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Saline lands and rice

Salinization of millions of hectares of land continues to reduce crop productivity severely worldwide

By S. M. Alam, R. Ansari, S. M. Mujtaba
and Aisha Shereen
NIA, Tandojam, Pakistan
Apr 23 - 29, 2001

Soil salinity is a major problem in arid and semi-arid regions, where rainfall is insufficient to leach salts and excess sodium ions down and out of the root zone. These areas often have high evaporation rates, which can encourage an increase in salt concentration at the soil surface through capillary rise. The presence of a cemented hardpan at varying depths and insufficient precipitation for leaching often adds to the problem. Newly established irrigation projects, with improper planning and management practices may also add salts to soils. Historically, soil salinity contributed to the decline of several ancient civilization. Despite the advanced technologies available today, salinization of millions of hectares of land continues to reduce crop productivity severely worldwide. Of the approximately 13 billion hectares total land on earth, about 1 billion are affected by salinity/sodicity. According to a report, saline/sodic soils cover about 26 % of the world's cultivated land. Incidentally, most of the developing and underdeveloped countries of South and Southeast Asia, Africa and South America are the worst affected by this menace. More than 80 million hectares of such soils are in Africa, 50 million hectares in Europe, 357 million hectares in Austral-asia, nearly 147 mha. in Central, North and South America. Similarly, a large bulk of about 320 million hectares of land in South and South East Asia is under the grip of salinity. It shows that no continent on our planet is free from salt-affected soils.

About 56 million are in densely populated South and Southeast Asia, where both food and arable land are scarce. Of the latter, 27 million ha are lands in the humid tropics climatically, physiographically, and hydrologically suited to rice, but lying uncultivated largely because of salinity. Saline soils contain sufficient salt in the root zone to impair the growth of cop plants. But, because salt injury depends on species, variety, growth stages, environmental factors, and nature of the salts. It is difficult to define saline soils precisely. Current definitions are based on salt content alone or in conjunction with texture, morphology, or hydrology. The most widely accepted definition of a saline soil is one that gives an electrical conductivity in the saturation extract (ECe) exceeding 4 mmho cm-1 (4 dSm-1) at 25C. pH below 8.5 and ESP (exchangeable sodium percentage) less than 15. These soils generally contain neutral soluble salts comprising of chlorides and sulphates of sodium, calcium and magnesium and they possess good physical condition and permeability, doe to flocculating effect of neutral salts. Sodic/saline-sodic soils, on the other hand, have high pH and exchangeable sodium. They have greatly impaired physical and chemical conditions due to defloccutating effect of sodium on soil, resulting in surface crusting, compaction of subsoil, reduced infiltration rate, poor hydraulic conductivity, etc. FAO/UNESCO mapped soils with ECe's exceeding 15 mmho cm-1 as strongly saline soils or solonchaks. A saline soil forms when the influx of salt is greater than the efflux. The balance depends on climate, geomorphology, relief, and hydrology.

The bulk of the world's saline soils, which generally occur in arid and semi-arid regions, exceed evapo-transpiration precipitation. Evaporation of salt solutions brought into poorly drained depressions by surface runoff, groundwater rise, artesian activity, or interflow leads to salt accumulation. Salinization occurs in arid and semi-arid areas due to waterlogging brought about by mismanagement and improper irrigation. Another type of salinity occurs in coastal areas subject to tides in both arid and humid regions. The main cause is intrusion of underground seawater. Saline soils vary widely in their chemical and physical characteristics, salt dynamics, and hydrology. The variables include salt source; nature and content of salts; lateral, vertical and seasonal distribution of salt; . soil pH; nature and content of clay; organic matter content; nutrient status; water regimes; relief; temperature; and soil toxicities. Strongly saline soils are barren. Less strongly saline soils in arid regions are characterized by patchy growth of halophytic grasses and shrubs, whereas Coastal saline soils of the humid tropics and subtropics are characterized by the presence of mangrove species. Most plants suffer salt injury at EC values exceeding 4 dS m-1. It is stated that many crop plants can stand much higher concentrations than generally supposed if the solution consists of physiologically baled salts as in seawater. Crop yield decreases markedly with increase in salt concentration, but the threshold concentration and rate of yield decrease vary with the species. Some of these differences are shown in Tables 1 and 2. These differences have implications for the management of saline soils breeding varieties for salt tolerance. Saline lands can be converted to crop lands by preventing the influx of salt water, leaching the salts out of the root zone, and correcting soil toxicities and nutrient deficiencies. Rice is the crop best suited to saline lands because the standing water that is necessary for leaching the salts causes chemical changes in the soil that benefit rice. The reclamation costs can be reduced by growing salt-tolerant rice cultivars.

Table 1. Some characteristics of saline soils.

Characteristic

Range

Texture

Sandy to clayey

PH

2.5 to 11

ECe (dS m-1)

4 to > 50

Salt (%)

0.1 to 5

Organic C (%)

<1 to > 30

Fertility

Very low to moderately high

Clay mineralogy

2:1 types to hydrous oxides

 


 

Table 2. Kinds of saline soils and Associated stresses.

Kind of saline soils

Accessory growth-limiting factors

Arid saline

High pH; deficiencies of zinc, nitrogen, and phosphorus

Neutral and alkaline coastal saline

Zinc deficiency, boron toxicity, deep water

Acid saline

Iron toxicity, phosphorus deficiency

Coastal acid sulphate

Iron and aluminum toxicities, phosphorus deficiency, deep water

Coastal organic

Deficiencies of nitrogen, phosphorus, zinc, copper,

toxicities of iron, hydrogen

sulphide, and organic substances; deep water.

Until about 1950, the increase in world food demand was met by an increase in the cultivated area. But from 1950 to 1975, world population increased faster than the cropped land area leading to a drop in per capita area from 0.241 to 0.184 ha. The pressure on the land is most severe in densely populated Asia, where population density is already extremely high, population growth is increasing and undeveloped land resources are limited. Overcrowding, food shortages, and land scarcity are compelling developing countries to bring under crops, lands lying idle because of salinity and other soil stresses. The population in South and Southeast Asia will grow at the rate of about 2.7% year during the next two decades. But food production rates are insufficient to keep pace with growth demand, especially in the poor populous countries. If present estimated trends persist, by the year 2005, the world population will likely be 6.3-6.7 billion and the food need 3 billion tons. But food production growth rates are barely sufficient to keep pace with population growth especially in the poor populous countries, and cereals deficit of 85 million tons in 1985 is projected for developing countries. Erosion, salinization, and denudation cause land degradation. About one-third of the world's crop land is severely crowded, and one-tenth of the 210 million ha of irrigated land has deteriorated, because of salinization. Increasing cost of fertilizers, chemicals, and electric power is a serious obstacle to increasing crop yields by intensifying agriculture, especially in developing countries. Use of modern technology enabled farmers in affluent countries to achieve impressive yield increases from 1961 to 1971. The introduction of new wheat and rice varieties helped even the Third World countries to benefit by the new technology. But yields started to plateau or decline in affluent countries, while the green revolution in rice bypassed most farmers of South and Southeast Asia. Thus, the new technology alone is not likely to prevent the huge food deficits anticipated a decade from now. Although, cultivar differences in salt tolerance have been known for over 50 years, it is only during the past decades that organized efforts have been made to select and breed salt-tolerant crop plants. Cultivar tolerance for salinity is a substitute for amendments on moderately saline soils and a supplement to amendments in strongly saline ones.

Salinity and sodicity has affected about 10 per cent of the total world land. Approximately, 20 mha of land deteriorates to zero production each year mainly due to salinization. According to another report, saline/sodic soils cover about 26% of the world's cultivated land. Incidentally, most of the developing and under developed countries of south and southeast Asia, Africa and South America lying in arid/semi-arid regions, are the worst hit by this menace. India for instance, has about 7 million ha of saline/sodic lands, while the Pakistan with a much smaller land area has about 4.2 million ha of such lands in the Indus basin only. The salt affected areas of Pakistan are almost exclusively located in the heart of its agriculturally the most important tract i.e. the Indus plain, thus causing serious economic and social problems for the people. The whole country may have as much as 6-7 million ha, resulting in national economic loss of about US$ 32 million annually in yields of wheat, rice, cotton and sugarcane.

In these countries, water logging is the main cause of upward flux of salts, which is due to intensive and continuous use of surface irrigation and has altered the hydrological balance of the affected area. Adding to the problem is a more serious and perpetual loss of water through seepage from canals and water courses: Water logging, even for short periods and with non-saline water, may have adverse effects on plant growth.

The coastal areas of Pakistan comprise southern part of Makran Division in Balochistan and southern part of Karachi, Thatta and Badin in Sindh. The region makes 40-60 km wide and about 1000 km long belt along the Arabian Sea. It lies between 23.45 and 25.30 N and lontitude 61.45 and 69.15 E. These areas are not suitable for rice cultivation. The salient future of coastal areas are as: extent 38750 km2, altitude 0- l000 m, population 13.5 million, climate hyper-arid, tropical, maritime, linguistic groups Balochi and Sindhi, soil diversified economic indicators marine fisheries, ship breaking, date palm, coconut-palm, livestock production, mangroves, tourism, ports and sea salt extraction. Of the 400,000 ha of coastal saline soils in the Philippines, 100,000 ha area under mangroves, 175,000 ha are in fish ponds that cost $4000 to $20,000 per ha, whose average annual production is 700 kg fish/ha. In Indonesia 558,000 ha of tidal swamps are planted to rice. The total cropped area of tidal swamp and spongy is 12.6% of the total rice crop area per year. Reclaimed tidal swamps produce 10% of Indonesia's rice. The government had opened 220,000 ha of tidal swamp lands for rice in one transmigration scheme alone. In the next 5 years, they plan to open another 250,000 ha. The 2 million ha of coastal saline soils of India occur in deltas in a strip ranging from a few kilometer to 50 km from the coast. Salinity is due to inundation by sea water and ingress of saline water at high tide.

Importance of rice

Rice is the second most important crop in the world after wheat, with more than 90% currently grown in Asia. About 120,000 varieties are grown across the world in an extensive range of climatic soil and water condition. It is grown on areas of 149.151 million hectares yielding 550.193 million tons of paddy with a yield of 3689 kg/ha. China is the major rice producing of the world followed by India, Indonesia and Bangladesh. However, yield per hectare is highest at 6.1 tons in Japan, followed by 5.1 tons per hectare in China. In Pakistan, the area under rice is 2.422 million hectares producing 4.671 million tons of paddy rice with an average of 2100 kg/ha. Punjab and Sindh are the two major provinces producing together about 80% of the total production. Rice a major summer monsoon is being grown as an important food/cash crop in Pakistan. After wheat, it is staple cereal in the country and abroad. This is a big source of foreign exchange earning for the country, around 900 million US dollars worth of rice was exported during 1999-2000. The areas production and yield of different varieties of rice are in Pakistan during 1998-99 were as Area (000 ha), Basmati (1215), IRRI (986), other (219) and total 2424. The productions (000 ha) were Basmati (1687), IRRI (2595), other (395) and total 4674. Similarly, the yields in kg/ha were Basmati (1387), IRRI (2625) other (1796) and total 1928). Among the four provinces, the Punjab is number one in area, production and acerage, followed by Sindh, Balochistan and NWFP. Pakistan earned (1998-99) about 26.83 billion rupees of total 4.7 million tons, 2.7 million tons consumed and 1.8 million tons and 0.2 million tons reserved for seed. It is a crop mainly of irrigated area and grown on about 10% of total cultivated land. Being an exportable commodity, it is immensely important in national exchequer. It is a crop mainly of irrigated area and grown on about 10% of total cultivated land. The production of rice in Pakistan contributes actively in the foreign earnings and also shares largely in the national income. The production of rice during past few years has general tendency to increase. The increased production of rice continuously helped in solving the food problem for the rapidly increasing population of the country. Because of the use of new varieties and certified seed accompanied with the better dosage of fertilizer resulted in increasing yield per hectare in all the provinces of the country.

Salinity affects the growth of rice in varying degrees at all stages of its life cycle starting from germination through to maturation. The effect may vary depending on the stage of plant development. Several studies indicate that rice is tolerant during germination, becomes very sensitive during early seedling stage, gains tolerance during vegetative growth, again becomes sensitive during pollination and fertilization and then become increasingly more tolerant at maturity. However, some workers maintain that flowering stage is not sensitive to salinity. In fact, effect of salinity is related to the stage of plant development at which salinity is imposed, salt concentration, nature of salt and the duration of salinization. Hence, to know the response of rice plant to salinity as a whole it is imperative that the effects be observed at various stages of its development, separately.

Rice varieties are tolerant to salinity during germination. Salinity delays germination, but does not appreciably reduce the final germination percentage. About 80-100% germination has been reported at EC of 25-30 mmho/cm at 25 C of saline solution after 14 days. High salt concentrations i.e. 2-4 % show strong inhibitory effect and markedly decrease the final germination. High salt concentrations (3-15 % NaCl) have been reported to inhibit germination, however, the seeds germinated when transferred to distilled water. It has been suggested that salinity has no harmful effect of germinability. Germination time increases with increase in salt concentration, because it is directly related to the amount of water absorbed by the seeds, which in turn depends upon the salt concentration. Thus osmotic stress may be responsible in delaying germination. Rice plants are more sensitive during the young seedling stage (2-3 leaves) than during germination. The electrical conductivity values for a 50 % reduction in germination one week after planting ranged from 20-30 mmho/cm, while the critical level of salinity for seedling growth was about EC 5 mmho/cm. Growth parameters such as dry matter, seedling height, root length and emergence of new roots decrease significantly at an electrical conductivity value of 5-6 mmho/cm.

During the early seedling stage, salinity causes rolling and withering of leaves, browning of leaf tips and ultimately death of seedlings at high salt concentration. In general, salt injury symptoms first appear on the first leaf (older) followed by the second, and then on the growing leaf. Salinity suppresses leaf elongation and formation of new leaves. Growth decreases with increase is osmotic pressure as water uptake decreases, sodium and chloride in the leaves and stems increases and the description of potassium and calcium by rice plants decreases. Most of the chloride is localized in leaf blades and stems, but of all the plant parts, the roots contain the least chloride. Photosynthetic function and chlorophyll content decrease in proportion to increase salt concentration. Size of stomata becomes smaller, which means that carbon dioxide concentration in the leaves containing higher sodium chloride is low, resulting in lowering of photosynthesis. Salinity also affects root development adversely. It has been reported that laternal roots and root elongation decrease with salinity. In short, the causes of salt injury to the rice plant are complicated. It seems that both osmotic imbalance and chloride ion may be responsible for the suppression of growth. The rice plant gains tolerance during vegetative growth. During vegetative growth, plant height, straw weight, number of tillers, dry weight of roots, root length, number of days from transplanting to flowering are all affected by salinity. The shoot growth is generally suppressed more than the root growth. With regard to root characteristics, salinity effects are more on the elongation of roots than the production of dry matter in roots. During vegetative period, the most common salinity effect is stunting of plant growth, whereas leaf withering becomes less apparent. Salt injury is more severe at high temperature (35C) and low humidity (64%) due to increased transpiration and uptake of water and salt by the rice plants. Salinity at the reproductive stage depresses grain yield much more than salinity at the vegetative growth stage. Rice at critical salinity levels may give normal straw yield but produce little or no grain. Usually, the decrease in grain yield is proportional to the salt concentration and the duration of saline treatment. When the plants are continuously exposed to saline media, salinity affects the panicle initiation spikelet formation, fertilization of florets, and germination of pollen grains and hence causes an increase in number of sterile florets. The greatest injurious effect is on the panicle. Salinity severely, reduces the panicle length, number of primary branches per panicle, number of spikelets per panicle, seed setting percentage and panicle weight, thereby reducing the grain yield. The weight of 1000 grains is also reduced. Salt injury also results in small grain due to reduction in grain length width and thickness.

Rice is moderately susceptible to salinity. The degree of injury, however, depends on the nature and concentration of salts, soil pH, water regime, method of planting, age of seedling, growth stage of the plant, duration of exposure to salt, and temperature. Most rice cultivars are severely injured in submerged soil cultures at an ECe of 8-10 dS m-1 at 25 C; sensitive ones are hurt even at 2 dSm-1. At comparable ECe's injury was less in sea water than in solutions of common salt, in neutral and alkaline soils than in acid soils, at 20 C than at 35 C, and in 2-week old seedling than in 1-week old plants.

Rice breeders have used genetic variability to produce cultivars that have high yield potential and that resist disease and insect damage and that tolerate cold, drought, and even floods. But apart from some sporadic work in Sri Lanka and India, little was done until recently to identify and breed cultivars adapted to adverse soil conditions such as salinity. For centuries, farmers have grown salt-tolerant cultivars on the saline soils of India, Burma, Thailand, Indonesia, the Philippines and Vietnam. But, because of lodging and susceptibility to disease and insect damage, yields are about 1 t/ha. Traditional cultivars selected and bred for salt tolerance during the past 40 years have not done much better. Recognition of the potential of saline lands for rice production in the densely populated countries of South and Southeast Asia prompted the inclusion of salt tolerance as a component of the Genetic Evaluation and Utilization programme of the International Rice Research Institute (IRRI). The GEU programme is an interdisciplinary effort on the part of plant breeders and problem area scientists to build into the germplasm tolerance for or resistance to the adverse conditions that small farmers (who constitute the bulk of rice-producers in developing countries) encounter in their environments and which they are often unable to correct. These adverse factors include pest damage, drought, floods and adverse soil conditions. Of the adverse soil conditions, salinity received the most attention, because of its widespread occurrence in current and potential rice lands.

Breeding rices for salt tolerance included these steps: Developing screening techniques, identifying salt tolerant germplasm, combining salt tolerance with good agronomic characteristics and pest resistance, testing thousands of rices generated by the hybridization programme for salt tolerance, selecting in the field salt tolerant breeding lines possessing other desirable traits, multilocational international testing, conducting yield trials under controlled field conditions at IRRI, testing in farmers fields. A study of the factors affecting salt injury indicated that: The discriminating salinity level is 8-10 dS m-1 at 25 C, a nearly neutral submerged clay soil treated with 0.5% common salt is a suitable medium, transplanting a 2-week old seedling raised in culture solution is better than direct seedling, the percentage of dead leaves is a good measure of salt injury. The greenhouse screening technique IRRI uses is based on those observations. About 4000 rices can be screened in 12 months on 100 m2 of bench space. Salt-tolerant cultivars identified in the greenhouse are used by IRRI's Plant Breeding Department as parents in the hybridization programme. Because plant breeders like to field test breeding lines under insect, disease, and other stresses, progeny from the breeding programme are screened for salt tolerance at IRRI on a 0.1 ha block of a nearly neutral clay treated with common salt to give an ECe of 8-10 dS m-1. In a year 1200 rices can be screened.

Salt tolerance studies are usually in grown chambers and greenhouse, with plants raised in plastic trays for small pots. The application of results obtained in such studies to field conditions, where the distribution of salt is neither uniform in depth nor constant with time, is difficult and requires knowledge of how plants respond to varying salinity. The salt tolerance of any crop is usually expressed as decrease in yield associated with a given level of soil salinity as compared with yield under non-saline conditions. The primary salinity factors influencing plant growth are the kind and concentration of salt present in the soil solution. Salt concentration in soil is usually determined by measuring EC of a soil saturation extract (ECe) obtained from the active root zone. Recently, simple, rapid and reliable instruments i.e. salinity sensors and four electrode probes, have been developed for measurement of electrical conductivity of-soil water ECsw. By these instruments, the electrical conductivity of soil water can be determined. At IRRI, field screening is done in a 0.1 ha plot. Salt is added from time to time to keep the EC. at 8-10 mmho/cm at 25 C. About 1,200 entries/ year can be screened. The tolerant lines are tested in the International Rice Salt and Alkali Tolerance Observational Nursery (IRSATON), which is a part of the International Rice Testing Programme (IRTP) for further screening in Southeast Asian countries in saline fields. About 32 entries have been rated as tolerant. The results of recent replicated yield trials in saline fields with rices bred for salt tolerance, good agronomic characteristics. Because the rice plant is susceptible to salinity at transplanting and gains tolerance with age, it is advisable that aged seedlings (6 weeks old) be planted in saline fields. During the early part of growing season and at flowering when rice is sensitive to salt, efforts should be made, if possible, to minimize salinity by frequent irrigation. In areas of high evaporation, frequent replenishment of water becomes essential to offset the increase in salinity of the surface water that results from evaporation. In the process of reclamation of highly saline soil, rice may be included in a crop rotation as a leached crop. Because, rice is a relatively shallow rooted crop, therefore, to establish a stand of rice, it is not necessary to move the salt to greater depth. The severity of the salinity problem would determine the frequency accumulated with which, rice may be included in the rotation. In some areas, the soil may need continual leaching. In such cases, it will not be possible to grow any crop except the more salt tolerant varieties of rice. Besides, using salt resistant varieties, the supply of other essential nutrients should be enough to obtain high yields on saline fields. Because of nutritional imbalances and differential plant response under saline conditions, development of a package of production technology (i.e. age of seedling, time of planting, number of plants/hill, spacing, type of fertilizer and its rate, application of micronutrients, etc.) is necessary.