Sulfur-Coated Urea Helps Wheat Fight Salt Stress and Use Nitrogen Better

Salt is quietly strangling one of the world’s most important crops. As irrigation and climate pressures push salts ever deeper into farmland soils, wheat farmers face a double bind: the very nitrogen fertilizer they rely on to sustain yields can make salt-stressed plants suffer even more. A new study published in BMC Plant Biology offers a surprisingly elegant way out of that trap. Researchers led by Muhammad Talha Aslam and Imran Khan at the University of Agriculture Faisalabad, working with collaborators across Pakistan, China, Saudi Arabia, Germany, Turkey, and Slovakia, tested whether specially coated urea fertilizers could help wheat withstand salinity, and found that a polymer-sulfur coating outperformed every alternative they tried.

The team set up a controlled pot experiment with three salinity levels: a non-saline control at 1.3 dS per meter, which reflects the natural background salinity of the soil they used, a moderate stress level of 6 dS per meter, and a severe stress level of 12 dS per meter. Across all three conditions, wheat plants received the same total amount of nitrogen, 287.50 milligrams per pot, delivered in three equal splits at sowing and at 25 and 50 days after sowing. The variable was the form of the urea itself: ordinary uncoated urea, polymer-coated urea, polymer-zinc-coated urea, and polymer-sulfur-coated urea, abbreviated PSCU. This design allowed the researchers to isolate exactly how the coating chemistry, rather than the nitrogen dose, shaped the plant’s response to salt.

The scale of the damage from salt alone was stark. Under high salinity with conventional urea, grain yield fell by roughly 35 percent and total crop biomass by about 23 percent, while shoot dry weight dropped by more than half. Behind those headline numbers lies a cascade of physiological failure. Salinity drove sodium ions into the plants, disrupted the uptake of potassium and other essential nutrients, and triggered electrolyte leakage as cell membranes lost their integrity. Photosynthetic pigments degraded, relative water content in the leaves declined, and the plants’ capacity to keep growing under what should have been adequate nutrition simply collapsed.

At the cellular level, the researchers traced much of this damage to reactive oxygen species. When salt stress disrupts photosynthesis and metabolism, plants accumulate these highly reactive molecules, which attack membranes, proteins, and DNA. One of the clearest signatures of severe stress in the experiment was the combination of elevated reactive oxygen species with reduced leaf water status and chlorophyll loss. Any fertilizer strategy that hopes to protect yields in salty soils must therefore do more than supply nitrogen; it must help the plant contain oxidative damage and maintain its internal water and ion balance.

All three coated urea formulations outperformed plain urea, but the differences among them were decisive. Polymer-sulfur-coated urea consistently delivered the strongest protection. Plants receiving PSCU showed reduced oxidative damage, better preserved chlorophyll, and the highest relative water content in their leaves. The coating also improved the uptake of nitrogen, phosphorus, and potassium, restoring a more favorable nutrient balance that salt stress had otherwise disrupted. In practical terms, the sulfur and polymer layers appear to slow the release of nitrogen and, in the case of sulfur, add a nutrient that plants can use to build amino acids and defensive compounds, giving stressed wheat a steadier and more useful supply of nutrition.

One of the most striking findings involved secondary metabolites, the specialized compounds plants synthesize to defend themselves. Under severe salt stress, plants fertilized with PSCU showed several-fold increases in flavonoids, total phenols, and tocopherol compared with plants on conventional urea. These molecules are not incidental byproducts; flavonoids and phenolics act as antioxidants that neutralize reactive oxygen species, while tocopherol protects lipid membranes from peroxidation. The coated fertilizer essentially armed the plants’ own chemical defenses, allowing them to mount a stronger antioxidant response precisely when salt stress demanded it most.

The benefits extended to how efficiently the crop used its nitrogen, a metric of enormous economic and environmental importance. The researchers evaluated multiple indices of nitrogen use efficiency, including partial factor productivity, nitrogen balance intensity, and uptake efficiency. PSCU maximized these indices across the salinity treatments, meaning that more of every unit of applied nitrogen ended up in the harvested plant material rather than lost or left stranded in the soil. Because nitrogen fertilizers are both a major farm expense and a source of nitrous oxide emissions and water pollution when used inefficiently, a coating that simultaneously boosts efficiency and stress tolerance addresses two problems at once.

The mechanism behind these gains likely involves the interplay between sulfur nutrition and controlled nitrogen release. Sulfur is a constituent of cysteine and glutathione, two cornerstones of the plant antioxidant system, and adequate sulfur supply is known to support the synthesis of stress-protective compounds. Meanwhile, the polymer layer moderates the dissolution of urea, reducing the sharp ammonia and nitrate pulses that uncoated urea produces and that can exacerbate ion toxicity in saline root zones. By smoothing nutrient availability over the growth period, the coating gives salt-stressed roots a more stable environment in which to function, and the plants respond with better membrane stability, water retention, and photosynthetic capacity.

The researchers are careful about the limits of their results. Even with PSCU, wheat under high salinity could not fully match the performance of plants grown in non-saline soil; the coating alleviated salt damage but did not erase it. That honesty matters, because no fertilizer formulation can substitute for drainage, leaching, salt-tolerant varieties, or sound irrigation management in severely affected fields. What the study does demonstrate is that, within the range of salinity tested, coating choice is not a minor detail but a first-order determinant of how much yield a farmer loses to salt. PSCU consistently outperformed both plain urea and the other coated formulations, suggesting that the specific chemistry of the coating, not just the act of coating, drives the benefit.

The implications reach well beyond one pot experiment. Much of the world’s irrigated wheat belt, from South Asia to the Middle East and North Africa, sits on soils with rising salt loads, and the region’s food security depends on squeezing more from every stressed hectare. Controlled-release fertilizers are already a growing market, and this study adds a compelling agronomic argument for formulations that pair polymer barriers with sulfur. Field-scale trials will be needed to confirm that the yield and efficiency gains hold under real farming conditions, variable soils, and different wheat varieties. But if they do, the message to farmers and fertilizer manufacturers alike is clear: when salt is in the soil, what wraps your urea may matter almost as much as the nitrogen inside it.

Subject of Research: Effects of sulfur and polymer coated urea fertilizers on salinity tolerance and nitrogen use efficiency in wheat

Article Title: Sulfur and polymer coating on urea enhances antioxidant defense, nutrient balance, secondary metabolites, and nitrogen use efficiency in salt‑stressed wheat

Article References: Aslam, M. T., Khan, I., Chattha, M. U., Ghafoor, S., Ismail, A. M., Alsunbul, M., Ur-Rahman, M. H., El Sabagh, A., Al-Ashkar, I., & Brestic, M. (2026). Sulfur and polymer coating on urea enhances antioxidant defense, nutrient balance, secondary metabolites, and nitrogen use efficiency in salt‑stressed wheat. BMC Plant Biology. https://doi.org/10.1186/s12870-026-09928-2

Image Credits: AI Generated

DOI: 10.1186/s12870-026-09928-2

Keywords: wheat, soil salinity, coated urea, nitrogen use efficiency, antioxidant defense, secondary metabolites, reactive oxygen species, nutrient uptake, controlled-release fertilizer, plant physiology, crop yield, sulfur nutrition