Tea powered iron nanoparticles help biochar fertilizers feed crops more slowly and sustainably

A new study published in Biochar presents a greener way to make slow-release fertilizers that could help farmers grow healthier crops while reducing nutrient loss to the environment.

Researchers developed a biochar-zeolite slow-release fertilizer coated with a biodegradable CMC/PVA film reinforced by iron nanoparticles made using tea extract. The resulting material, named CMC/PVA/0.5Fe-SRF, was designed to release nutrients more gradually, retain soil moisture, and improve fertilizer efficiency.

“Conventional fertilizers often release nutrients faster than plants can absorb them, which leads to waste, water pollution, and greenhouse gas emissions,” said corresponding author Chengfang Song. “Our goal was to design a fertilizer that works more like a timed delivery system, supplying nutrients steadily while using environmentally friendly materials.”

The team used green tea extract as a natural reducing agent to synthesize tea extract iron nanoparticles, or T-FeNPs. These nanoparticles were then incorporated into a coating made from carboxymethyl cellulose and polyvinyl alcohol, two film-forming materials known for their biodegradability and compatibility with agricultural applications. The coated fertilizer core contained zeolite, nitrogen-phosphorus-potassium fertilizer, and rice straw biochar, combining nutrient supply with porous materials that can help retain ions and water.

In soil leaching tests, the best performing formulation, CMC/PVA/0.5Fe-SRF, reduced cumulative nitrogen release to 58.47 percent and phosphorus release to only 15.82 percent over 30 days. This was substantially lower than conventional NPK fertilizer and also better than coated fertilizers without iron nanoparticles. The researchers found that the tea derived iron nanoparticles helped fill pores in the coating, making the membrane denser and more hydrophobic. This created a more difficult pathway for water to enter and for dissolved nutrients to escape.

“The iron nanoparticles act like tiny reinforcements inside the coating,” said corresponding author Bing Yu. “They strengthen the barrier, slow down water penetration, and provide active sites that can bind phosphate. This gives the fertilizer a more controlled nutrient release profile.”

The fertilizer also showed promising results in tomato cultivation. Plants treated with CMC/PVA/0.5Fe-SRF reached the greatest height, produced the highest fresh and dry biomass, and maintained strong root growth. Compared with conventional NPK fertilizer, the optimized coated fertilizer increased fresh biomass from 17.6 g to 20.77 g and dry biomass from 2.03 g to 2.88 g. The researchers suggest that sustained nutrient supply, improved soil water retention, and the micronutrient role of iron all contributed to the growth benefits.

Post-harvest soil analysis further showed that the iron reinforced slow-release fertilizer improved soil nutrient retention. The treatment increased total nitrogen, phosphorus, potassium, cation exchange capacity, and organic matter related indicators, while maintaining stable soil pH. These results suggest that the material may support both crop productivity and soil health.

The study also evaluated economic and environmental potential. The estimated production cost of the optimized fertilizer was US$562.02 per ton, and the authors calculated that improved nitrogen use efficiency could help reduce fertilizer related greenhouse gas emissions. In East Asia alone, the potential reduction was estimated at 35.69 million tons of CO₂ equivalent under a fertilizer substitution scenario.

Because the coating uses tea extract, biodegradable polymers, rice straw biochar, and zeolite, the approach aligns with circular agriculture and green chemistry principles. The authors note that further field trials will be needed to validate performance under diverse soils, crops, climates, and farm management systems.

“Our findings show that sustainable fertilizer design can combine plant based chemistry, nanotechnology, and biochar engineering,” Song said. “This strategy offers a practical pathway toward fertilizers that are more efficient for farmers and less burdensome for the environment.”

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Journal Reference: Wu, M., Ruan, Z., Wu, Y. et al. Green-synthesized iron nanoparticles enhance CMC/PVA coatings for biochar‑zeolite slow‑release fertilizers. Biochar 8, 80 (2026).   

https://doi.org/10.1007/s42773-026-00592-1  

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About Biochar

Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field. 

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