Mechanisms of salt tolerance in crop plants and salinity management

By Md Tariqul Islam*, NewAge, August 27, 2009

SALINITY affects the physiological processes of plants and it is the most important factor which severely affects crop production. These adverse effects may be attributed to non-availability of water, disturbance in nutrient uptake causing deficiency and ion-toxicity to plants. Salinity and sodicity stresses are ever-present threats to crop yields, especially in countries where irrigation is an essential aid to agriculture. As per FAO/UNESCO soil map of the world, a total of 953 m ha covering about 8 per cent of the land surface is suffering from salinity/sodicity. The salt affected soils are reported to occupy 42.3 per cent of the land area of Australia, 21.0 per cent of Asia, 7.6 per cent of South America, 4.6 per cent of Europe, 3.5 per cent of Africa, 0.9 per cent of North America and 0.7 per cent of Central America. Recent estimates indicate that 6.74 m ha area in India is affected by soil salinity and alkalinity. In Bangladesh, 2.85 million hectares of land are saline affected and there is a great possibility to bring these areas under cultivation with salt tolerant crop varieties and proper reclamation and management.

Saline soils are often recognised by the presence of white salt encrustation on the surface and have predominance of chloride and sulphate of sodium, calcium and magnesium in quantities sufficient to interfere with growth of most crop plants. Soil with neutral soluble salts has saturation paste pH less than 8.5. The electrical conductance of saturation extract of saline soils is more than 4 dSm-1 at 25 degrees Celsius and exchangeable sodium percentage is less than 15. The sodium adsorption ratio of the soil solution is generally less than 15. However, soil salinisation with neutral soluble salts of sodium invariably result in soil solution SAR greater than 15. Such soils are termed saline-sodic.

Parent material, weathering and release of salts, shallow groundwater and capillary rise, indiscriminate use of irrigation waters, ingress of seawater along the coast, salt-laden sands blown by sea winds, lack of natural drainage, etc are the sources and causes of accumulation of salts in soil. On the other hand, continuous accumulation of salts from tidal flooding, upstream withdrawal of sweet water, cyclone and tidal surges, prawn culture, irrigation with saline water, capillary movement of groundwater, etc may cause coastal salinity.

Soils containing excessive salts of sodium carbonate and sodium bicarbonate and having sufficient exchangeable sodium to interfere with growth of most crops plants are called alkali. These have pH of the soil saturated paste more than 8.5, ESP 15 or more and ECe limitless if resulting from salts capable of alkaline hydrolysis.

The alluvium (parent material) rich in plagioclase feldspars under hydrolytic dissolution release high amount of sodium. Weathering of alumino-silicate minerals through carbonation provides solutions of bicarbonates and carbonates of alkali in addition to silica and alumina. The bicarbonates and carbonates migrate with the subterranean and surface waters and accumulate in un-drained areas under arid and semi-arid conditions to form alkali soils.

Rice plants tolerate salinity stress by 3 mechanisms acting upon singly or jointly. These mechanisms are exclusion, dilution, compartmentalisation or maintenance of high potassium-sodium ratio.

Exclusion: It refers to the restricted uptake of sodium ions by tolerant rice varieties.

Dilution effect: Usually the tolerant varieties grow faster than non-tolerant varieties under saline condition. It was experimentally found that lower shoot sodium content in the Pokkali variety is not due to any better control of sodium transport by its roots but is directly attributable to the dilution effect of its rapid vegetative growth.

Compartmentalisation: When a rice plant is exposed to saline condition, the older leaves die due to high amount of sodium accumulation while younger ones remain green and growing. This physiological behaviour of rice plant is called compartmentalisation which is a useful feature of gramineae.

High potassium-sodium ration: It is now established that salt tolerant varieties maintain a higher potassium-sodium ratio compared to that in non-tolerant variety. Relatively higher amount of potassium than sodium ions is probably required in panicles for the protection of growing panicles from the toxic effect of sodium ion. It was experimentally revealed that growing panicles of tolerant genotypes maintain higher potassium-sodium ratio compared the growing panicles of BR11 salinity sensitive variety at reproductive phase.

Strategies to manage coastal salinity: Irrigation with harvested rainwater and cultivation of salt tolerant crop varieties are most useful techniques in many countries.

Strategies to manage sodic soils: The chemical amendment based technology has been developed to reclaim the alkali/sodic soils. Various components of this technology includes; field levelling, bunding, soil sampling to know the sodicity status for working out amendment dose, application of gypsum/pyrite as per requirement of the soil and its mixing in upper 10cm soil, keeping water ponding for 5-7 days, following rice-wheat rotation for the first 3 to 4 years and growing sesbania during summer as green manure crop after wheat harvest in April. By adopting this technology about 1.3 m ha area has been reclaimed in the states of Punjab, Haryana and western UP.

Salt-tolerant varieties: A sizeable part of the salt-affected area in India is in possession of small and marginal farmers who are themselves poor. Under such situations, chemical amendments based reclamation technology without government subsidy is not sustainable. Development of salt tolerant varieties of important field crops is an option of great promise for utilization of such areas, as most of these varieties give significant yield without or with little application of chemical amendments. Several varieties of field crops like rice, wheat and mustard have been developed which have potential to yield reasonable economic return both in high pH alkali soils and also in saline soils. In case of rice, the most promising varieties include CSR10, CSR13, CSR19, CSR23, CSR27, CSR30 and CSR36. These varieties can be cultivated in soils with pH and EC range from 9.4 to 9.8 and 6-11 dS/m.

Reclamation of saline waterlogged soils: Sub-surface drainage technology has been developed to lower the water table in saline waterlogged areas. The system consists of a network of concrete or rigid PVC pipes along with filter installed manually or mechanically at a designed spacing and depth below soil surface to control water table depth by draining excess water and disposing it out of the area by gravity or by pumping from an open well, called sump. The first approximation of the area covered under sub-surface drainage in India.

Alternate land use systems: A sizeable part of the salt-affected soils in India is constituted by the village community lands, lands along the roads, railway tracks and other government lands reserved for specific purposes. Reclamation of such area for crop production is posing problems because of community rights on such land resources. These sites offer ample opportunities to raise salt tolerant trees, bushes and grasses to produce fuel wood, fodder and energy. An alternate technology of raising multipurpose forest tree plantation, fruit trees, agroforestry systems and other high value medicinal and aromatic crops seems quite feasible. Several salt tolerant forest and fruit species have been identified which can be grown in highly sodic and saline soils. The promising forest species include Prosopis juliflora, Acacia nilotica, Tamarix articulata and Casuarina equisetifolia.

Agroforestry: Several grass species have mechanism to tolerate high salt concentration in the root zone soil. Some of these highly tolerant grasses either exclude the absorption of salts from the soil and/or deposit the absorbed/translocated salts at points within the plant system which do not allow them to interfere in metabolic processes. Such grasses have been identified. Grasses like Leptochloa fusca has the potential to yield high biomass even at pH level of 10.4 and more. Similarly, Bricharia mutica is another salt tolerant grass, which can be grown even under prolonged waterlogged and salt situation. Several experiments have been conducted at CSSRI, Karnal and elsewhere to study the performance of these grasses in association with salt tolerant trees like Prosopis juliflora and Acacia nilotica in a unified agroforestry system. A field study conducted at Gudha experimental farm for six years indicated that Leptochloa fusca has the potential to yield about 20 tonnes/ha of green biomass per annum when planted with Prosopis juliflora in a soil having pH of 10.4. Leptochloa grass has a special characterstic that it starts disappearing when sodicity level in the soil decreases. Thus, allowing the regeneration of other moderately salt tolerant grasses and other annuals. The results of this experiment clearly indicated that sodic soils can be reclaimed by growing Prosopis juliflora and Leptochloa fusca for five years. During this period, the surface soil is reclaimed and salt tolerant crops like Berseem (Trifolium alexendrinum), Egyption clover, oats and senji can be grown without the application of amendments.

Medicinal and aromatic crops: A number of medicinal and aromatic crops have been screened for salinity and sodicity tolerance in India. Crops like Isabgol (Plantago ovata) and Matricaria can be successfully cultivated in soils having pH of 9.5 and EC between 8-10 dS m-1. Similarly, dill (Anethum graveolens), a spice crop and Salvadora, a non-edible oil tree can be grown in saltaffected vertisols very successfully. Industrial species like Euphorbia and mulethi (Glycyrrhiza glabra) also have good scope for cultivation in salty environments.

Soil reclamation: Salt tolerant trees and grasses when planted either in association or as sole plantations reclaim the sodic soils over a considerable period of time. The mechanism for sodic soil reclamation by trees involves; a) dissolution of native calcium carbonate present in precipitated form in sodic soils by the biological activity of tree, grass roots, b) addition of leaf litter and turn over of old roots which increase organic carbon in the soil, c) penetration of water into the otherwise impermeable soil through the holes created by the decayed roots which facilitates reaction with CO2 evolved from root respiration and thus producing carbonic acid. This acid, though weak in reaction initiates the process of dissolution of native CaCO3. The free Ca in the soil solution available through this reaction replaces the Na ions on the exchange complex and d) initiation of biological activity in the soil due to improved organic matter contents, moisture and fertility regime. The degree of reclamation depends upon the kind of tree species, planting density, the adopted management practices and fencing provided to the plantation to check encroachment by humans and animals. Several experiments have been conducted in the past to study the reclamation effects of trees on physical, chemical and biological properties of sodic soils. The results of a long-term field experiment after 20 years (planted in 1970) in a soil having pH of 10.3 indicated that Prosopis juliflora has the maximum impact on reducing soil pH and EC and improvement in soil organic carbon and plant nutrients.

Bio-drainage: Bio-drainage refers to a technique of lowering groundwater table in waterlogged areas through the use of raising tree plantations. This technique removes excess soil water through the process of transpiration by trees using solar radiation energy. It is a kind of preventive technique to avoid the development of salinity and water-logging problem in canal command areas. The technique is highly useful when the soils are still in the process of salinisation due to rise in ground water level. However, if the soils are already salinised it has limited scope. Several species of trees have been screened to study their capacity to transpire water from different salinity and water table depths. The most promising species identified for bio-drainage include Eucalyptus, Populus, Casurania and Bambusa. Several programmes are in progress throughout the country to reclaim waterlogged areas in canal commands through bio-drainage.

*Dr Md Tariqul Islam is a senior scientific officer at the Bangladesh Institute of Nuclear Agriculture, Mymensingh

A sea of tears: the flooded people of South Bangladesh

With ocean levels rising, and shrimp farms proliferating, villages in south Bangladesh are being flooded by the sea. There is no water to drink, so people must search for it daily, writes Tahmima Anam.

Guardian, June 20, 2009

If you look at a map of Bangladesh, you will see that the southern coast has a meandering, indistinct border. This is the home of the Sundarbans, the world’s largest mangrove forest, with its strange, submerged trees, its Royal Bengal tigers, and its mythical figures such as Bon-Bibi, goddess and protector of the forest. It is where the delta ends and the sea begins.

Water has been the making and unmaking of Bangladesh. It is the reason the rice grows thick and fast, why the rivers ripple with fish, why the land is carpeted with green. But the water is also cruel. Every year, torrential rains flood villages and farms; rivers break their banks, swallowing great chunks of land, destroying the homes, and the dreams, that are built upon it.

Now, through disasters both man-made and natural, water is wreaking a new kind of havoc. Due to rising sea levels in the Bay of Bengal, and because the government has encouraged the unchecked growth of shrimp farms, the villages scattered along the south-western coast are being flooded with salt water. Large tracts of land, previously green with paddy, are now hot and stagnant pools, hospitable only to the cultivation of shrimp. The shrimp farms are lucrative, but they employ fewer people than the rice farms they have supplanted, leaving many households without an income. The briny water also has ruinous effects on the ecosystem. Nothing grows in these districts any more: the fish have died, along with the birds that depended on them. The cows have nothing to eat, so there is no milk; the tigers are fleeing inland and attacking humans. Worst of all, there is no fresh water to drink.

Munem Wasif’s photographs capture the desperate search for drinking water that has become a daily struggle for the villagers of southern Bangladesh. Their wells and fresh water sources contaminated, they spend the better part of their days in the search for water. Women make the long trek to the nearest source, kolshi flasks heavy on their hips. Children are taken out of school to help with water collection. Some villagers have taken collective action: every day, they lead small boats through the forest, gathering water and supplying their entire village. Others have no recourse but to pray – to the skies, to God, to Bon-Bibi – for the sweet, life-giving water that once coursed abundantly through this land.

• Munem Wasif visited Bangladesh with the support of Prix Pictet, which aims to communicate issues of global significance through photography, and WaterAid, which manages clean water and sanitation projects in the developing world

Farmers resist saline water based shrimp farming

The Daily Star, may 11, 2009

Defying a call from local lawmaker, goons hired by shrimp enclosure owners cut the dam of Chhoto Kainmari Canal in Mongla upazila to release saline water into the adjoining areas on Saturday.Photo: STAR

Farmers in Mongla upazila have vowed to resist shrimp cultivation by using saline water in croplands.

Hundreds of men and women of Kainmari, Chakrakhali, Kalibari, Dasherkhando and Kanainagar villages of Chandpai union continued movement for the second day yesterday and prevented shrimp enclosure owners from releasing saline water into the paddy fields through sluice gates.

Also on Saturday, hundreds of farmers of these villages foiled an attempt to release saline water into their fields with help of hired goons.

Meanwhile, security has have been stepped up at several villages of Chandpai union under Mongla upazila in Bagerhat district fearing bloody clashes any moment between farmers and owners of shrimp enclosures.

According to Vice Chairman of Mongla Upazila Parishad Mohammad Noor Alam, the owners of shrimp enclosures have hired goons to release saline water into the fields in the aforesaid villages defying the call by local lawmaker Begum Habibun Nahar to refrain from using force.

The hired goons on Saturday noon cut the dam of Kainmari Canal to forcibly release saline water into a field.

“The farmers have vowed to resist the hired goons. So the situation in these villages have become highly tense,” said OC Feroze Ahmed at Mongla Police Station.

Shubhash Mandol, Gurudas Dakua, Niva Halder and Alta Bagchi of Kalikabari village told this correspondent yesterday that release of saline water in the fields has become a big threat to their livelihood. Because it damages fertility of the fields, they pointed out.

Mongla Upazila Chairman Md Iddris Ali Ijardar said anti-salinity movement is gaining momentum as farmers are now more aware about the evils of saline water-based shrimp cultivation.

Meanwhile lawmaker Habibun Nahar warned that any ill motive against the greater interest of the people would be severely dealt with.

She said this as some owners were reportedly still planning to use force and dismantle the sluice gates with the help of hired goons.

Hundreds of villagers were guarding sluice gates and shrimp enclosures of Mongla villages since start of their current protest on Saturda

Saline-tolerant variety can help boost rice production

Says minister

The Daily Star, April 23, 2009. Dhaka

Cultivation of BR-47, a saline-tolerant rice variety, in the southern coastal region could bring about significant changes in rice production, Food and Disaster Management Minister Dr Abdur Razzaque said yesterday.

“There are around 2.8 million hectares of agricultural land in the coastal districts. If high yielding varieties like BR-47 are developed, there could be a dramatic change in rice production,” he said.

The minister was speaking as the chief guest at the closing ceremony of a two-day seminar titled ‘Stress Tolerance Rice for Poor Farmers in Africa and South Asia (STRASA)’, at the Bangladesh Rice Research Institute (BRRI) in Gazipur.

Farmers have already started cultivating the BR-47 variety in limited scale, which has shown huge success, he said at the seminar organised by BRRI.

“You cannot increase the yield of the existing varieties to a great extent. Therefore, development of stress-tolerant varieties, which can tolerate drought or submergence, is important,” said the minister.

The developed countries need to invest more in advance agriculture research, he noted.

BRRI is on the way to develop submergence-tolerant and drought-tolerant varieties and will be able to release those in near future, said BRRI Director General Dr Firoze Shah Sikder.

Chairing the closing session, Bangladesh Agriculture University Vice Chancellor Dr Abdus Sattar Mondal said it is necessary to seriously look into the nutritional standards and livelihoods.

Referring to the recent fall of rice prices, Mondal said the government must provide incentives to farmers to maintain better rice production.

International Rice Research Institute Program Leader Dr David Mackill, Liaison Scientist for Bangladesh Dr MA Hamid Miah and STRASA South Asian Regional Project Coordinator Dr Uma Shankar Singh also spoke.

Khulna Shrimp Cultivation Area: Thousands suffer from drinking water crisis

The Daily Star, April 19, 2009. Amena Khatun, back from Khulna

A few decades of shrimp cultivation has rendered the surface and ground water sources too saline for consumption in two upazilas in Khulna and an acute drinking water crisis has arisen in the area affecting thousands of locals.

People living in the Dakop and Paikgachha upazilas are suffering from such a severe shortage of safe drinking water that it has resulted in widespread diarrhoea and dysentery in the area.

Residents of Kamarkhola, Kalinagar, Saharabad, Joynagar, Satgharia villages under Dakop upazila and Madhukhali, Hanirabad, Radhanagar, Alokdip, Munkia, Dighalia, Bainchapara village in Paikgachha upazila are so hard hit by the problem for the last couple of months that they live in fear of massive health disaster at anytime.

Dr Akbar Hossain from Dakob upazila health complex said that around 12-15 people are seeking admission in the complex each day and the numbers are increasing rapidly. Most of those admitted are children under the age of five and are suffering from diarrhoea and dysentery.

“They all suffer from acute respiratory infection (ARI) from consumption of impure water,” he said.

Locals say high levels of salinity in surface and ground water has almost become a permanent problem here. Even the water pumped out by deep tube-wells is too saline for drinking.

They complained that the local administration is yet to take any constructive initiatives to solve this problem.

According to Dakop Upazila Nirbahi Office, a total of 2216 ponds in 106 villages under nine unions of the upazila cater to villagers drinking water needs. But most of the ponds have dried up in recent months due to the soaring temperature and especially because the area has not experienced any rainfall this year.

Chhabi Rani Mandal, 40, a housewife from Kalinagar village, told this correspondent,

“Every day I walk 6 kilometres to fetch one pitcher of drinking water from nearby village. On days it is not possible for me to do so, my children are forced to drink the muddy and salty water from the almost dried up pond nearby. They are all suffering from dysentery now.”

Jhumpa Roy, a student of class five at Kamarkhola Government Primary School ,said she gets to drink a glass of water during the six hours (8.00 am to 2.00 pm) at school. Jhumpa, Shavan, Rupa, Mohon and many other children of the school say that that’s the best glass of water they get to drink in a day.

Shavan said, “Often students get into a fight when the water in the containers dip to the bottom levels as children think they will not get their share that day.”

Asim Roy, headmaster of the school, said, “Most of the students suffer from dehydration due to the small amount of water they get to drink.”

“I have applied several times for funds to build a concrete water reservoir to the Thana Education Officer (TEO) but didn’t get any response,” he said.

Dakop Upazila Nirbahi Officer Mohsin Ali however said that his office is about to launch a project soon to set up water reservoirs in each school to partly overcome the drinking water crisis. He added that since the government has limitations to respond to such schemes quickly enough, private organisations should come forward to help.

Shamaresh Roy, Chairman of Kamarkhola Union said, “We hope to dig a fresh pond but the present crisis would continue until the monsoons if the government doesn’t come forward with alternatives.”

World Vision, a non-government organisation working in the area, has dug two ponds while and excavated 20 others to help solve the crisis, but that doesn’t entirely solve the crisis faced by the majority of the people in the area.

Increased salinity changes vegetation pattern

Pinaki Roy, back from Dakop, Khulna, The Daily Star, July 14, 2008

Geowa plants are abundant on the banks of Khalsi canal. Most of these species are less than a decade old; many others are growing for only couple of years. Not only geowa, some golpata (nypah) and other saline-tolerant plants are visible here and there along the canal.

The canal is named after Khalsi village in Dakop upazila in Khulna where the residents are mostly Hindus.

During a stroll around the village it was seen geowa is the main tree there. Locals say vegetation pattern of Khalsi has entirely changed in just 20 years. Just two decades ago, like other typical Hindu majority villages, Khalsi was full of Tulsi, joba, sheuli and coconut, banana, mango, wood apple and other fruit trees essential for worshipping the Hindu gods and goddesses.

But now concentration of salt has become so high in the land that only saline tolerant plants can survive it.

“We had 45 coconut trees. But those started dying slowly since saline water entered the village. Now we have some other fruit trees with no produce,” said housewife Mira Ray.

During the 80s when shrimp farming was becoming popular, influential people forced small farmers to lease out their land for the venture. A number of the farmers started shrimp farming at will, but many others were forced into the business flooding their cropland with saline water.

Initially, shrimp farming yielded a good profit. But soon it became a losing concern due to virus infection and the farmers realised that producing crops was a better option.

“I remember just immediate before first shrimp farming we got 1,000 mounds of rice from 60 bighas of land. Now our land has increased to 94 bighas and produce comes down to only 100 mounds,” said Dr Achintya Bhowmik, principal of a local graduation college.

Krishna Pada Mandal echoed the same. He cultivated rice in 10 bighas and harvested only 11 mounds rice whereas a farmer from saline-free land got over 100 mounds from the same quantity of land.

Now villagers want to get rid of saline water and saline water-based shrimp farming like those of Bazua who have successfully rid their cropland of saline water and are making huge money cultivating rice, watermelon and pumpkin.

But influential shrimp farmers want to continue the business anyhow. The villagers allege an influential businessman from Dhaka is producing shrimp taking lease of their land at Tk 1,000 yearly for per bigha.

The villagers say the businessman cultivates shrimp on only 40 acres of land through irrigating saline water from the Kazibachha river. But about 700 acres of land around is getting increased amount of salinity.

The villagers say local civil and police administration is working for the influential shrimp cultivators, who are also leaders of major political parties. These leaders are filing cases against marginal farmers and police are taking bribe from them, they claim.

A farmer who is protesting against the salinity aggression said if someone wants to cultivate rice in an acre of land, they have to spend Tk 21,000 for power-tiller, Tk 2,400 for seeds, Tk 3,000 for labours and Tk 1,000 for irrigation. 

In the end, they will possibly get around 45 mounds of rice from single-crop land which is worth no less than Tk 40,000 at current market price. And in this area farmers can grow three crops annually. 

But now farmers who have one acre of land are hardly getting only Tk 5,000-6,000 a year.

SUCCESS OF BAZUA VILLAGE 
Locals of Bazua union, just three kilometres down from Khalsi have successfully got rid of saline water and are cultivating crops there. Vegetation pattern is different there as geowa and other saline water varieties are hardly visible. 

The farmers in Bazua said they get around Tk 75,000 by selling watermelons in one acre of land in addition to Tk 25,000 from over 30 mounds of rice.

Parimal Mandal from Bazua said they are doing fine cultivating sweet water varieties. 

Gouranga Prasad Roy, convener of salt-water prevention committee in Bazua, said the small and landless farmers protested against the shrimp farming.

“Commoners and landless people don’t want shrimp farming as it only makes the rich people richer. But by cultivating watermelon and rice the marginal farmers are making some money themselves.”