A-Ok Starter Plugs for starting seeds and rooting of cuttings prior to plugging into larger Gro-Blocks. Strip of 15 MM 40/40, 1.5 in. x 1.5 in. x 1.5 in. polywrapped blocks. Grodan rockwool is an excellent quality stonewool substrate with a very high water holding capacity. Rockwool is absolutely free of disease, salts, and weed seeds. It has no Caution Exchange Capacity (CEC) so the grower can take advantage of a nutritional program specifically tailored to the crop being grown. Rockwool blocks containing plants are set on top of the slabs and the roots grow through the block and into the slabs. Nutrient solution may be applied through a spray or drip emitter either directly to the slab or through the growing blocks. Blocks are commonly used for germination of seed and rooting cuttings. The vertically oriented fibers in blocks promote good downward root growth.