Ithaka Institut

Composting with biochar

Terra Preta like soil substrates

The Ithaka Institute runs it's own compost facility in St. Leonard where we have undertaken since 2010 numerous trials to develop biologically active soil substrates with biochar. We have investigated the effects of biochar on the composting process, tested feedstock blends and composting techniques, measured greenhouse gas emissions and nutrient efficiency and examined biochar alteration through the composting process.

In Terra Preta, charcoal particles have been in contact with decomposing organic substances, dissolved organic carbon, nitrogen and other nutrients and their (microbial) turnover, in addition to living plant roots, their exudates, symbionts and degraders. These turnover processes involving organics and minerals seem to be a prerequisite for beneficial biochar effects. Therefore, the combined use of biochar and compost, where such interactions are accelerated and more intensive compared to soil, was a logical step for us and at least worth to investigate closer. 

To use biochar as an amendment during the composting process quickly proved to be a successful strategy to achieve high quality soil substrates for agricultural or horticultural use. However, simply adding biochar to compost does not guarantee that badly made composts will turn into a miraculous fertile substrate!

 Have a look on how we do it in the following video.


These main principles of quality composting have to be observed:

  • Start with a balanced feedstock C/N of around 35
  • Add sufficient amounts of clay (~ 5% vol) and rock powder (>1 % weight) to promote aggregate formation
  • Avoid anaerobic conditions for more than 1 - 3 days and minimize thus putrescence and prevent detrimental shifts in the decomposing and humifying microbial community.
  • To achieve this, compost windrows should not have diameters of more than 3 m (see Fig. 7.1)
  • Windrows should be turned at least every three days during the heat rotting phase for oxygenation (Fig. 7.1) and later once a week, totalling to 7 to 10 windrow turnings.
  • Low-bulk density materials such as lignin rich green clippings, saw dust or biochar should be added for aeration and liquid absorption (‘bulking agents’)
  • Water content should be controlled and remain at about 40% (dwt)

Our standard feedstock blend contains cow, chicken and horse manure, green clippings, pomace, soil with high argil content, rock powder and varying proportions of biochar from 5 to 80%.

Further information on how to produce Terra Preta like substrates can be found in the Ithaka Journal: Ways of Making Terra Preta


Trial 2011 (collaboration with Claudia Kammann, University Gießen)

Three treatments of 6,7 - 8,2 m3 with three replications: composting without biochar, composting with 20% biochar, composting with 20% biochar preceded by lactic fermentation. Gas samples for GC-analysis of CO2, CH4 und N2O emissions were taken every other day. Compost samples were analysed every second day for NH4, NO3, NO2 and Ntot. Temperature, CO2, CH4, H2S and humidity were measured daily.

The final substrates were used for field trials in vineyards (Ithaka), in vegetable gardens (Ithaka), cereals (Ithaka), melon and orchards (INRA-France) (Mythopia). At the Universities of Giessen and Geissenheim, at the Colleges Bingen and Geneva and at the FIBL Institute various pod trials were set-up with these biochar-compost substrates. Investigations of the composted biochars are currently underway at Universities in Zurich, Giessen, Sidney, Palermo and Seville.

Results are in preparation for several publications. The biochar-compost trial is the subject of  a chapter in the up-coming book on European Biochar Research. The best biochar-composts of the 2011 trial gave birth to the Swiss Terra Preta substrate now produced by Swiss Biochar and sold in Switzerland and Germany with increasing succes.


Trials 2012-2013

We produced biochar-composts with increasing biochar content from 10%, 20%, 30%, 40% to 50% with the biochar addition being done at the beginning of the composting process. We measured temperature, CO2 and took substrate samples but did not take gas measurements. 

The resulting substrates were part of the Smart-City-Project. The substrates were tested in vertical bio-beds for bio-filtration of street effluents and heavy metal decontamination. The substrates were further used in field trials in winegrowing and orchards. Some of these biochar-composts were part of the window garden project with > 250 participants in Switzerland, Germany and Austria.  


Trials 2014

We are excited to start additional new trials investigating the following areas beginning in May 2014:

  1. influence of the type and proportion of clay amendments in the original feedstock blends
  2. frequency of windrow turning during the heating phase
  3. influence of biochar on the redox dynamic within the windrow
  4. measuring of GHG emissions and N-cycle related microbia

The 2014 trials will take place in collaboration with the College of Geneva and the University of Tubingen.