Ithaka Institut

Biochar Gardening

Over the span of two years more than 200 hobby gardeners took part in a biochar trial coordinated by the Ithaka Institute. Different types of vegetables were planted on two 10 m² garden plots, one with compost only and the other with compost + biochar. The analysis of the results showed very interesting differences, once again underscoring the importance of specifically applying biochar.

In early 2010, the Ithaka Institute launched a project in which hobby gardeners were asked to carry out tests with biochar in their gardens. Participants were each provided with 10 kg of biochar taken from the same batch made from green cuttings by the Swiss Biochar company. In addition the gardeners were given detailed instructions on how to apply the biochar as well as a standard questionnaire for recording their findings. By the end of 2011, 65 individual tests had already been evaluated. On the basis of the experience gained in the first year of tests the instructions and questionnaires were revised in early 2011 and a further 150 test packages sent out. A number of the previous year’s participants repeated the test, using the revised instructions and the same plot of land. By the end of 2011 the Ithaka Institute had received 144 questionnaires from participants. The test findings were evaluated individually and then summarized in a meta-analysis.

The evaluation of all 144 tests showed a wide range of results, even after removing the extremes. In some cases yields were more than twice as high, while at the same time there were cases where there was a significant negative impact on yields. This showed that biochar had a major influence on the interaction between plants and the soil. The better these interactions are understood, the more specifically biochar can be applied. The average for all tests showed a 7.5% improvement in yields.

The most important outcome of the whole test can be seen when looking at the results per plant family (see Fig.). Whereas brassicaceae (cabbage plants), cucurbitaceae (cucurbits) and apiaceae (umbelliferae) did particularly well, the opposite was true for solanaceae (nightshade plants). No trend in either direction was noticeable for legumes or asteraceae (lettuce).


The very positive tendency seen in the effect of biochar on brassicaceae, cucurbitaceae and apiaceae confirms the findings of the first test year. The fact that the different effect of biochar applies not just to various types of plants but also apparently to whole families, is quite understandable, as within a plant family there is a tendency for similar specialisations to be found with regard to certain environmental conditions and metabolism commonalities. 

The complete results of the biochar garden trial can be found in the Ithaka Journal: Biochar Garden Trial - Results 2011