Thanks everyone for your replies. Biochar or charcoal sounds like a promising idea.
Anyone know where I can purchase biochar? Perhaps though it will e more convenient and or cost effective to just buy charcoal and crush it up.
Brad
Go the charcoal method, just make sure its 100% natural wood charcoal made from mesquite or other woods. I looked into biochar the prices were very high. Go to a local store that sells grilling supplies and find out if they sell the 100% natural wood charcoal.
What I did was just crushed up the blocks with a hammer, I put a thick cloth/old towel over it to crush. Here is what it looked like after I was done with it.
Here is a picture of some of the chunks in the mix.