You may choose to start your seeds in rockwool rather than soil if you plan on using rockwool for your entire grow and want to transfer the small cubes into larger cubes as the plant gets bigger. Rockwool is also cleaner than soil. There are no insect eggs and contaminants in rockwool.
To start seeds in Rockwool, first presoak your seeds for 24 hours in room temperature water. After the pre-soak, move your seeds with tweezers into your rockwool cubes. Do not allow the taproot to become long before transplanting to rockwool. The ideal time to transplant is right after the taproot emerges from the seed and is only 1cm or less long.
Be careful when working with Rockwool. The fibres are hazardous to your lungs when the cubes are dry. You should wear a mask and gloves when handling. Rockwool is not biodegradable or reusable.
Rockwool is not pH balanced and so you have to do that prior to planting. Rockwool cubes must be pre-soaked for 24 hours in water that has been pH adjusted to 5.1.
Unlike starter soil, rockwool has no nutrients in the medium whatsoever. You have to add the nutrients. To do this you should supplement the water you use with nutrients to make it 300 ppm. 300 ppm is the ideal amount of nutrients for seeds and seedlings.
Rockwool cubes have pre-drilled holes where you set the seed. Try to place the tap root facing down into the hole because it will be hard to reposition the seed after the taproot gets long. Use tweezers and a chopstick to help you set the seed. Act very gentle.
After your seeds have been transplanted in Rockwool, put them in a sealed tupperware container with a heat mat underneath until the sprouts emerge. It should take around 48 hours to get sprouts. Once you have sprouts, remove the cubes from the tupperware container and put them under fluorescent lights.
A difficult aspect of rockwool is keeping it evenly moist. Cubes must be kept evenly moist, but not so drenched that they will waterlog and rot the seed.
Dr. Seeds does not recommend rockwool for beginners because it is more difficult compared to starting seeds in soil. However, if you are up to the challenge and willing to mind the precautions, Rockwool does provide some special benefits over soil and is an effective medium for seed sprouting.