Beginner Gardening: Good Growing Practices - an Overview for Beginners, 1 by tapla

tapla wrote:
No need for more than a tablespoon, but as much as you wish. If the particles hold together after freezing, it should be ok to use. Again though, make sure the particle size is appropriate, and if you're using it to make the gritty mix, you screen the fines out.

More about the gritty mix if you follow the link provided. The important part of putting soils to work FOR you instead of against you is understanding how they work and what affects how much water they hold. That's all explained Here:

http://davesgarden.com/community/forums/t/1073399/

Al