Soil based mediums are the most commonly used form of growing medium. This is because they are cheap, easy to manage and fairly straight forward to maintain. However, if your soil is not tailored for the growth of cannabis, then no matter how advanced your skills are, your cannabis plants are not going to reach their full potential. As you gain experience knowledge you will begin to gain a better understanding of what your plant needs and create a soil composition that is well suited to your cannabis.
When you use a soil based medium your cannabis will grow long winding roots. These roots search through the soil and absorb water and nutrients from it. The soil will also go through drier periods that will allow for air circulation, this gives the roots an opportunity to breath and preform essential respiration.
When using soil based mediums, you will want to pay attention to its pH level. pH is a measure of acidity, raging from 1-14, with 1 being very acidic, 7 being neutral and 14 being very alkaline. Cannabis needs a pH level of 7 in order to thrive.
You will also want to bear in mind the nutrient ratios of bought soil. These are expressed as NPK on the soil packet: nitrogen (N), phosphorous (P) and potassium (K). These are three essential nutrients required by your cannabis plants - and they will be required in different amounts of each. Soil packets will usually express the ratios in the format of 20:20:20. In this example it expresses that the soils is made up of 20% of each nutrient. Different soils will have different ratios. For a more detailed explanation of the ratios cannabis requires, see our article on Nutrients and Fertilizers. For an explanation of exactly what these nutrients do, see our article on Nutritional Elements.
The types of soil based mediums

Never use soil from your garden unless you are growing outdoors. Soil from your garden will contain bugs, diseases and pests that can compromise the controlled environment of an indoor grow room. Always buy soil from a store when growing indoors.

Clay is a very tough, dense soil type that contains hydrates aluminosilicates. These become flexible as water is added making clay easy to shape. Cannabis struggles to grow in pure clay soil as it has fairly poor drainage and can be tough for roots to get through. It is often mixed with other soil types to balance composition.

The use of sand is usually ineffective for the growth of marijuana. They are not very good at retaining water and nutrients, that drain away to quickly for your cannabis to fully utilize. Sand can be used effectively to balance out the composition of more dense soil types.

Silt soils are much like sand, except they look more like clay. They too, cannot hold water for very long but are quite apt and retaining nutrients. Silt can make for a good addition to a mixed soil composition for both balancing out denser, less drainable soil, as well as adding nutrient retention properties to your mix.

Humus is an organic substance made from decomposed plant matter. It is mixed with manure and loam to make compost. Due to the nature of humus you run the risk of introducing the occasional bug into your grow room, even if you buy it from a store. Most humus brands claim to be bug free, but this is very hard to achieve.

Loam is your standard shop bought soil. It is usually compromised of all of the previously mentioned soil types, often in a composition that is highly beneficial to most plants, with good nutritional value and the correct density for the correct level of moisture retention and drainage. Each packet of loam should have its composition break down on it, you will want one that is mainly humus, clay and sand. Loam is highly recommended grow medium for your cannabis plants.
Shop bought moss is great at absorbing and retaining water and nutrients. It can be mixed into your grow medium's composition, but it easily breaks down after it has been watered a few times, meaning it has to be replaced often.

As touched on, you do not want a soil that is to light or to dense. Cannabis thrives in a balanced soil that is not to wet (dense) or dry (light). Wet soils will not allow enough oxygen to the roots as it remains water-logged, whereas dry soil drains too quickly, make it hard for the cannabis to obtain any moisture. Soil packets should indicate whether they are a particularly wet or dry soil. Read the packets and aim for a middle ground.