Substrate hydroponics uses solid, soilless material soaked in water and nutrients to grow plants. Horticultural-grade rock wool is ideal for these purposes because it is capable of holding large amounts of water and enough oxygen to properly aerate plant roots. Rock wool can be purchased in small cubes for starting seedlings or large starter sheets for larger plants. And it is easy to use, even for those who are just beginning to explore the world of substrate hydroponics.
Fill a container large enough to easily accommodate the rock wool starter sheet(s) with water at a level just slightly higher than the height of your rock wool starter sheet.
Bring the pH of the water in the container down to 5.5. The easiest way to do this is with lemon juice and pH testing strips. Add 1 to 2 tbsp. lemon juice to the container at a time, stir and then test the water's pH with the testing strips. Stop when the color indicator on the pH testing strip reads 5.5. This low pH is necessary to counteract the high alkalinity of the rock wool, which can be as high as 7.8.
Mix in the plant food you plan to use on your plants at roughly 1/4 strength.
Submerge the rock wool and leave it to soak for at least one hour and no longer than overnight.
Remove the rock wool starter sheet from the water, place it in the sink or in your tub and flush the liquid in the starter sheet out by pouring a fresh batch of the lemon juice/plant food solution (as prepared in steps 1 through 3) over the top surface of the starter sheet.
Plant your seeds in the provided holes in the starter sheet(s). If there are no holes provided, use a sharp knife to create a hole that is 1/4 inch deep. Insert the seed deep enough so that it is not exposed to the light. It should fit snugly into the hole. If it doesn't, line the hole with a little crumbled rock wool taken from the outer edge of the sheet.