Ithaka Journal worked with home gardeners from across Switzerland to conduct a 2 year home garden test plot study. In the study, home gardeners aged the biochar in their own compost, and applied the biochar on a 1 Meter square test plot, then compared the results with an identically planted 1 Meter square control.
The results were mixed, and there were a lot of reasons given by the authors for this. The biggest variable that they isolated in the study was the compost. Apparently the quality of the compost varied quite a bit, and could easily have accounted for a lot of the mixed results.
Additionally, the biochar amended compost was mixed into the top few inches of the planting beds. Some crops are fairly shallow rooted, and that get good benefits from this type of amendment, but most plants will do better if the biochar is blended with good compost and soil and applied to the whole root zone. We've seen better results when the planting holes are amended.
the Methodology in full: