What are soil blocks? I first learned about soil blocks after watching The Real Dirt on Farmer John movie. He uses them to start seeds and then I saw that Eliott Coleman recommends them. A block of soil is made and with the right mix will hold together during plant root growth. Once the roots grow into it, it will be more solid. It eliminates the need for flats, peat pots, paper pots and any other pot which can get root bound. In the soil block, roots will air prune and not go beyond outside. It is a messy operation, but for those who like to tinker with things in their workshop, I made a 3/4" blocker to try out. A soil block is supposed to help minimize transplanting shock. Remember that soil blocks can dry out faster than soil in flats or pots. Keep an eye on them. You will have to water 2 times a day at least. Also, this small block can disintegrate quickly if you pour a lot of water on it. Use a mist sprayer only.
With this mini soil blocker, it is possible to make 144 and they would fit under a 1' x 1' space of a grow light. This is what makes this nice. You can start many seeds in a small area. You only have about 2 weeks growing time maximum, then you must transplant into something larger like a 2" soil block, recycled cups or flats. Transplant shock should be minimized because the roots are not disturbed. You can see my other instructable on making the 2' soil blocker if interested in that. See here http://www.instructables.com/id/Making_a_Soil_Blocker/
. After it outgrows the 2" container, anything bigger than that, I would just transplant into the bottom of the half gallon square milk carton or orange juice carton cut 5" high. I don't think there is a need to make a 4" soil blocker, but you could.
This is good for the home hobbyist. If you experience good success and want to try more, I would invest in a professional soil block maker that can do multiples at a time.
See this site http://www.pottingblocks.com/
for some great advise and ideas on the soil block method. I am not affiliated with it. The gentleman who runs the site is very knowledgeable and helpful in returning emails. He also has supplies and square soil block makers at reasonable prices. I learned a lot from all of the content he shares.