You don’t have to buy new soil whenever you want to plant cannabis plat in a pot. If the soil is disease and insect-infested, you can’t use it, but if it isn’t, you can reuse it. By revitalizing and reusing good old soil, you will be keeping your costs on growing cannabis low. Cannabis plants suck in a lot of nutrients with the help of soil microbes when they are planted. During the vegetative stage, they require nitrogen, and during the flowering stage, they need calcium and magnesium. So it is safe that after harvesting your cannabis, the plant may have taken all the nutrients in the soil.