CSIRO Division of Soils, Canberra ACT
The importance of dryland salinity has been recognised for 40 years or more by scientists and for 10 years or so by Governments. If members of the community have not heard an overview of dryland salinity by now, they never will. What is more important is that they should have a very clear understanding of the underview of salinity - because it is what goes on underneath the soil surface that dominates the whole salinity story.
What Is Dryland Salinity?
Dryland salinity is a collective term covering the accumulation of soluble salts at or near the soil surface to the degree that crop and pasture production is reduced or eliminated. Once this cycle is initiated it proceeds along an inevitable pattern of: plant death - bare, exposed soils - erosion of the surface soil by wind and water - and zero productivity. Once the physical and chemical properties of the soil surface have been destroyed it is a difficult and expensive process to re-establish productive vegetation.
Recognising dryland salinity is not a simple process because it covers a whole spectrum of symptoms ranging from an 'incipient' phase, in which plant productivity is simply reduced to varying degrees, through to the 'terminal' phase of complete plant death. Figure 1 demonstrates the danger of waiting to see visible signs of salinity before remedial actions are implemented. It also demonstrates the limitations of remote sensing techniques at this stage of scientific technology.
Figure 1. Detecting soil salinity
The following classification scheme shows the range and complexity of morphologies that are associated with various dryland salinity situations.
. . . . Irrigation salinity . . . . . .
..................................... . . . Salt pan
. . . . Dryland salinity . . . . . . Salt seepage
............................... . . . . . . Seepage scald
............................... . . .. . . . Dry scald
The various end members can have a range of other descriptive attributes signifying their position in the landscape, such as sodicity, texture, vegetation status, etc - all of which have quite important implications for land management.
One of the major features of dryland salinity problems is the lack of uniformity of saline conditions across a landscape, i.e. spatial variability. This is tied to the lack of uniformity of soil properties and to the highly variable manner in which water moves through a soil regolith. Figure 2 demonstates the vertical and horizontal variability of salt distribution which is commonly encountered at different scales. Such variability cannot be predicted accurately simply from soil -type descriptions.
Figure 2. Spatial variability of salt distribution
Sources Of Salts
There are three basic sources of soluble salt (Figure 3). The first is from the chemical weathering of soil and rock materials; the second, from salt water entrapped in the pore structure of marine sediments that have been uplifted to form part of our land surface; the third, from cyclic and aeolian material that is redistributed through the earth's atmosphere.
Figure 3. Sources of salts
Figure 4. Land slope classes in the Murray-Darling Basin (T. Dowling, 1990)
The Murray-Darling Basin is a classic example of the first two. Figure 4 shows the Murray and Darling Basins surrounded by uplands. Much of this rock material was originally of marine origin, i.e. it was laid down by sediments eroded from the then existing land surfaces and transported out to sea. The same processes are occurring right now. These marine muds were uplifted (and compressed in the process) to form part of our present continent. Depressions in that land surface, such as the Murray and Darling basins, were gradually filled by erosional products originating from those uplifted land masses. This included the soluble salts originally entrapped in the porous marine materials as well as salts originating from at least three major marine intrusions into the Murray Basin. At the same time the original border material underwent a series of intense periods of chemical weathering in which the primary minerals were altered to secondary minerals (such as clays) plus soluble salts. For all intents and purposes there is now virtually an infinite amount of soluble salt in our landscapes.
That fact in itself is not of major concern. It is only when the salt is mobilised towards the soil surface that it becomes a problem to plant productivity and survival.
A less obvious form of salt mobilisation and redistribution is that carried in the atmosphere either as salt spray from the oceans (cyclic salt) or from dust generated from the land surface (aeolian material). Over long periods of time it can have a very significant effect on the near-surface properties of the land. The more dramatic aeolian processes have resulted in the layers of pelletised clay known as 'parna'. Others resulted in the lunette dunes of gypsum and other saline evaporites that are associated with ancient salt pans. Even today New Zealand receives aeolian material from dust storms generated on the Australian mainland.
A number of studies have shown that salt from the oceans is carried inland for distances of many hundreds of kilometres. Figure 5 shows the atmospheric (cyclic) salt outfall recorded over a 3 year study by Blackburn and McLeod (1983). Incidentally it should be remembered that cyclic salt is not just comprised of sodium chloride. Calcium, magnesium and sulphate frequently exceed sodium and chloride ions.
Figure 5. The distribution of cyclic salt in the MDB (kg/ha)
How Does Salt Move?
A part of this answer has already been described for the atmospheric phase in terms of cyclic salts and aeolian materials.
In the land phase, water is responsible for the movement of soluble salts. Figure 6 is an idealised version of the hydrological cycle showing all the known water inputs and outputs. Most important is the fact that water enters the soil across the whole landscape - not just parts of it. As water progresses through the system it carries with it salt from the various sources discussed above. It is not as simple as the diagram suggests because of the highly variable physical properties of the soil and rock mantle.
Figure 6. The hydrological cycle
Figure 7 demonstrates some of the aquifer structures that may be encountered, either singly or collectively, in any part of a landscape. Again it is a highly idealised diagram. The important things to notice are that the surface soil, when saturated, has the characteristics of an aquifer; old prior stream material is a common form of aquifer; quite extensive aquifers may be perched on top of a restrictive clay layer within a single catchment; and, regional aquifers underly a number of catchments. Aquifers may also be confined or unconfined.
Figure 7. Types of aquifers
The resultant salt distribution arising from all these combinations and permutations can be summarised as 'recharge' or 'discharge' profiles (Figure 8). In the former, soluble salts are leached down the profile whilst in the latter they are leached upwards. The overriding management problem is not one of preventing either one or the other but rather of managing them to maintain a healthy environment for plant production and surface streams.
Figure 8. Recharge and discharge salinity profiles
Watertables are surrounded by a considerable amount of folklore and mystery - ranging from fast flowing underground streams to the 'flat world' theory. Of course neither is true as the saturated zone within a landscape is the result of many input and output processes that proceed at varying rates over relatively short distances.
An 'unconfined' water table tends to follow the shape of the land surface and is more reminiscent of a lumpy water mattress than a sheet of water. Parts of the landscape let water move rapidly downwards from the surface to the saturation zone and so create a slowly-dispersing mound in the watertable. Trees and other deep-rooted vegetation can suck water from the saturated zone or prevent incoming water from ever reaching it. This results in cones of depression in the watertable - in much the same way as a pump does.
Seasonal weather changes and changes in atmospheric pressure can cause the saturated zone to move upwards and downwards by tens of centimetres over a period of days. Thus there is an ever-changing mobility associated with watertables. Examples of real-life shapes and movements are given in Figures 9 and 10.
Figure 9. Regional groundwater surface (data from R.M. Williams, 1990)
Figure 10. Change in pressure head
Salt concentrations are also highly variable, depending on the salinity of the material with which the groundwater is in contact, the rate at which water moves through different parts of the landscape, and with the concentration effects of capillary evaporation and salt exclusion by plant roots. It is little wonder then that total salt concentrations can vary from 10 - 1000 fold in any one region (Figure 11).
Figure 11. Frequency distribution of total soluble salts in the MIA
Effect Of Vegetation On Dryland Salinity
The physical environment in which water and salts move within the landscape has been briefly discussed. This is the same physical environment in which crops, pastures and forests are produced and evaluated. They are all dependent on very specific water quantities, water qualities and the distribution of those factors in both space and time in order to express their real growth potential. The best we can hope for is some sort of evaluation under average conditions. Hence the message is to not get too dogmatic about individual performance data obtained from very small areas over short periods of time.
The other side of the question is whether vegetation is a tool that can be used to improve the near-surface environment for other more valuable crops. When regarded as biological pumps the answer is in many cases, yes. In fact the biological pumps themselves can be species having economic and environmental value.
To do this we need to return to Figure 8 where it is clear that it is the unsaturated zone that is the key to salinity control. Three criteria must be met:
sufficient water must enter and be stored in the profile to sustain plant growth;
sufficient water must pass through the root zone (leaching fraction) to ensure that salts concentrated by plant water uptake are periodically leached out of the root zone;
the saturated zone must be kept below the depth of capillary contact with the soil surface.
Original plant communities were structured in a manner that largely met these objectives. However, they were not always successful, as evidenced by quite large tracts of saline lakes and halophytic swamplands prior to European settlement. But they do demonstrate the importance of permanent and mixed plant communities as compared with monocultured annuals interspersed with periods of little or no plant production.
Mixed plant communities (trees, shrubs and grasses/legumes) imply mixed rooting depths and hence a series of soil water interception mechanisms. This is a good model but not one that readily fits in with traditional broadacre agricultural practices - particularly in terms of machinery accessibility.
One plant community geometry that is commonly advocated for groundwater recharge and salinity control is planting trees and shrubs on areas having potentially high rates of recharge. This is valid but only to the extent that the area susceptible to high recharge affects the total water entering the saturated zone.
Another is agroforestry, which has a much sounder theoretical basis when it is remembered that all parts of the landscape are potential recharge zones. It also has a much sounder basis in terms of erosion control and more productive controlled grazing practices. However, it cannot be implemented without giving thought to competition in soil water extraction by mixed communities (Figure 12). Spacings between the permanent vegetation and strategies for using 'short-term' and 'long-term' species in the inter-row areas need careful consideration. It becomes a plant-engineering problem with the emphasis not so much on how much water a plant uses but rather its pattern of usage in a competitive environment and in a usually very variable physical environment.
Figure 12. Soil water extraction by trees, shrubs and grasses
By understanding what goes on beneath the soil surface, dryland salinity can be controlled. However, this will only be achieved when both researchers and landowners recognise that they are dealing with a highly variable system. This will probably require specific treatments for each part of the landscape. Furthermore, it is the whole of the landscape that needs to be managed - not just a few paddocks here and there.
This adds up to some drastic changes both in research direction and in farming practices. Soil salinity has wiped out whole civilisations in the past. There is no guarantee that large farming communities in Australia won't suffer a similar fate in the not-so-distant future if they don't learn how to manage soil water for salinity control.