Saline Soils: A Landscaping Challenge

Written by Jochen Wiede (dipl. Ing. Landscape Architect), 2005

 

 

Table of contents: 

1. Problems with salinity

2. How to cope with soil salinity?

3. Useful web resources

4. About the author

 

 

Arid and semi-arid landscapes such as those found in most parts of the United Arab Emirates (UAE) exist within the context of an extremely sensitive ecological balance. Man and indigenous plants historically managed to adapt to this water-scarce environment without disrupting the pre-existing equilibrium in this delicate ecosystem. Urbanization and drastic increases in population, however, are seriously threatening sustainable natural resources in the country. A number of institutions in the UAE, including universities and municipalities, are trying to identify means of tackling threats to existing natural resources that have been brought upon by the rapidly rising levels of water consumption. Modern urban ways of life have destroyed the historical pact that man used to have with nature.

The UAE has one of the world's highest levels of domestic waste and water consumption per capita. Ambitious programs for afforestation, landscaping within urban and rural contexts, as well as intensive food production are transforming the arid lands of the UAE in a way that is totally alien to its older inhabitants. This situation offers a unique chance in devising a rural development scheme with a string of self-sufficient villages. A project of this kind could be just as attractive financially as those huge offshore developments in Dubai and Abu Dhabi. It would fuse traditional ways of life with the most up-to-date technologies. Alternative energy sources would be used for domestic waste and sewage treatment as well as for desalinating seawater. Sustainable land use practices for landscaping, agriculture, and food production would maximize water-use efficiency. Traditional architecture would be attractive as a part-time retreat for all levels of society or as homes for low-income locals.

At present, fossil, non-renewable groundwater resources are being depleted to an alarming extent. Renewable groundwater tables of inland aquifers that depend on rainfall are drastically falling in level and increasing in salinity due to non-sustainable irrigation methods and the infiltration of seawater. Municipal waste traditionally has been dumped in landfill sites, and at best subjected to only little treatment. These sites are endangering the groundwater as a result of the various pollutants they contain that are seeping into the groundwater. Areas used for farming, landscaping, or afforestation are showing alarming levels of soil salinity. A good number of the millions of trees planted during the last decade are dying off or being damaged as a result of drought or salinity stress. Dust storms and land erosion are becoming increasingly numerous where there is increase in soil salinity.


 Problems with salinity

Although soil salinity poses a threat to plant life, there are methods of coping with it. Many indigenous plants in this area, including the date palm, have a certain tolerance to soil salinity. Some desert plants have developed protective mechanisms through modifying their growth habits or through biochemical adaptation. This phenomenon may be studied in relation to many plants in the semi-desert zone and in the ‘Subkha,' an unfertile stony area that divides the coastal flats from the semi-desert plateau.

Rising salinity levels in soils, however, are seriously affecting even date palm production in the region. Efforts initiated by ICBA, the International Centre for Biosaline Agriculture, to select and develop date palms with a higher ability at tolerating salinity levels are prone to failure if salinity in irrigation water causes salinity levels in the soil to rise, and no other soil amendment methods are implemented.

Soil salinity usually has either a naturally developed primary source or a secondary one. Soil salinity is naturally developed near coastal flats or in arid zones, when the water table of saline groundwater rises up to two to three meters from the soil surface and capillarity transports salt to the topsoil. Secondary soil salinity is mostly due to inefficient irrigation methods. Use of saline water for irrigation, discrepancy between root uptake and evapotranspiration, poor design of irrigation system, or poor subsoil drainage can result in over-concentration of salts.

Salinity in soils may be categorized in the following manner:

Extremely saline sites: Over 16 dS/m (deciSiemens per meter) or 9,600 ppm/mgl (parts per million/per milligram). The salinity level may rise up to that of seawater (30,000 ppm/mgl).
Some of the groundwater in the north of the UAE has reached salinity levels of 25 dS/m, or half of that of seawater. Plants adjusted to this level are halophytes like mangrove plants, desert saltbushes like haloxyon, or trees including various types of Tamarix, Acacia, and Prosopis (Mesquite).

Very saline sites: 8 - 16 dS/m or 4800 - 9600 ppm/mgl.
This is equivalent to soil salinity levels in the ‘Subkha' area and in depleted inland aquifers. Plants such as the date palm and casuarina trees can deal with these salinity levels in native well-drained soils and under sustainable levels of water supply, i.e. where there is no increase in soil salinity due to the application of very saline irrigation water.

Moderately saline sites: 4 dS/m - 8 dS/m or 2400 - 4800 ppm/mgl.
This is the salinity level of most groundwater - and irrigation water from wells - in the UAE, and increasingly also in terrains that are irrigated regularly. Many perennials, shrubs, and trees can sustain growth in these soils provided that adjusted irrigation methods and soil amendments (see below) are used to keep salinity levels in check.

Slightly saline sites: 2 dS/m - 4 dS/m or 1200 - 2400 ppm/mgl.
These sites mostly are found inland in desert or semi-desert situations where there is no regular irrigation with saline water. Creating situations of long-term vegetation in such sites, which for the most part are very sandy and have poor soils that are devoid of organic matter and deficient in vital nutrients, is very difficult. Revegetation efforts are unlikely to succeed without improving micro-organic life in the soils and the soils' colloidal properties (the ability of the soil to hold, by absorption, various plant food elements, such as nitrogen, potassium, calcium, magnesium and many trace elements, and to release or exchange these elements under plant growth conditions).

Fragile and unfertile sandy soils are easily affected by drought and salinity problems as one attempts to establish plants there. Salinity problems result in the disruption of biochemical processes between soil particles and plant roots. These disruptions include the following:

  • Salinity disrupts the ion exchange mechanism between soil moisture and plant cells. As a result, plant cells dry out, plants wilt, irrigation must be increased, and therefore salinity steadily rises as has been mentioned before under secondary soil salinity conditions.
  • Salinity changes the selective capacity of plants to ‘feed' on soil particles. Harmful quantities of nutrients or trace minerals (such as boron, copper, manganese, and zinc) consequently can damage or kill the plant. Fertilizing plants under these conditions will aggravate this problem.
  • Salinity changes the electrochemical balance of soil particles. It destroys physical soil properties, reduces its draining capacity, and increases evaporation and soil erosion.

 

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