What is the best input mixture to produce cohesive, strong bricks when using Open Source Ecology's compressed earth brick press? I've heard that lime and sand should be added to help brick cohesiveness. What proportions? Are there other additives?
They've included the best mixture in their earth brick press wiki:
Bricks are made of 20%-30% clay, the rest being sand and silt. A combination of mechanical and electrostatic forces is responsible for the high strength of compressed earth blocks.
For stabilized bricks, chemical binding forces of the binder - such as cement - are responsible for the strength.
You'll have to learn or estimate the composition of your soil, as Chris writes and add the missing component to get the right balance.
As with most enthusiastic activities, you'll have to use "trial and error" to get the best results. And both OSE and SE communities will be happy to hear back from you about your progress.