger influence than rainfall and salts tend to concentrate high in the profile. Plants generally need to be more salt and drought tolerant.

Annual rainfall levels can also provide an indication of the upper limits of productivity, assuming that pastures are capable of using all the rain that falls. In Southern Australia rainfall predominantly falls in the winter months, however summer storms bring significant falls in some areas. Saltland pasture plants are adapted to a range of different growing seasons and can use rainfall directly or indirectly (through accumulated groundwater).

Types of salinity

It should be noted that there are different types of salinity, which occur with or without the influence of groundwater. This website is principally concerned with saltland pasture systems for areas that are affected by groundwater discharge from shallow saline watertables, and many of the species discussed are able to make productive use of the soil moisture that rises from shallow groundwater.

Some of the saltland pasture species discussed may also be suited to another major type of salinity, ‘dry saline land’, which occurs without the influence of groundwater.

Groundwater flow system (GFS) type

Knowledge of GFS type is essential for determining the focus for salinity management (eg. recharge control or living with salt).  The GFS usually determines the severity and scale of salinity, which directly affects the suitable saltland pasture options and the scale of establishment.

The most severe salt (in severity and extent) is usually associated with discharge from broad, flat ‘regional’ groundwater flow systems. These collect groundwater from long distances over large areas, over long periods of time. These systems are the most difficult in which to reverse salinity trends. Therefore living with salt, through the establishment of saltland pastures, is a high priority.

Local GFS which collect groundwater within smaller catchments have discharge areas that occur in smaller, more localised areas. Because areas of saline discharge are smaller, saltland pastures may need to be managed in the same paddock as other more traditional (non-saline) pastures. Local GFS are also easier to remediate through catchment recharge reduction and other salinity management options. Somewhere between ‘regional’ and ‘local’ scale GFS are ‘intermediate’ flow systems.

Some sites may experience groundwater discharge resulting from interactions between deeper, larger scale flows, and shallower, more local groundwater flows.

Potential Mitigation Works

More productive species may eventually be established (than initial site conditions would suggest) if mitigation works can be undertaken. These may include (where appropriate):

  • Shallow (surface water) drainage/ management to alleviate seasonal flooding or prevent stagnation of floodwaters.
  • Groundwater drainage to lower watertables.
  • Ameliorating soil limitations (eg. fertility, acidity, sodicity).

Where limitations can be addressed, it may be possible to grow a wider range or more productive saltland pasture species. Boosting productivity (and hence water use) can also help alleviate the severity of salinity in some areas.