A brief description from pottingblocks.com:

Potting Blocks, also known as soil blocks, are free-standing compressed cubes of potting soil which hold their shape without any container. Potting Blocks are made from a zinc coated stainless steel Soil Block Maker, much like an ejection mold. The block maker metal form is packed into a tub of pre-moistened potting soil and then discharged into nice, firm, blocks with a pre-drilled seed or transplant holes formed right into the top. Potting Blocks are used for seed starting or germination, and transplanting. They have an amazing success rate due to the volume of soil compressed in the cube. The roots are naturally “air pruned” due to the air barrier of the “container-less” cube. They become the growing medium and the container! They are used for everything; herbs, flowers, vegetables, cuttings, and other transplants.

Potting Blocks have many advantages over traditional potting methods. First, they eliminate transplant shock! The
seedling and root system stays intact and protected, a “home away from home”. They will not become “‘root-bound”. They eliminate root circling. They replace plastic pots, trays, inserts,etc. They contain more cubic volume of soil than pots of the same top dimensions. They promote great air circulation. They have a major increase in space utilization than round pots. And, studies in Europe have shown that Potting Block transplants are superior in performance than container-bound transplants.

I’m definitely going this route next year. I love the idea of not having to sterilize and reuse old pots, or buy new ones each year. Use this with a capillary mat to provide water when needed and you’ve got a great compact arrangement without dealing with actual pots. I especially love that you can get a 5/8″ blocker to start small seeds and then a 2″ blocker that will make an indentation large enough to slide your pre-germinated 5/8″ block right in!

In the image below, the top square is the 5/8″ block made with the small blocker, slid into the middle block, which is made with the medium size blocker, and that one is slid into at 4″ block, made with the largest size blocker.

I’d be able to fit so much more under my grow lights this way, and I can keep an eye on the moistness of the soil, which I’m having a problem with monitoring in my seed starting tray.

I’ve seen some people have made their own, however I am not that crafty. So hopefully someone will buy me a couple sizes for my birthday. 🙂