New method purifies ash containing harmful substances for use as forest fertilizer

The Heleä project "Solutions for a green transition by increasing the beneficial use of ash and ash fertilization" examined more efficient and sustainable ways to utilize ash which are generated in bioenergy production.  

- The use of ash is an important way to promote forest growth, strengthen domestic fertilizer security and support climate and circular economy goals, says Senior Lecturer Rose Matilainen from the University of Jyväskylä.  

The purification process made the ash usable as fertilizer 

An intermediate report on the project, published earlier in August 2025, found that several ash samples analyzed exceeded the limit values of the concentrations of harmful substances. The highest levels found in arsenic and cadmium, and in individual cases also in lead and zinc.  

- In the project, we developed a dissolution method based on a natural organic acid that effectively removes arsenic and cadmium but keeps the nutrients in the ash. Most of the treated ash subsequently satisfied the nutrient requirements for forest ash fertilizers, explains Postdoctoral Researcher Emilia Virtanen from the University of Jyväskylä.  

With this method, the purified ash can be used as a forest ash fertilizer. The method allows valuable material to be recycled instead of going completely to waste.

Minimal environmental impact

The research also examined the solubility of nutrients in ash under laboratory conditions and in a field experiment. The results show that the nutrients in fly ash produced from wood combustion in power plants dissolve slowly in acidic conditions and in the natural environment. This supports the long-term fertilizing effect of the ash.  

- In the field experiments, the concentrations of wood and mixed ash remained almost unchanged during approximately 1.5 years of exposure. There was no evidence of nutrient migration to surrounding areas or surface waters, which indicates a low environmental impact, says Virtanen.  

The Heleä project is coordinated by Jamk University of Applied Sciences. The University of Jyväskylä and the Finnish Forest Centre is also participating in the project.

- The University of Jyväskylä is responsible for researching the quality of ash. We analyze the nutrients and valuable metals contained in ash using methods developed and optimized by the university, says Virtanen. 

The Heleä project is co-funded by the European Union and is part of the Just Transition Fund (JTF) funding program. The project is also funded by Alva Oy, Finsilva Oy, Keuruun Energia Oy, Loimua Oy, Puulaakson Energia Oy, the City of Saarijärvi, Yara Suomi, MHY Karstula-Kyyjärvi and MHY Keski-Suomi.