Salinity poses distinct physiological threats to plants.


By Dr. S.M. Alam
Sep 16 - 22, 2002



About one half of the world's people are concentrated on a mere 5% of the earth's surface, whereas 57 % of the land supports only 5% of the people. The main reason is the prevailing climate, which may be hospitable or hostile man's occupancy. In the short run, there are other reasons as well. There may be found in the history, politics and physical infrastructure. The worldwide, salt affected soils hinder the development of agriculture production. Frequency of salt affected soils make their utilization necessary by irrigation, suitable amendments and also by introducing certain salt tolerant species.

Use of physical and biological means to reclaim salt affected soils of unproductive inland salt basins and deteriorated agricultural lands have wide scope of study. Rainfed agriculture, lack of irrigation water sources, shallow and brackish groundwater suggest that the salt affected lands of the country may better be utilized for the cultivation of salt tolerant crops. Cultivation of saline lands with salt tolerant species may prove to be valuable biological device for soil desalination and reclamation.

Salinity refers to high concentrations of soluble salts predominantly sodium, which when present in the exchange complex of the soil particles, make the soil sodic. Saline soils are unproductive, strangely though they contain all the nutrient elements. It is interesting that the sea, which is saline is the abode of two-third of the plant population of the earth the plants capable of absorbing all the required, elements are also fixing the small amounts of dissolved carbon dioxide under the low and diffuse light intensity. On land too, many wild plants cope up quite successfully with salinity the halophytes growing luxuriantly along the coast, estuaries and salt marshes. The fact therefore, is inescapable that there is no fundamental biological incompatibility between plant life and salinity. The problem is on the other hand with those, which we use as crop plants and their intolerance to salinity.

Salinity poses two distinct physiological threats to plants. The low osmotic potential of the saline medium makes the plant difficult to maintain their intracellular osmotic potentials. The second effect of salinity is of nutritional nature the presence of high and potentially toxic concentrations of ions in the medium. The ions responsible for salinity, namely sodium, chloride, sulphate, bicarbonate and magnesium may cause toxicities and nutrient deficiencies by damaging the membrane by displacing other ions from the active absorption sites, and by other mechanisms not yet understood.

Two approaches are generally resorted to solve the problems of salinity. It is a common practice to modify the soil conditions to suit the crop plants using soil ameliorants and good irrigation water to reduce salt concentrations and favouring better growth. The second approach is to exploit the genetic potential of plants for their adaptability to adverse soil conditions. It is estimated that an area of about 6.5 million ha in Pakistan have different degrees of salinity and alkalinity hazards. It is paradoxical that while halophytes (salt loving: plants) can grow well in saline conditions in estuaries and salt marshes, crop plants are highly sensitive and make it impossible to exploit the gigantic reservoir of water and mineral nutrients of the oceans all over the world.

Salinity and aridity are the two oldest enemies of agriculture. It is believed that about 7 per cent of the total surface area of the world is salt affected. The development of soil salinity is far more extensive in the extreme arid and arid lands than in semi-arid and humid lands. Based on the climatic maps, the approximate areas of extreme arid, arid and semi-arid regions of the world are 2,24, 8.42 and 8.20 million square miles, respectively, i.e. about 36 % of the gross land area of the world. The worldwide extent of saline and alkali soils is about 322.9 million hectares (Australia 84.7, Africa 69.5, Latin America 59.4, Middle East 53.1, Europe 30.7, Asia and Far East 19.5 and North America 16.0), which is about 26 % of the world's cultivated land. In Pakistan, approximately, 6.5 million ha of land is reported to be affected by salinity that is constantly increasing. It is commonly accepted that while under saline conditions, the growth of most of the crop lands is inhibited, because of total ion activity, growth and development of wild halophytes remain either unaffected or is stimulated.

With the increasing world population and the need for increased crop production, the non-productive salt affected lands may be used to produce non-conventional crops of economic value and also for food crop like pear millet for the saline belts of Thar. With the present research concentrating on reclaiming saline areas, it is desirable to choose those species which are well suited to saline habitats and some most economical means of reclamation to make the salt affected soils productive.

The worldwide agricultural production is greatly hindered by a number of environmental hazards, major among which are salinity and aridity. A boom in the total food production of country will remain a dream only until a well managed multi-angled approach in the cultivation techniques is adopted to improve and protect our agroecosystem. Solely rained cropping, erratic rainfall, poor soil fertility status, presence of excessive salt in the soil either because of the natural phenomena or because of the ill-planned large irrigation projects, etc. are the major factors directly linked with the crop fortunes of Pakistan. Severity of these factors is further pronounced in the arid and semi-arid regions. One of the major limitations in increasing agricultural production in these regions is the brackish to saline natures of underground water. As a result of irrigation in such regions, the level of ground water rises, bringing the ground water within the capillary range; thus the ground water is drawn up by capillary action, evaporates, leaving behind the soluble salts on the surface. Gradually, as a result of evaporation the concentration of salt in the soil is increased to such an extent that salt crusts are formed. Presence of excessive salts in the soil makes it unsuitable for agricultural purposes.

The halophytes are well-known for salinity tolerance and are capable to produce 10 to 35 t/ha of biomass under salinity. Halophytes commonly exhibit quite high concentrations of several inorganic ions in various plant parts. In view of the fact that halophytes are unaffected by the high NaC1 content of the soil certain mechanisms appear to have developed in these plants that enable them to adapt to saline conditions. Adaptations of halophytes of salty conditions include high tolerance for the negative effects of salinity as well as a positive reaction towards it. Means or ways of adaptation may vary with species as well as ionic composition to the medium. While, selecting the suitable plant species to be grown in saline habitat in large scale, it appears wise to choose those species, which have got some economic importance also, besides salt tolerances potential.

Management of salt affected soils: Because of the threatening over population, new ways need to be developed to convert hostile areas into hospitable ones to be productive desert areas of the country. Saline and alkaline conditions reduce the value and productivity of soil. For agricultural practices, such soils are regarded as a class of problem soils that require special remedial measure and management practiees. Keeping in view, the increasing demand for various agricultural, forestry, industrial and livestock products for internal consumption and export and decreasing proportion of land available per capita, it becomes necessary that each piece of land is to be put to its best use. The point to raise an awakening for policy-makers is that the true aim of improvement of the so-called wastelands should not be one sided, in the way that bumper crop production should not be at the cost of further deterioration of soil.

In many countries, the frequency of salt affected soils make their utilization necessary, by irrigation, application of chemical amendments or by planting salt tolerant plants (biological desalination). The USA has made rapid strides in establishment of salinity tolerance differences amongst a large variety of crops, water and salt movement and leaching requirement, computation of regional salt balance, use of poor quality of irrigation water etc. The USSR and Egypt have gained valuable experience on various reclamation methods including large scale artificial drainage systems. Australia has successfully demonstrated a useful technology for raising productivity of arid salt affected land using tolerant species and special planting techniques. Israel has made considerable headway in the use of poor quality waters. Tunisia, under an internationally aided programme, has successfully concluded a project on development of location specific technologies on use of brackish and saline waters. In Pakistan, numerous and diversified methods are used in the fight against salinity according to various soil types. Among the major practices are the use of chemical amendments, especially gypsum, fertilizers, leaching and growing of dhaincha along with crops.

In Pakistan, reclamation of saline and alkali soils was practiced much before any scientific knowledge on the nature and properties of these soils became actually available. Every attempt was based on the trial and error method with little or no knowledge of the basic principles involved in the process of reclamation. Some of them, which were successful, were later on, proved to be technically sound. Some others turned out to be mere temporary expedients and the land reverted to the original condition sooner or later; and yet many others failed to show any improvement on the land so treated.

The methods to be adopted for reclamation have necessarily be based upon a proper understanding of the causative factor which led to the development of the saline and alkali condition in the soil. According to experts for any reclamation technique to be met with: i) salt or alkali must be completely removed from the zone; ii) the land must be prepared from reverting to the original condition; and iii) the repair of the damage, already done to the soil, should be substantial. The few major principles underlying the various methods that are employed to reclaim saline and alkali soils are: i) the hydro- technical amelioration; ii) the biological amelioration; and iii) the chemical amelioration. Identification and utilization of crop plants that are well suited for saline agriculture with increase the rational food productivity.

In the reclamation and subsequent management of saline and alkali soils, the irrigation drainage and leaching are the very old processes of removing salts from the salt affected soils. The primary objective of any irrigation method is to supply water to the soils so that moisture will be readily available at all the times for crop growth (an alternative to lesson the effect of physiological drought), but even then soil salinity is definitely an influencing factor. Drainage is the process of removal of excess water from soil and thereby removing the excess water soluble salts also along with water. Leaching is accomplished by ponding an appreciable depth of water on the soil surface by means of dikes or ridges and thus establishing downward water movement through the soil. This has been found the most effective procedure that can be used for removing excess soluble salts from soil.

Biological improvement of these soils includes the addition of organic matter to the soil, reducing the soil water evaporation by shelter belts, cultivation of perennial salt tolerant plant species or crops, if annual crops are cultivated, introduction to fill in crops and by incorporation of crop residues, farmyard manure and press mud, etc. Addition of organic matter benefits saline and alkali soils principally in two ways: i) Improvement in water infiltration, and ii) release of CO2 during decomposition. Even growing a crop has biological amelioration action, since during respiration CO2 is produced. Under composted or partially composted material, by releasing CO2 during decomposing, exert calcium solubilizing action in the soil, thus, increasing the soil fertility status. Biological amelioration operations compensate for the loss of soil nutrients which being soluble in the water are lost with downward percolating leaching water.

The choice of a chemical amendment may be influenced by the time required for its reaction in the soil. In general the cheaper amendments are slower to react. From the standpoint of efficiency in replacing exchangeable sodium, it is advantageous to leach most of the soluble salts out of the soil before applying chemical amendments. As a result of removal of soluble salts, depends upon the soil permeability. Inorganic chemicals, which are used to reclaim alkali soils can be: i) soluble calcium such as calcium chloride or gypsum; ii) slowly soluble calcium compounds like, limestone, rock phosphate or basic slag; and iii) acidifying materials (for solubilizing the insoluble native soil calcium), like sulphur, iron sulphate, iron pyrites, aluminium sulphate and sulphuric acid, etc. Owing to its high solubility in water, CaCl2 is probably the most readily available source of soluble calcium, but it is seldom used because of its cost. Sulphuric acid, iron and aluminium sulphates are quick-acting amendments. H2SO4 is often heap enough for field applications, but the use of other two chemicals is not economically feasible. Because of their relatively low costs, gypsum and sulphur are the most common amendments used for reclamation. From the plant physiological point of view, lime has just the opposite effect from sodium on the cell plasma. It should, therefore, be added to salt affected soil in adequate amount for the plant growth.

In the country, there are many trees, shrubs, grasses and vegetable crops, which can grow under saline conditions. Some of these plants are:

TREES: Acacia selerosperma; Acacia amplixwpa; Acacia victoriae; Acacia nilotica; Acacia acuminata; Acacial cambagei; Acacia salicina; Acacia calcicola; Acacia coriacea; Acacia saligna; Acacia bivenosa; Acacia subtessarogona; Acacia kempeana; Acacia aneura; Acacia cunnighamii; Acacia holosericea; Acacia adsurgens; Acacia validinervia; Cassia nemophila; Cassia sturtii; Casuarina glauca; Casuarina obesa; Eucalyptus striaticalyx; Eucalyptus microtheca; Leucaena leucocephala; Prosopis juliflora; Prosopis chilensis; Prosopis cineraria; Prosopis tamarogo.

SHRUBS: Atriplex numularia; Atriplex amnicola; Atriplex lentiformis; Atriplex undulata; Atriplex crassifolia; Beta vulgaris (fodder beet); Hasawi rushad; Kochia indica; Lotus carniculatus; Macroptilium atropurpureum; Medicago sativa; Sesbania formosa; Sesbania rostrata; Sesbania aculeata; Suaeda fruticosa; Trifolium alexandrinum; Trifolium resupinatum.

GRASSES: Cynodon dactylon; Desmostachya bipinnata; Dicanthium annulatum; Echinochloa colonum; Echinochloa crusgalli; Echinochloa turnerana; Hordeum vulgare; Leptochloa fusca; Lolium multiflorum; Sorghum vulgare; Sporabolus arabicus; Panicum antidotale; Panicum maximum; Pennisetum purpureum; Polypogon monspeliensis; Sorghum halepense.

VEGETABLES: Aster tripolium; Brassica carineta; Brassica compestris; Brassica juncea; Brassica napus; Coreandrum sativam; Eruea sativa; Lactuca sativa; Medicago falcata; Spinacea levacen; Trigonella faenum-graeeum.

In actual practice, the reclamation can be made much more effective and speedy, by combining various ameliorative methods, since the interaction between them brings in more spectacular results than when they are applied singly. However, a prerequisite to these operations of the specific area to be reclaimed, must be fully analysed e.g. the physical nature of soil, its chemical composition, source of salt, depth and nature of underground water, subsidiary source of irrigation water and its nature, etc. to achieve the real success. Certain saline and alkali soils, by their nature are amendable to easy reclamation techniques, while with some others, many intricate problems are there which make their reclamation difficult. Implications may differ from place to place reclaim salt affected soils depending upon the varying nature of the factors, mentioned above and therefore, same remedial measure at one site may turn into complete failure at another site.

Looking at the acute salinity problems along with water shortage, if an attempt is made to check the complete evaporation from soil surface by cultivating salt tolerant perennial plant species continuously for a period of 2-3 years, simultaneously by the addition of organic matter to the soil in the form of cattle dung, which costs almost nothing, the fertility status of soil is also improved, along with a reduction in surface salt content of the soil. After this sort of improvement, if a salt tolerant crop with shallow root system is shown in the following rainy season with almost nil or a little addition of extra water to irrigated the soil, obviously a success can be expected. After the crop harvest, soil surface should immediately be protected by raising the same salt perennial plant species., native of these areas. The proposed plan appears equally good for the soils of both the sites.

To rest the efficacy of the operation undertaken to reclaim the salt affected soils, selection of the test crop is the most important factor. Many crop failures may result by growing crops that have low salt tolerance, and this leads to draw a wrong conclusion and the undertaken reclamation measure is considered ineffective. The salt tolerance of a crop may be appraised according to following three criteria, suggested by USDA: i) The ability of the crop to survive on saline soils; ii) the yield of crop on saline soils; and iii) the relative yield of the crop on saline soil as compared with its yield on non-saline soil under similar growing conditions.