Soil blockers make different size blocks of soil for planting into. As roots don't grow out the sides into the air they tend to stop and grow down. I've got 3 size blockers that make 20mm 40mm & 100mm blocks. The 20mm ones are good for fitting more seedlings on heating mats and with a square insert for the 40mm blocks they can be placed into the next size for planting out. If the seedlings are keep longer they fit into the 100mm blocks.
I've been using worm casting for the 20mm blocks and a sifted mix of potting mix, castings & blood & bone for the larger blocks.
When made with worm castings the blocks can go into an AP system without washing the soil off the rooots.
