Grodan Stonewool (rockwool) is made from molten rock spun into cotton candy-like fibers and then compressed into cubes, blocks, or slabs. Where other rockwool brand manufacturers cut all sizes from one batch of wool, Grodan is unique by making different types of fibers. The purpose is to support the requirement of the plant in the different plant stages. Grodan has a tremendous capacity for absorbing nutrient solution while retaining plenty of oxygen for rapid plant growth. Grodan comes in graduated sizes. Seedlings started in cubes are easily transferred to blocks and then onto slabs, if desired. Seedlings or cuttings started in the Grodan cubes can also be transplanted into more traditional planters and growing mediums. Grodan Slabs – The final stage of planting with growing blocks are the Grodan slabs. These slabs come in clearly marked UV resistant poly sleeves for ease of use. Recommended for larger, long term crops with extensive roots like peppers, tomatoes, or cucumbers. Typically, seedlings in blocks are placed onto slabs. Grodan Expert is the slab that drains the fastest because of its vertical fibers. By far the most popular slab for the hydroponic hobbyist market. Grodan Master is a special wet/dry slab. The top layer is thicker, which stays wetter, while the bottom layer stays drier. In effect it defies gravity, allowing more even distribution of moisture throughout the slab. This item does not qualify for fee shipping This item is marked Oversized, you will be notified if extra shipping charges apply befor the order is processed.