Soil acidity rising in Bangladesh as crop yields fall, SRDI scientists warn

Soil scientists in Bangladesh warned on 5 December 2024, World Soil Day, that acidity was rising in the country's farmland and pulling crop production down. In a report published that day, the Bengali daily Ajker Patrika quoted specialists of the Soil Resource Development Institute (SRDI) as saying that farmers were not getting the yields they expected while their production costs were climbing, and that the country's food security could come under threat if soil acidity was not brought under control.

Md Nazmul Hasan, an acid soil management specialist, told Ajker Patrika that acidity was increasing in the soil of every region of the country except the Ganges basin. “The higher the acidity, the lower the crop output,” he said. “If soil acidity is high, wheat, maize, potato, vegetables, pulses and other Rabi crops will not give the expected yields.” Rabi, as Banglapedia describes it, is the agricultural season that begins in November, when the south-east monsoon ends, and runs to the end of March, with dry, sunny weather that is cool from December to February. Its major crops include wheat, maize, barley and boro rice; potato and sweet potato; mustard, sesame and groundnut; chickpea, lentil and grass pea; and winter vegetables such as cabbage, cauliflower, brinjal and tomato.

Dr Md Nurul Islam, principal scientific officer of SRDI and director of the institute's acid soil management programme, told the paper that if acidity was not brought under control at once, the amount of acid in farmland would keep increasing day by day. Farmers' spending on fertiliser would rise, he said, but they would still not get the yields they wanted. He named dolomite lime and organic matter as the main remedies, which are described later in this report.

The warning came from the government's own soil survey body. SRDI is the institute that samples, tests and maps the soils of Bangladesh, and its acid soil management programme tracks how the reaction of farm soils has changed between national surveys. The figures the scientists cited came from that programme.

World Soil Day is held every year on 5 December to draw attention to the importance of healthy soil and to argue for the sustainable management of soil resources, according to the Food and Agriculture Organization of the United Nations (FAO). An international day for soil was first recommended by the International Union of Soil Sciences in 2002. The FAO Conference endorsed the day in June 2013, and in December 2013 the UN General Assembly designated 5 December 2014 as the first official World Soil Day. The date was chosen because it was the official birthday of King Bhumibol Adulyadej of Thailand, who sanctioned the event.

The theme FAO and its Global Soil Partnership set for 2024 was “Caring for Soils: Measure, Monitor, Manage”, which, FAO said, underlined the importance of accurate soil data and information for understanding soils and for decisions on sustainable soil management for food security. The 2024 observance was the tenth global celebration, and the official event was hosted by the Kingdom of Thailand, the first held outside FAO headquarters. The theme fitted the Bangladeshi warning closely: the alarm over acidity rested on SRDI's measurements of soil samples taken across the country over more than two decades.

SRDI is an attached department of the Ministry of Agriculture. According to Banglapedia, the national encyclopaedia, it began in 1961 as the East Wing Directorate of the Soil Survey Project of Pakistan, set up with FAO and UNDP help to make a quick inventory of the soil and land resources of the country. Between 1963 and 1975 it completed a reconnaissance soil survey of the whole country except the Sundarbans and the reserve forests of the Chittagong Hill Tracts. The institute took its present name in 1982-83, and in 1993 the Analytical Services Division of the Bangladesh Agricultural Research Institute and its four regional laboratories were merged into it.

Its stated missions include making an inventory of the country's soil and land resources, classifying them by their potential, building a detailed database on soil, land, water and agro-climate, and identifying and managing problem soils. Banglapedia lists among its units regional offices and laboratories, fertiliser testing laboratories, district offices and two research centres, along with 12 mobile soil testing laboratories that test soil on farms and give farmers balanced fertiliser recommendations. It also prepares the land and soil resource utilisation guides used by extension workers at upazila level.

According to the information of SRDI's acid soil management programme quoted by Ajker Patrika, the share of the country's arable land that is strongly to very strongly acidic has grown with each survey. In 1998 it was 28.23 per cent. By 2010 it had risen to 41.23 per cent, and in 2020 it stood at 45.67 per cent.

SRDI's research put Bangladesh's total cropland at about 85.86 lakh hectares in 2020. Of that, the soil on about 2.78 lakh hectares was very strongly acidic, with a pH below 4.5, and on about 36.44 lakh hectares it was strongly acidic, with a pH between 4.5 and 5.5. That means, the report said, that the soil of nearly half of all cultivated land, 45.67 per cent, was strongly to very strongly acidic. The same hectare figures for 2020 had been set out more than a year earlier, on 26 October 2023, by Md Nurul Huda Al Mamun, a principal scientific officer at SRDI's regional laboratory in Faridpur, in an article in the Daily Observer.

Al Mamun's article added the institute's 2020 findings on two of the nutrients that acid soils lose first. The 2020 study, he wrote, showed that cultivated land with very low to low calcium covered about 2.10 million hectares, or 24.53 per cent of the total cultivated land of Bangladesh, and land with very low to low magnesium about 1.05 million hectares, or 12.31 per cent. He also wrote that the soils of the northern part of the country in particular were short of calcium, magnesium and other alkaline elements, which he named as one of the reasons acidity was rising.

Researchers measure acidity through soil pH, the strength of hydrogen ions in the soil, on a scale of 1 to 14. A low pH means many hydrogen ions and an acid soil; around 7 the soil is neutral, and above 7 it is alkaline. The classes SRDI uses, as Ajker Patrika and Banglapedia set them out, are:

• very strongly acidic: pH below 4.5
• strongly acidic: pH 4.5 to 5.5
• slightly (or moderately) acidic: pH 5.6 to 6.5
• neutral: pH 6.6 to 7.3
• slightly alkaline: pH 7.4 to 8.4

Crop growth and yield depend to a large extent on whether the soil is acidic or alkaline, the researchers said, and the ideal pH for crops is taken as 6.5. Writing in October 2023, the SRDI scientist explained that plants cannot take nutrients from the soil in the right amounts when its reaction is out of balance, even if those nutrients are present, so the first thing to check when a deficiency appears is whether the soil pH is right. Nutrients are most readily available to crops, he wrote, when the pH lies between 5.6 and 7.3, from mildly acidic to mildly alkaline, a range that also favours the soil micro-organisms that release nutrients.

Banglapedia's article on soil fertility describes the average pH of Bangladesh soils as lying on the acidic side of the scale, between 5.5 and 6.5. The Gangetic alluvium, especially the calcareous soils, has a pH above 7.5 and at times reaching 8.3, because it contains free carbonates. Soils of plateaus, raised lands and hills are usually acidic. Lowland rice is little affected by the original soil reaction, the encyclopaedia notes, because the pH of a flooded soil tends to settle between 6.5 and 7.5 on submergence. The crops the specialists named, wheat, maize, potato, vegetables and pulses, are grown in the dry Rabi season.

The causes listed by SRDI's programme were climate change, heavy use of urea fertiliser, the loss of calcium and other alkaline elements from the soil, growing many crops a year on the same land, and burning crop residues as fuel instead of letting them mix back into the field.

Banglapedia explains the chemistry behind the urea point. When urea is applied it first raises the soil pH slightly, but at a later stage its breakdown product, ammonium, is oxidised to nitrate, and hydrogen ions are formed in the process, making the soil more acidic. Acid soils also form naturally in humid regions such as Bangladesh, because rain washes calcium, magnesium and other basic elements down out of the topsoil, and hydrogen and aluminium are the two ions mainly responsible for soil acidity. Most river alluvium is near neutral to moderately alkaline when it is first laid down, the encyclopaedia says, but unless fresh silt keeps arriving, the topsoil of non-calcareous deposits usually becomes moderately acid within about 50 years, and calcareous topsoils lose their lime and may themselves turn acid.

In his 2023 article the SRDI scientist gave four routes by which acidity builds up: acid rain, whose water has a pH of around 5.7; the carbon dioxide released by roots, which forms carbonic acid in the soil; the uneven uptake of mineral ions by plants; and chemical fertilisers whose ammonium releases hydrogen ions as it turns into nitrate. Banglapedia records how much nitrogen fertiliser the country uses: urea sales of 2,121 thousand tonnes in 2000-01 and 2,657 thousand tonnes in 2010-11.

At a seminar on acid soil management held at the Department of Agricultural Extension's office in Rajshahi on 10 March 2024, speakers, as Jugantor reported, added the excessive use of chemical fertiliser and pesticide on cultivated land, rivers drying up and the falling level of underground water to the list of reasons the damage was growing.

In strongly acidic soil, the SRDI research said, phosphorus, calcium, magnesium and molybdenum run short while aluminium, iron and manganese build up to excess, so crop growth is held back and yields fall. FAO describes acid soils, those with a pH below 5.5 for most of the year, in the same terms: they are associated with toxicities, aluminium above all, and with deficiencies such as molybdenum, along with other conditions that restrict plants.

Phosphorus is a clear example. Banglapedia says its availability depends on pH, and puts the available phosphorus of Bangladesh soils between low and medium, with about 6 per cent of the land severely deficient, 50 per cent strongly deficient and 44 per cent moderately deficient. The strongly acidic Madhupur Tract soils, it notes, are mainly phosphate fixing, meaning they lock up phosphate, and are low in potassium, sulphur and calcium.

Acidity also affects the soil's structure, its balance of water and air, and the work of the micro-organisms that break down organic matter, Banglapedia says. For the farmer the result is the one Dr Nurul Islam described: more fertiliser bought for the same field, but a crop that still falls short of what the land should give.

Most soils in greater Rangpur and Dinajpur, the hill areas of Sylhet and Chattogram, and the Barind and Madhupur Tract regions were strongly to very strongly acidic, the report said, and farmland there was not giving the expected yields. Banglapedia's description of the fertility of each agro-ecological zone matches that map:

• Old Himalayan Piedmont Plain: strongly acidic in the topsoil and moderately acidic below, with rapid leaching of nitrogen, potassium, sulphur, calcium, magnesium and boron.
• High Barind Tract: acidic to strongly acidic, with low organic matter and low general fertility; the North Eastern Barind Tract is strongly acidic.
• Madhupur Tract: strongly acidic, low in organic matter, phosphate fixing and low in potassium, sulphur and calcium.
• Northern and Eastern Hills: acidic, with low organic matter and low general fertility.
• Akhaura Terrace: strongly acidic with low organic matter.
• Sylhet Basin: mainly acidic; the Northern and Eastern Piedmont Plain is slightly to strongly acidic.

The Barind Tract is the largest Pleistocene land unit of the Bengal basin, covering about 7,770 square kilometres across most of greater Dinajpur, Rangpur, Pabna, Rajshahi, Bogura, Joypurhat and Naogaon. Banglapedia describes it as drought-prone, with rainfall averaging about 1,971 mm, most of it in the monsoon. The Madhupur Tract, a terrace of 4,244 square kilometres stretching from south of Jamalpur to Fatullah in Narayanganj, has soils developed largely on Madhupur clays, which are nutrient poor, somewhat acidic and red or brown in colour.

The Ganges basin was the exception the specialists named. Banglapedia describes the High Ganges River Floodplain as slightly alkaline and the Active Ganges Floodplain as mildly alkaline, the calcareous Gangetic soils having a pH that can reach 8.3. Tea, which Banglapedia says is grown only in the Sylhet and Chittagong regions, is treated differently on these acid hill soils: the encyclopaedia notes that liming is needed for crops other than tea where the pH falls below 4.5, a condition most prominent in acid sulphate soils and hill soils.

The World Soil Day warning followed others from SRDI scientists in the months before it. In his Daily Observer article of 26 October 2023, Al Mamun called soil acidity one of the many problems in crop production, wrote that it was increasing because of the unplanned use of chemical fertilisers, and said steps were needed to reduce the excessive and high acidity of the soils of the northern, Barind, Madhupur and hill regions if the desired yields were to be obtained.

At the Rajshahi seminar of 10 March 2024, chaired by the chief scientific officer of SRDI's Rajshahi divisional office, A F M Manjurul Haque, speakers said acidity had risen on more than about 46 per cent of the country's land. Jugantor reported that scientific officers had carried out research on soils at several places in Rajshahi division and had succeeded in reducing acidity by applying dolomite lime, but that they stressed lime alone was not enough: fertiliser and pesticide had to be used by the rules, the use of organic manure and vermicompost increased, and marginal farmers made aware of the problem.

On 31 August 2024, the state news agency BSS reported from a two-day training workshop that SRDI's Rajshahi divisional office organised for sub-assistant agriculture officers. Soil health experts there said that managing soils with lime and organic matter had become essential and could raise crop yields by 25 per cent on average in the high Barind area, because most of its land had turned acidic and crop production was suffering badly. In his keynote, Dr Nurul Islam, a principal scientific officer of SRDI, said liming and organic fertilisation had been found effective in raising yields while protecting soil health in the high Barind.

A December 2022 report by the environmental news site Mongabay had already drawn attention to the wider decline of Bangladesh's soils, listing soil acidity, deforestation, nutrient leaching and too little manure among the causes. It cited SRDI as saying that good soil holds at least 2.5 per cent organic matter, while most of the country's soil holds less than 1.5 per cent.

Applying dolomite lime to the land can be a good way to bring soil acidity down, Dr Nurul Islam told Ajker Patrika. For this, he said, generally 4 to 8 kg of dolomite can be applied per decimal of land, though the dose may be higher or lower depending on the texture of the soil. He added that more emphasis must also be put on adding organic matter to the soil. FAO describes incorporating lime or dolomite into the cultivated topsoil as an effective way to improve acid soils, and notes that lime also supplies calcium and magnesium to soils short of them, the two nutrients that SRDI's 2020 study found very low to low on large areas of farmland. Liming raises the pH, FAO adds, which frees nitrogen-fixing bacteria to work and increases nitrogen fixation.

Al Mamun's 2023 article set out how SRDI advises the lime to be used. If the soil pH is below 5.5, he wrote, treating it with lime is essential, and the recommended dose of dolomite in strongly acidic soils is 4 kg per decimal, or 400 kg per acre, or 1,000 kg per hectare. After one application the land does not need dolomite again for three years. His method:

• Divide the total dose for the field into two equal halves.
• Spread one half along the field in a north-south direction and the other half east-west, so the lime is spread evenly.
• Mix it well into the soil by cross-ploughing and levelling with a ladder.
• Wait at least seven days before sowing or planting, because sowing seed straight after liming can lower germination and stunt the roots and stems of seedlings.
• If the land is dry, apply the lime to the fallow field, plough and level it, give a light irrigation, and then sow after at least seven days.

He also listed other measures for acid soils: keeping water standing in a paddy field brings acidity under control within about three weeks, applying fresh wood ash or husk ash reduces the effects of aluminium poisoning, and good-quality organic manure in adequate amounts is beneficial. Banglapedia notes that most Bangladesh soils respond better when organic matter is added along with chemical fertiliser, and gives the recommended rate of fresh or partly decomposed cow dung as 5 to 10 tonnes per hectare, with green manure crops such as Sesbania rostrata also encouraged. The organic matter status of Bangladesh soils, it says, is one of the lowest in the world.

FAO's guidance on acid soils points the same way: an integrated approach combining liming matched to each field, acid-tolerant crops, efficient fertiliser use, suitable rotations and crop diversification, with soil tests every two to three years to work out how much lime a field needs, and a high organic matter content to offset part of the harm acidity does. Because the right dose depends on a field's pH and texture, farmers who suspect acid soil can have a sample tested at an SRDI laboratory or one of its mobile soil testing laboratories, and should ask their upazila agriculture officer before buying lime.

Photo: Reaid Hossain / Wikimedia Commons (CC BY-SA 4.0)

Sources: Ajker Patrika (5 December 2024), Soil Resource Development Institute, Daily Observer (26 October 2023), Jugantor (10 March 2024), Bangladesh Sangbad Sangstha (31 August 2024), Mongabay (30 December 2022), Food and Agriculture Organization of the United Nations (World Soil Day; FAO Soils Portal), Banglapedia (Asiatic Society of Bangladesh)