Instructables

Step 6: Soil Block mix

3 buckets brown peat (standard peat moss, use a premium grade)
1/2 cup lime. Mix ingredients together thoroughly.
2 buckets coarse sand or perlite
3 cups base fertilizer (equal part mix blood meal, colloidal phosphate, and greensand). Mix.
1 bucket garden soil
2 buckets well-decomposed compost. Mix ingredients together thoroughly.
* From The New Organic Grower, by Eliot Coleman

Or you can use any good organic potting soil. I found a bag of Master Nursury Gardener's Gold Organic potting mix and it works great. I did make my own blend as above too, but it was expensive and time consuming trying to locate greensand, colloidal phosphate (gave up - could not find), etc.

Add 1 part water to 3 parts soil mix. Mix to a soft putty or wet cement (not soupy). A soil block needs to have the soil packed tight. Rinse out blocker in bucket of water after each block is made. This will help release each new block made.

Remember that soil blocks can dry out faster than soil in flats or pots. Keep an eye on them. Good luck!