Biochar Compost

When we established Europe's first biochar field trial in 2007, we mixed charcoal with compost by intuition — there was no scientific protocol, just the logic of feeding soil the way farmers always did for the last 10'000 years. That intuition turned out to be right, and proving why it works became one of the institute's most consequential lines of research.

In 2007, when we laid out the mother of modern biochar trials in the Mythopia vineyard, there was no published method for preparing biochar for soil application. We purchase rather ill-defined charcoal - there was no certified industrial biochar yet - and mixed it with compost before application, for the simple reason that compost was what we usually used to built soil. Years later, as the scientific community began publishing pot trials using large quantities of pure biochar with disappointing or inconsistent results on plant growth, it became clear that this early intuition had landed on something fundamental.

What followed was a prolonged and at times philosophical divide in biochar science: those who tested pure biochar at high rates (at very small scale) and found limited effects, and those who combined biochar with nutrients who saw more consistent agronomic benefits. The latter combination is now absolute standard practice. But understanding why it works and, especially, what role is played by organic nutrient compounds, required systematic investigation.

The institute's first peer-reviewed publication (Schmidt et al. 2014) reported the results of the Mythopia vineyard trial, including the first documented co-composting of biochar - not simply mixing finished compost with biochar, but adding biochar at the start and composting everything together. This distinction matters: during co-composting, microbial activity deposits an organic coating on the biochar's inner and outer surfaces that transforms its chemical behaviour. Kammann, Glaser & Schmidt (2016) demonstrated that co-composted biochar captures and retains nitrate that would otherwise leach, providing a slow-release nutrient reservoir unlike standard NPK applications.

The mechanistic understanding came through the PhD research of Nikolas Hagemann, now director of Ithaka Germany, who conducted his doctoral work at the institute on biochar composting. Hagemann et al. (2017), published in Nature Communications, showed that the organic coating formed during co-composting fundamentally changes biochar's surface chemistry — creating functional groups that enhance cation exchange capacity, nutrient retention, and microbial colonisation. Further publications (Hagemann et al. 2017, 2018) detailed how the composition and maturity of the compost organic matter determine the coating's properties and, consequently, the biochar-compost's agronomic performance.

Based on this research, the institute designed the formulations for Swiss Terra Preta — a biochar-compost substrate that became one of the most successful biochar-based products on the market, used in home gardening, professional horticulture, and urban tree planting. The Stockholm model for urban tree substrates, now adopted by cities across Europe, is built on the same principle: biochar-compost as a structural and biological substrate for tree root zones in sealed urban soils.