I’ve been studying biochar for a year now (see my biochar notes), and just this week the first major book on the subject arrived, Biochar for Environmental Management.  It’s academic, and dense with science, but totally fascinating.  Imagine my surprise in chapter 5 when i encountered a reference to this paper:

Hoshi, T. (2001) ‘Growth Promotion of Tea Trees by Putting Bamboo Charcoal in Soil’, in Proceedings of 2001 International Conference on O-cha (Tea) Culture and Science, Tokyo, Japan, pp 147-150

It even turns out to be online!  See the paper in English as pdf, and the English website which has a more detailed version (in Japanese) with pictures, which shows all the stages from harvesting the bamboo and making the chacoal, to spreading the char in the field and measuring the bushes.

A quick summary: A 10-year test begun 1998, which in year 3 the tea was already showing 20%/40% greater height/volume, using only a rather small amount of char (100g/m2 a year, or 500g/m2 once).  The composition of the harvested tea was the same, so the main benefit is reduced need for fertilizer.

For those of us trying to grow tea organically/sustainably, in poor soils like our Hawaiian clay, this is huge.  They didn’t test with large amounts of char (1-3 kg/m2 is typical of tests on other crops) so more might be even better, and it should be fully complementary to all the other organic approaches (EM, IMO, compost tea, etc.) so there is a lot of exciting things to try!