Coastal Bangladesh Soils Reveal Stark Nitrogen Crisis in New Nutrient Maps

In the tidal floodplains of southwestern Bangladesh, where rice paddies meet the brackish fingers of the Bay of Bengal, the health of the soil determines whether families eat. A new study of the Khulna region has now produced some of the most detailed pictures yet of what lies beneath those fields, and the findings are strikingly uneven. Nitrogen, the nutrient that crops crave more than any other, is so depleted that every single soil sample tested fell into the very lowest category. Meanwhile, potassium, sulfur, magnesium and calcium are so abundant that adding more of them would be not just wasteful but potentially harmful. The research, published in Discover Soil, combines laboratory soil chemistry with geographic information system mapping to give farmers and policymakers a nutrient atlas of one of the country’s most agriculturally stressed coastal zones.

The study was led by Sabiya Akhter of Khulna University, working with Md. Akramul Islam and Shaikh Magfur Rahman. In December 2021, the team collected 22 soil samples from the top 15 centimeters of soil across Gangarampur Union in Batiaghata Upazila, Khulna district. Sampling points were laid out on a systematic grid roughly one kilometer apart to ensure consistent geographic coverage, and at each point five subsamples were gathered at random and blended into a single composite sample. Portable GPS devices recorded the exact coordinates of every location, allowing the chemical results to be tied precisely to the landscape. The samples were air-dried in the shade, gently crushed, sieved to two millimeters to remove stones and root fragments, and shipped to the Soil, Water and Environment Discipline laboratory at Khulna University for analysis.

The analytical suite covered the macronutrients that govern crop growth: total nitrogen, available phosphorus, available sulfur, available potassium, available calcium and available magnesium, along with soil organic matter. Total nitrogen was measured colorimetrically, a choice the researchers justify because available nitrogen forms fluctuate wildly with environmental conditions at the moment of sampling, whereas total nitrogen represents the stable reserve of the soil and is widely regarded as a reliable fertility indicator. Available phosphorus was extracted with sodium bicarbonate solution using the Olsen method and quantified with the molybdophosphoric blue color technique. Sulfur was assessed by turbidimetry, with barium chloride generating the turbidity that a spectrophotometer reads at 420 nanometers. Potassium, calcium and magnesium were extracted with neutral ammonium acetate, and organic carbon was determined by the classic Walkley and Black wet oxidation method, converted to organic matter using the conventional Van Bemmelen factor of 1.724.

The numbers that emerged reveal a soil system under acute strain. Total nitrogen ranged from a mere 0.001 percent to 0.016 percent, with a mean of 0.0103 percent, and every sample landed in the very low category. Organic matter fared somewhat better, spanning 0.38 to 4.19 percent with a mean of 2.47 percent, but roughly half the samples still fell into the low class. Available phosphorus was the most erratic nutrient of all, ranging from 2.17 to 113.39 parts per million with a coefficient of variation of 136.7 percent, the highest of any measured parameter, and about half the samples were classified as low. Sulfur, by contrast, was remarkably consistent and overwhelmingly abundant, with 77.27 percent of samples in the very high category and a modest coefficient of variation of 34.28 percent. Potassium and magnesium were rated very high in every sample, while calcium mostly sat in the optimum range.

To turn these point measurements into a landscape-scale picture, the team used inverse distance weighted interpolation within ArcGIS 10 software. This technique estimates values at unsampled locations by weighting nearby measurements more heavily than distant ones, producing smooth nutrient distribution maps across the union. The researchers deliberately chose this method over more sophisticated geostatistical approaches such as kriging because, with only 22 sampling points, the reliability of variogram modeling degrades and simpler interpolation can actually outperform it. The resulting maps show nitrogen uniformly in the very low class across the entire area, phosphorus concentrated in the very low and low classes, sulfur exceeding 45 parts per million throughout, and potassium above 0.375 milliequivalents per 100 grams everywhere. Magnesium mirrored potassium, while organic matter clustered in the low to medium range.

The nutrient index, a weighted scoring system that condenses the frequency distribution of samples into a single fertility rating, confirmed the pattern. Nitrogen scored 1.0, the lowest possible value, and phosphorus scored 1.95, placing it in the medium band. Sulfur reached 5.31, potassium and magnesium both hit the maximum of 6.0, calcium scored 3.31, and organic matter registered 2.4. In practical terms, the soils of Gangarampur are starving for nitrogen and phosphorus while drowning in sulfur, potassium and magnesium. This imbalance matters because applying fertilizer blindly, without knowing which nutrients are already plentiful, wastes money, pollutes waterways and can even suppress yields through nutrient antagonism.

The reasons behind this skewed profile are rooted in the region’s geography and history. The study area sits within the high Ganges alluvial floodplain and the Ganges tidal floodplain, where periodic flooding with tidal water delivers seawater-derived sulfur, potassium and magnesium directly to the fields. Coastal soils formed from alluvial and deltaic deposits also contain potassium-rich minerals such as feldspar, mica and illite, which weather slowly and release potassium into the soil solution. Nitrogen, however, has no such geological subsidy. The authors attribute its collapse to excessive continuous cultivation, rapid decomposition of organic matter in the hot humid climate, minimal application of manure or compost, and gaseous losses of ammonium nitrogen to the atmosphere. With mean annual rainfall of 1,693 millimeters, leaching and denitrification strip what little mineral nitrogen the soil can hold.

Phosphorus deficiency has its own chemistry. The alkaline, calcareous coastal soils tend to lock phosphorus into insoluble forms that plant roots cannot access, a process known as phosphorus fixation. Salinity intrusion from the sea, an escalating problem in the Khulna region as sea levels rise and upstream freshwater flows decline, further suppresses phosphorus availability. Previous work in coastal Bangladesh, including surveys dating back to the 1980s, has consistently documented nitrogen deficits in saline zones, and the new maps confirm that decades of high-yielding variety rice cultivation, sustained by heavy applications of urea, triple superphosphate and muriate of potash, have failed to rebuild the underlying nutrient capital. Organic matter, the engine of soil fertility, remains low across much of the area, a decline the researchers link to erosion, altered land use including shrimp farming, and the removal of crop residues.

The correlations among nutrients add another layer of insight. The analysis found a positive relationship between nitrogen and calcium, but negative correlations between nitrogen and potassium, and between sulfur and magnesium. Such antagonistic patterns suggest that the abundant cations supplied by tidal flooding may interfere with nitrogen uptake or retention, complicating any simple fertilization recipe. The authors argue that the region’s excessive precipitation, driving erosion and leaching, combined with the sandy fraction inherited from the parent material, underlies the generally poor fertility rating for nitrogen and phosphorus. Their central recommendation is unambiguous: nitrogen nutrition should be the top priority, with careful attention to phosphorus, before any crop planning in the region.

The study is candid about its limits. Twenty-two samples taken from a shallow 15-centimeter depth cannot capture every nuance of spatial variability across a landscape of nearly 38 square kilometers, and the findings should be generalized with caution. The authors call for long-term monitoring to track how soil fertility changes over time, the integration of remote sensing and geostatistics to sharpen future maps, and the coupling of soil data with crop performance data to enable truly targeted nutrient management. Even so, the work delivers a practical tool for farmers, extension services and policymakers in a region where agriculture accounts for the majority of livelihoods and cropping intensity runs at roughly half the national average. By showing exactly where nitrogen must be rebuilt and where potassium and sulfur can be left alone, the nutrient atlas offers a blueprint for cutting fertilizer waste, reducing environmental degradation and steering the coastal fields of Khulna toward the sustainable, food-secure future that both the global sustainable development goals and local families demand.

Subject of Research: Spatial assessment of soil macronutrient status and fertility in coastal agricultural soils of Khulna, Bangladesh

Article Title: Spatial distribution of soil macronutrients and nutrient index for sustainable agriculture in the Khulna region of Bangladesh

Article References: Akhter, S., Islam, M. A., & Rahman, S. M. (2026). Spatial distribution of soil macronutrients and nutrient index for sustainable agriculture in the Khulna region of Bangladesh. Discover Soil, 3(1), Article 137. https://doi.org/10.1007/s44378-026-00296-9

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00296-9

Keywords: soil fertility, macronutrients, nitrogen, phosphorus, nutrient index, spatial distribution, GIS mapping, Bangladesh, coastal agriculture, sustainable agriculture, soil organic matter, Khulna