The journal Environmental Surfaces and Interfaces recently featured a comprehensive review by Jesper T.N. Knijnenburg and Kaewta Jetsrisuparb on the thermal transformation of phosphorus during biochar production. Their research examines how the co-pyrolysis of biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More with various phosphate additives can be engineered to create advanced fertilizers with specific nutrient release profiles. By analyzing the interactions between phosphate salts, feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More composition, and heating conditions, the authors provide a framework for designing materials that improve the efficiency of phosphorus use in global agriculture. This approach is increasingly vital as traditional phosphate rock deposits face depletion and excessive fertilizer runoff continues to cause environmental issues like water eutrophication.

The study identifies that the type of cation used in the additive is the primary factor determining how quickly nutrients are released from the biochar. Additives containing monovalent cations, such as potassium or sodium, typically result in biochars with high water solubility, where the phosphorus is immediately available but also prone to leachingLeaching is the process where nutrients are dissolved and carried away from the soil by water. This can lead to nutrient depletion and environmental pollution. Biochar can help reduce leaching by improving nutrient retention in the soil.   More. In contrast, incorporating divalent cations like magnesium or calcium promotes the formation of poorly soluble species that release nutrients much more slowly. For example, increasing the calcium-to-phosphorus ratio to one-to-one in certain biochars was shown to reduce phosphorus release to only 2 percent after 240 hours. This stability ensures that nutrients remain in the soil longer, supporting multiple growth seasons rather than being washed away by rain.

Pyrolysis temperature and the chemical state of the phosphate additive also play crucial roles in defining the final material structure. The researchers found that higher temperatures and a higher degree of protonation in the starting materials favor the condensation of simple orthophosphates into more complex forms, such as pyrophosphates and polyphosphates. These condensed forms are often more resistant to immediate dissolution and can even help lock away heavy metals in the soil. While traditional water-soluble fertilizers provide a quick burst of nutrients, these engineered biochars offer a steady supply. In plant trials, while commercial fertilizers sometimes showed better results in the very first growth cycle, biochar-based versions resulted in higher cumulative yields over the long term due to reduced soil fixation.

The composition of the original biomass feedstock further influences the effectiveness of the modification process. Feedstocks naturally rich in calcium, such as poultry manure, can interact with added phosphoric acid to form crystalline calcium and magnesium phosphates without the need for additional mineral modifiers. The review highlights that the choice between wet and dry mixing also impacts the final product, with wet mixing providing better penetration of phosphorus into the internal pores of the biomass. Ultimately, the ability to tailor these release kinetics allows for the creation of fertilizers specifically suited for different soil types and crop needs. By transforming organic waste into stable, nutrient-enriched carbon, this technology offers a circular economy solution that enhances both soil health and environmental protection.

Source: Knijnenburg, J. T. N., & Jetsrisuparb, K. (2026). Co-pyrolysis of biomass with phosphate additives: linking phosphorus transformations to nutrient release for tailor-made biochar-based fertilizers. Environmental Surfaces and Interfaces, 1(1), ES155.