Saline Soils: A Landscaping Challenge
Written by Jochen Wiede (dipl. Ing. Landscape Architect), 2005
Table of contents:
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.
How to cope with soil salinity?
A burgeoning population, as well as booming tourism and building sectors, together with rising demands for landscaping in the urban parts of the UAE will outmatch natural sweet water reserves. Without drastically conserving water, reducing personal water consumption or increasing the amount of desalinated seawater, increasing soil salinity levels eventually will negatively affect growth in the UAE. One way of conserving water is the reuse of graywater. Case studies in California have shown that using limited amounts of filtered graywater from private household sources (no kitchen discharge, only environmentally friendly additives in washing machines) is an appropriate way to irrigate fruit or ornamental trees and shrubs, since salinity buildup in the soil under such circumstances is minimal. It is important when using graywater irrigation to apply the water directly within the root zone. This may be achieved through drip pipes that are located 25 to 40 cm below the soil surface. In terms of sustainable irrigation, there are a number of measures to be taken in order to reduce the amount of water to be used and prevent salinity buildup in soil. Treated sewage effluent if used for irrigation should not exceed salinity level of 0.7-3.0 dS/m (420-1800 ppm/mgl).
These measures include the following:
- Reducing loss of soil moisture by means of mulching and wind-breaks, no surface irrigation
- Selecting similar ecotypes of plants within one mode of irrigation
- Irrigating at night and on demand according to plants requirements
- Selecting and installing drip irrigation systems according to site and plant requirements and with monitoring possibilities
Concerning landscaping techniques, the key to improving sandy soil structures for sustainable plant life and combating salinity is to surrogate missing soil colloids such as we naturally find in humus and clay particles and to break up deeper soil levels, that is for tree and larger shrub growth, down to a depth of about 150 cm. Good results have already been achieved with soil amendments and controlled irrigation in land rehabilitation, afforestation, and amenity landscaping.
Important soil additives include synthetic and organic chelators that substitute missing natural soil colloids. All possibilities in combining some of these commercial products with local means of mulching still have to be exhausted. Polymers such as potassium-based polyacrylamids are promising products. Also alkali hydro silicates, potassium- or calcium-based alginates, and fossil humates might be important additives. These humates are highly compressed natural organic forms of stable humus complexes, rich in humic and fulvic acids. They will play a major role in improving poor soil structures, combating soil salinity, and enhancing biochemical processes between plant and soil. In certain soil textures and within certain planting techniques, it is advisable to add water retaining volcanic sands or clay minerals. Inoculation of poor soils with certain soil bacteria and mycorrhizal fungi should, therefore, be part of soil amendment strategies.
Long-term success in establishing plant growth under desert or semi-desert conditions can only be seen in the context of soil improvement that takes into consideration the close monitoring of soil acidity, salinity, and nutrition deficiency and the use of appropriate water supply. Until a self-sufficient carbon and nutrient cycle is established, replenishment of organic matter such as humate-based chelators in solid or liquid form and other water retaining agents will have to be considered. A disadvantage of some of these commercial soil amendment products is that they function only for a few years before they decompose or dissolve. Success also hinges on maximal root growth stimulation within the first years and specific plant adaptation on the site. Only an appropriately chosen plant will then be able to fend for itself.
The higher salinity becomes the less stable a soil complex will be to support sustainable plant growth. Irrigation with seawater for instance, as it has been considered for specific sea-related developments, will break down soil amendment agents even faster, and consequently will break down the highly specialized biochemical mechanism between soil and plant. Only adapted plants like halophytes, such as mangroves, and plants of salty marshes can tolerate seawater irrigation for long due to their adaptation to a uniform soil structure on marshy land with sediments of silt.
The task of dealing with this special soil-plant relationship under difficult climatic conditions demands an interdisciplinary approach. The needs and the growing habits of plants have to be understood to determine what is required in terms of soil preparation, as well as planting techniques and maintenance. Also necessary is an up-to-date knowledge of soil amendment and irrigation techniques. Most importantly, landscape architects need to have a very good knowledge of the plants they are using.
The following list includes examples of indigenous and introduced trees, shrubs, grasses, and perennials that have a record of growing reasonably well in improved sandy soils within specific ranges of salinity and drought exposure.
Acacia ssp. (A. tortilis, A. Arabica, A. ligulata, A. raddiana, A. cyanophylla)
Atriplex ssp. (A. nummularia, A. sembacceata, A. leucolada, A. halimus, A.canescens, A. littoralis)
Azadirachta indica, Batis maritima
Calotropis procera, Callistemon salignus
Calligonum comosum, Coccoloba uvifera
Casuarina ssp. (C. equisetifolia, C. glauca, C. obesa, C. cristata)
Conocarpus ssp. (C. lancifolia, C. Leucocarpus, C. erectis)
Eucalyptus ssp. (E. camaldulensis, E. microtheca, and many Australian varieties)
Fagonia indica, Hamada elegans
Haloxylon ssp. (H. salicormeum, H. persicum)
Hibiscus tiliaceus, Lawsonia inermis
Leucaena glauca, Leucaena leucocephala
Moringa peregrina, Moringa optera
Melaleuca leucadendron, (more Australian types)
Prosopis ssp. (P. spicigera, P. juliflora)
Phoenix dactylifera (regional types like nakhal, khasab, shahla) and other palms
Pistacia atlantica, P. palaestina
Pithecelobium inga dulce
Rhizophora mangle (Mangrove)
Rhazya stricta, Salvadora persica
Salicornia europaea, Simmondsia jojoba
Tamarix ssp. (T. aphylla, T. articulata, T. stricta)
Arundo donax, Cyperus conglomeratus, C. papyrus, Scirpus maritimus
Cynodon dactylon, Juncus gerardii, J.maritima
Limonium sp. (L. axilliaris, L. sinnuatum)
Ipomoea biloba, Pennisetum divisum
Panicum turgidum, P. persicum
Paspalum vaginatum, Stipagrostis plumosa
Useful web resources
This is the web site for the International Centre for Biosaline Agriculture (ICBA), an applied research and development center located in Dubai, United Arab Emirates. ICBA aims at developing and promoting the use of sustainable agricultural systems that use saline water to grow crops. The web site includes, among other things, newletters and articles dealing with the problem of salinity in irrigated agriculture.
This is a section of the web site for the Center for the Study of the Built Environment (CSBE). The section documents the results of a project that CSBE carried out between 2002 and 2004 and that aimed at investigating, implementing, and promoting graywater reuse at the small and medium scales in the domestic sector in Jordan.
This is the web site for the Canadian Organic Growers Inc. (COG), an education and networking organization representing farmers, gardeners, and consumers in Canada. The web site includes, among other things, on-line publications on organic farming and gardening, including The Canadian Organic Grower magazine.
This is an article entitled "Natural Vegetation of the UAE" by M.I.R. Khan. The article is published in bulletin no. 11 (July 1980) of the web site for the Emirates Natural History Group (ENHG), a non-profit organization with an interest in the natural history of the United Arab Emirates and Oman.
This is a report entitled "Monitoring Graywater Use: Three Case Studies in California." The report deals with a graywater study that was initiated in 1996, and conducted by the California Department of Water Resources with the City of Santa Barbara and East Bay Municipal Utilities District. The report includes case studies with observations on the operation of three graywater systems in the cities of Santa Barbara, Danville, and Castro Valley over a two-year period.
About the author
Jochen Wiede is a German landscape architect and planner. He heads and owns the Basel-based firmWiede Landscape Design (known in Switzerland as Wiede Grünplanung). The firm has served as a consultant on various projects in Switzerland as well as in other countries. Wiede has been involved in creating communal and regional landscape plans that focus on agriculture, forestry, nature reserves, and green belts. He has carried out designs for public spaces, parks, leisure and school grounds, and sport fields. In addition, Wiede has carried out projects for road-, lake-, and river-side landscaping; reclamation of disused and derelict lands; planting schemes in severe climatic and soil conditions; and management of nature reserve zones.
Wiede is focused currently on challenges related to landscape development in arid climates, particularly in the Gulf region.
Jochen Wiede can be reached at the following email address:email@example.com.