Here is an easy and quick method to make a large amount of biochar:
This uses the same concept as the japanese cone kiln for biochar production, but in a long format. The long dimension makes for a great savings in energy spent cutting wood if you are starting with long material. Even wood that is too long for the trench can usually be burned off instead of cut. Should be good for biochar production on ranches and homesteads with quantities of long material like poles, branches, bamboo, and and other long stuff, but could be adapted to smaller batches and smaller material by blocking part of it off. Pretty cheap and pretty versatile.
For more on Cone Kilns and similar stuff: http://backyardbiochar.net/
Turkeysong Experimental Homestead: http://turkeysong.wordpress.com/