|Saline Agriculture: Salt-Tolerant Plants for Developing Countries (BOSTID, 1990, 130 p.)|
Scientists exploring seashores, estuaries, and saline seeps have found thousands of halophytes with potential use as food, fuel, fodder, fiber, and other products. Many have already been in traditional use, and there are also a number of plants that, although not halophytes, have sufficient salt tolerance for use in some saline environments.
Although economic consideration of halophytes and other salt tolerant plants is just beginning, they are now receiving increased attention in arid regions where intensive irrigation has led to salinized soils or where water shortages are forcing use of marginal resources such as brackish underground water. This report will examine some of the plants that may be suitable for economic production in saline environments in developing countries.
There are four sections in this report. They highlight salt tolerant plants that may serve as food, fuel, fodder, and other products such as essential oils, pharmaceuticals, and fiber. In each of these sections, plants are described that have potential for productive use. Each section also contains an extensive list of recent papers and other publications that contain additional information on these plants. A list of researchers currently working on these plants or related projects is included at the end of each section.
Many halophytes survive saline stress by accumulating salt in their vegetative tissues. The salt levels in the leaves and stems of these plants can limit their direct consumption as food, but their seeds are relatively salt-free, which may allow production of starchy grains or oilseeds.
For example, the seeds of Zostera marina, a sea grass, were used as food by the Seri Indians of the southwestern United States; in recent tests, these seeds were ground to flour and used to make bread. Seeds of Distichlis palmer), Palmer's saltgrass, were harvested from tidal flats at the head of the Gulf of California by Cocopa Indians. The seed, about the same size as wheat, has also been used for making bread.
The production of vegetable oils from seed-bearing halophytes appears promising. A number of these seeds have an oil content comparable with that of better known sources of vegetable oils. A Salicornia species is being evaluated as a source of vegetable oil in field trials in the United Arab Emirates, Kuwait, and Egypt. Since many developing countries import vegetable oils, the opportunity for domestic production on currently unusable lands warrants investigation.
It may be possible as well to use the salt-containing vegetative parts of some halophytes to produce salt-free leaf protein. In this process, any inorganic salts in the leaves are separated from the protein. Leaf protein production may help improve the nutritional quality of foods in developing countries.
There are also traditional food crops that are grown commercially using underground brackish water for irrigation. These include tomatoes, onions, and melons. Asparagus also appears to grow well with brackish water irrigation.
More than a billion people in developing countries rely on wood for cooking and heating. In most developing areas, the rate of deforestation for fuelwood and for agricultural expansion far exceeds the rate of reforestation. Increasing needs for agricultural land to feed growing populations make it unlikely that land suitable for food crops will be used for tree planting.
One alternative, therefore, is to use marginal or degraded lands to produce more fuelwood.
Fuelwood and building materials can be produced from salt tolerant trees and shrubs employing land and water unsuitable for conventional crops. Fuel plantations established on saline soils or irrigated with saline water would allow more fertile land and fresh water to be reserved for food or forage production. With careful planning, trees and shrubs can help rehabilitate degraded lands by stabilizing the ecosystem and providing niches and protection for other plants and animals.
In Australia, a consortium of business and academic groups is developing a program to market salt-tolerant trees for fuel and pulp. The project will screen Australian tree species for growth rates, salt tolerance, and drought tolerance. Root fungi associated with these trees, which help the trees obtain nutrients from the soil, will be screened for salt tolerance and their influence on tree growth Trees with superior growth on saline soils will be tissue cultured and inoculated with salt-tolerant root fungi. These cloned trees will then be tested for field performance in Australia and developing countries.
Halophytic grasses, shrubs, and trees are all potential sources of fodder. Trees and shrubs can be valuable components of grazing lands and serve as complementary nutrient sources to grasses in arid and semiarid areas.
Among the grasses, kallar grass (Leptochloa fusca) tolerates waterlogging and recovers well from cutting and grazing. Its economic value as fodder for buffalo and goats has already been demonstrated in Pakistan and is now being examined in other countries. Members of the Spartina genus (cordgrasses) have also been used as fodder. These tough, long-leaved grasses are found in tidal marshes in North America, Europe, and Africa. The salt-tolerant grass Sporobolus virginicus has also been used as cattle forage.
Distichlis spicata has been used as forage for cattle in Mexico. Introduced in the area of a dry salt lake outside Mexico City, D. spicata reduced windblown dust while serving as cattle feed.
In arid and semiarid zones, trees and shrubs for fodder have several advantages over grasses. They are generally less susceptible than grasses to fire and to seasonal variation in moisture availability and temperature. Usually less palatable than grasses, they can provide reserve or supplementary feed sources.
Among the shrubs, saltbushes (Atriplex spp.) grow throughout the world. They tolerate salinity in soil and water, and many are perennial shrubs that remain green all year. They are especially useful as forage in arid zones.
Among trees, Acacia species are widely used in arid and saline environments as supplementary sources of fodder. Acacia pods provide food for livestock in large areas of the semiarid zone of Africa. Acacia Cyclops and A. bivenosa tolerate salt spray and salinity.
They grow on coastal dunes as small trees or bushy shrubs. Pods and leaves of both are consumed by goats.
Leucaena leucocephala is a tree legume widely cultivated in tropical and subtropical countries. Leaves, pods, and seeds are grazed by cattle, sheep, and goats. In Pakistan, it has been grown on coastal sandy soil through irrigation with saline water. When seawater comprised 20 percent of the irrigation water, yields were reduced by 50 percent, however.
The leaves and pods of mesquite (Prosopis spp.) have been used as forage for cattle, goats, sheep, and camels in countries throughout the world - P. juliflora and P. cineraria in India, P. chilensis in South America, P. glandulosa in the United States, and P. pallida in Australia.
About 20 years ago the Chilean government began to improve the salt-afflicted Pampa del Tamarugal in the northern part of the country by growing tamarugo (P. tamarugo). In some cases, these trees were planted in pits dug through the salt into the soil. Although watering was required for the first year, after that the plants survived by capturing moisture from the ground and air. About 23,000 hectares are now covered with tamarugo forest. The tamarugo leaves and fruit are used as feed for sheep and goats.
FIBER AND OTHER PRODUCTS
Salt-tolerant plants can also be used to produce economically important materials such as essential oils, flavors, fragrances, gums, resins, oils, pharmaceuticals, and fibers. They may also be marketed for use in landscape gardening, and for their foliage or flowers.
In India, peppermint oil and menthol have been produced in saline environments. The salt- tolerant kewda, a common species of screwpine, is used to produce perfume and flavoring ingredients.
Sesbania bispinosa, commonly known as dhaincha in India, is an important salt-tolerant legume and fodder crop. In addition to use of the stalks as sources of fiber and fuel, the seeds yield a galactomannan gum that can be used for sizing and stabilizing applications, and a seed meal that can be used for poultry and cattle feed.
Grindelia camporum is a salt-tolerant resinous perennial shrub. It produces large amounts of aromatic resins that have properties similar to the terpenoids in wood and gum rosins, which are used commercially in adhesives, varnishes, paper sizings, printing inks, soaps, and numerous other industrial applications.
Jojoba (Simmondsia chinensis) is a perennial desert shrub with seeds that contain a unique oil similar to that obtained from the sperm whale. This oil and its derivatives have been used primarily in cosmetics, but broader use in lubricants and waxes will probably develop if prices come down. Jojoba is relatively salt tolerant. In Israel, jojoba is growing well near the Dead Sea with brackish water irrigation.
Phragmites australis, common reed, is an ancient marsh plant that has served in roofing, thatching, basketmaking and fencing, as well as being used for fuel. It grows throughout the world in watersaturated soils or standing waters that are fresh or moderately saline. In Egypt, two salt-tolerant rushes, Juncus rigidus and J. acutus, have been investigated with particular emphasis on their potential use in papermaking.
Many attractive halophytes can be used as landscape plants, especially in areas with constraints on the use of fresh water for watering or irrigation. In Israel, salt-tolerant trees and shrubs are sold for amenity planting. In addition, other salt-tolerant plants have potential for cut-flower production.
Although the salt-tolerant plants described in this report typify those that are currently being evaluated or appear to deserve additional attention, the inventory is far from complete. Many other species may have equal or greater potential. In some cases in this report, specific companies or products are identified. This is for convenience and does not constitute endorsement.