A soil block is a cube of a soil mixture and water that is compressed into a mold forming a block. It is used for indoor seed starting. Instead of using container, the compressed soil block serves as both the container and the soil medium all in one. Soil blocking is done using a tool found at many farm supply stores. They are also available to purchase online. Soil blocks do come in different sizes. It is important to find one that fits your growing needs. If you plan to have dozens of starts, you might want a tool that will make several small blocks at once. Some blockers make 6 large cubes at a time.
To use a soil blocker, it is important to have a good growing media. Since the cubes need to be compressed tightly to hold their shape, a bag of standard potting mix will not work. Most people who use the soil blockers will make their own mixture. Most recipes include a mixture of: peat moss or coconut coir, perlite or course sand, compost, and soil. A good recipe can be found here. If buying a premade mixture, Pro Mix is one of the better options on the market for seed germination and soil blocking. Pro Mix can be found at Alaska Mill & Feed and other stores throughout Alaska. Keep your soil block mix dry and covered until you’re ready to begin sowing seeds.
When you are ready to use it, you will want your soil very wet- the consistency of cake batter. Peat moss and coco coir are highly absorbent and mixing thoroughly is key. When the mixture no longer takes in water and the water starts to pool in places, you can stop adding water. Let the mixture sit for a few hours to continue to absorb. Press the blocking tool into your soil mixture, giving it a good twist before lifting up. You should see moisture come out of your cubes. Make sure that soil has filled each mold before gently extracting the cubes. Your cubes are then ready to plant in. Each block gets one seed so you will know how many starts you have. Gently press the seed into the soil following the instructions on the seed packet. Since all the water was added to the block, you do not need to water directly after planting the seeds. However you do need to check the moisture daily and water as needed. An overly dry block will crumble as well as an overwatered block. The key to keeping your soil blocks from falling apart is to water from the bottom. Do this by placing the blocks in a tray and watering the tray itself.
Using a soil blocker has many benefits. It is economical because you do not have to buy new inserts for your propagation trays every year. Trays and inserts can wear out easily and often need repurchased. You save space on storage as well with reducing the number of trays or containers used. The plants benefit since the roots are pruned by the air. Once the roots hit the air, they stop growing, until the plants are transplanted into the garden or bumped up to the next size soil block. This inhibits the plant to become root-bound. By the plant not being root bound, when time to transplant, they the roots aren’t needing to be tampered with. This lessens transplant shock. You can simply place the soil block in the ground and not disturb the roots as you would trying to remove them from plug inserts.