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Chapter VI. Rehabilitation of saline environment

1. Introduction
2. Aims of saline environment rehabilitation programmes
3. Salt-tolerant shrub resources
4. Plant selection
5. Establishment
6. Highlights of section

1. Introduction

Under natural conditions, salt affected soils support salt-tolerant vegetation, including trees, shrubs and grasses. Sheep, goats and camels utilize the vegetation for grazing, and the shrubs are also cut and used as fuel. As a result of heavy grazing and fuel gathering, many salt-affected areas become denuded.

Bare salt-affected soils also occur as a result of changes in landscape hydrology due to either land development for agriculture or the installation of irrigation and drainage schemes. The bare saline areas which result from development schemes or overuse are usually regarded as wastelands but they are capable of producing biomass useful as forage or fuel. In this section, salt-tolerant shrubs are considered.

2. Aims of saline environment rehabilitation programmes

Rehabilitation of salt-affected soils may serve a number of purposes:

- feed for grazing animals - Many salt-resistant plants provide a valuable reserve feed for drought conditions or fill regular gaps in feed supply caused by seasonal conditions. Chenopod shrubs are used extensively in natural stands and may be planted on denuded saline areas. They provide shelter as well as feed for grazing animals (Figure 6.1).

- reduction of soil erosion and degradation - Establishing a plant cover on otherwise bare saline areas contributes greatly to reducing wind and water erosion. Several Atriplex spp. exhibit a prostrate growth habit and natural stem layering, excellent characteristics for preventing soil erosion. Moreover, the layered stems are more able to resist heavy grazing (Figure 6.2).

- fuel production - Shrubs in natural saline areas are cut and used for fuel. Some species planted for forage production may be useful also as fuel; alternatively, species useful only as fuel may be planted.

- improvement of aesthetics - There are extensive tracts of land that are salty and unattractive around many cities in arid areas. -These areas provide a source of dust which blows over the city. Dust problems are also evident in areas such as airport grounds, highway developments and the fringes of irrigation projects. Chenopod shrubs, such as Atriplex cinerea which grows about 0.5 meter high and up to 6 meters in diameter, provide a possible means of preventing the dust and improving living conditions and aesthetics.

- wildlife conservation - Salt affected wastelands and overgrazed range country provide a very poor habitat for wildlife in terms of food, cover and breeding places. Species grown for cover include the large Atriplex lentiformis which is known as quail brush. Fuel species include salt- and drought-tolerant Melaleuca spp. which are also useful for nesting and shelter.

- to use shallow saline groundwater - There are vast areas in arid countries with shallow saline groundwater. These areas can be used for planting salt-tolerant species.

Figure 6.1 Grazing in a saline environment.

Figure 6.2 Atriplex omnicola produces natural stem layers which contribute to grazing and erosion control.

3. Salt-tolerant shrub resources

There are three broad groups of salt tolerant shrubs:

- Samphires (glassworts) - Include the genera Salicornia, Arthrocnemum, Halocnemum, Halosarcia and Allenrolfea. These plants occur on highly saline sites which are in many cases waterlogged at some times of the year. Usually saline groundwater is sufficiently shallow for the capillary fringe to intercept the surface for many months of the year. The stems of these leafless plants are succulent and highly saline but some samphires are eaten by sheep when other feed is scarce. Some samphires develop a substantial woody frame.

- Saltbushes (or goosefoots) - Include the genera Atriplex, Chenopodium, Rhagodia and Halimione. These leafy shrubs are typified by having a gray-green colour due to the development of salt bladders on their epidermal cells. They are highly salt-tolerant but occur in less waterlogged situations than the samphires. Saltbushes vary greatly in palatability but are an important component of arid and semi-arid shrub pastures in many countries. Atriplex spp. are used as fuel and some species produce a strong woody frame.

- Bluebushes and saltworts - Include the genera Salsola, Kochia, Maireana, Sarcobatus, Suaeda and Enchylaena. These shrubs have succulent leaves and vary greatly in their palatability to animals. They range in salt and waterlogging tolerance from those which occur in association with samphires and saltbushes to those which are less salt- and waterlogging-tolerant than most saltbushes.

4. Plant selection

Rehabilitation of saline environment depends on careful selection of salt-tolerant species. To this end the adaptation of a given plant species to environmental parameters such as climate, salinity, and site hydrology should be considered.

The most important differences in salt-affected soils at any one site relate to site/water relations. For coastal sites, depth and frequency of tidal inundation or depth to the watertable are major factors affecting species distribution. In endoreic basin, seeps and areas with high watertable, species zonation relates to the depth of watertable or susceptibility to inundation or surface water logging as well as to salinity. In Table 6.1 species are classified according to climatic zones and salt-affected land types on which they are reported to grow.

5. Establishment

Shrubs may be established in the field either as plants or cuttings or by sowing seeds. Cutting establishment is suitable for Tamarix spp. but insufficiently reliable for other shrubs. Planting of seedlings is reliable if adequate soil preparation is provided. Soil preparation techniques for salt affected areas should have two main thrusts: the first is to control groundwater where its presence influences the accumulation of salt at the soil surface. This can be achieved by drainage, deep furrowing and ridging; the second is to encourage salt to move downwards in the soil instead of accumulating at the surface. This can be achieved by selecting "niches" in the area where salt leaching is ensured naturally or by establishing artificially such "niches" where young seedlings or seeds can be established.

An illustration of the above is the "niche technique" developed in Australia for seeding salt-tolerant shrubs on salt affected areas. The technique consists of making a furrow and a ridge and establishing a "niche" on the ridge (Figure 6.3).

The furrow is intended to catch water and cause water to be stored in the subsoil close to the growing shrub to aid survival and growth. The ridge allows the planting site to be raised above the general ground level to avoid waterlogging or flooding problems and to help leach salt from the niche. The niche provides a sheltered planting site with compressed side slope for run-off to concentrate in the niche. The niche can be seeded or planted. When sowing is done, mulching can promote water penetration, salt leaching and reduce evaporation and soil crusting (Figure 6.4).

Figure 6.3 Cross-section of furrow, ridge and planting niche

Figure 6.4 Atriplex undulate established by the niche technique.

In many instances, it is also possible to establish shrubs by direct seeding. Research in several countries has demonstrated that shrub establishment by direct seeding into salt-affected soils faces many hazards. Problems arise due to:

- Low salt tolerance at germination;

- Poor utilization of incidental rainfall;

- Specific temperature requirement;

- Poor soil structure;

- Period of excess water;

- Insect attack and frost damage.

The best establishment method for each saline environment must be determined by field testing.

The following principles should be observed:

- The possibilities of surface waterlogging, flooding and erosion should be minimized by protecting the treatment area with appropriate soil conservation works.

- Salt levels in the surface soil may be reduced by cultivating before the first rains to encourage water penetration and salt leaching.

- The sown area should be protected against grazing by domestic animals or wildlife including rodents.

- Measures should be taken to counteract wind erosion hazards if present.

- Insect problems should be detected by regular inspection and dealt with promptly.

- A well-proven seeding method should be used.

6. Highlights of section

Under natural conditions many saline environments support salt-tolerant shrubs which are useful as forage and/or fuel.

Revegetation of bare salt-affected soils using these shrubs may provide forage or fuel, reduce run-off, erosion and groundwater levels and improve the aesthetic and wildlife values of an area.

Salt-tolerant shrub reconnaissance should be made to select suitable sites for planting. Species for planting in saline environments are listed and methods of planting in specific sites are discussed.

Cheaper establishment techniques (direct seeding) should be tested and the productivity of the shrubs measured to provide the basis for an economic evaluation of large-scale planting.

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