Some of the main things that you need to supply plants are air, water and light energy. Two of these necessities are controlled by the growing medium. I am referring to air and water. The roots of the plants must have access to fresh oxygen, while the vegative parts require co2. To assist the roots in oxygen intake, you must have the soil adequately airated. Also, water must be able to flow through without oversaturating the soil.
Use a mix of peat moss, black earth, vermiculite, perlite and Sheep Manure at a volume ratio of 3-1-1-1 consecutively. First mix the peat moss, vermiculite, perlite and black earth together thoroughly. At this stage you can dampen the soil by adding some water while mixing. Second, add the sheep manure evenly through the mix(do not leave any chunks of manure in the mix). If you do not have access to the sheep manure, use another high phosporus content such as bat guano at a much lower rate. If water is not easily accessible, you can use a wetting agent (which holds water) from your local hydroponic store. Just follow the directions and mix it into your soil/manure.
Use a mix of peat moss, vermiculite and perlite(wetting agent if desired) at a ratio of 2-1-1 for your indoor gardens. This peat mix provides the perfect environment for marijuana roots to thrive by allowing access to oxygen and water consistantly.
Dampen the mix evenly before filling pots to avoid a water mess.
Pre-mixed garden mediums(promix-BX) that are sold at Home and Garden stores are usually good. Be sure to read the bag to see that the ingredients needed are all included. Feeding Caution - some have fertilizers already in the mix.