Cannabis can be grown in many different growing mediums each with differing aeration, water retention, and pH.
Substrate qualities
Each subtrate be it soil, coco coir, rockwool or anything else has a few key metrics that can be used to determine how to use it and if it is suitable for growing cannabis.
Water holding capacity (WHC)
A subtrate's water holding capacity is the amount of water it can hold when fully saturated. Imagine watering soil until it begins to drain, this is full saturation. Its important to expose the root zone to the right water content, for example, high water content (overwatering) can drown roots by starving them of Oxygen leading to rot, and low water content (underwatering) can deprive the plant of the nutrients. The ideal amount to water a plant depends on the WHC and field capacity of the subtrate.
Air porosity
The amount of air a subtrate holds after it has been fully saturated and drained. The subrate will hold air inside of pores in the material. Large or course particles create larger pores and increase air porosity like perlite. Finer particles like coco coir and rockwool, create smaller pores that hold more water but less air. Air porosity is the total air these pores can hold. It is generally expressed as a percentage of the subtrate volume.
Field capacity
The amount of water a subtrate holds when fully drained. In practice, with proper drainage in place, the water content available to the root zone will be the field capacity of the subtrate.
Soil
The traditional growth medium. Easily available, the pH is mostly determined by the water applied, although soil provides a buffer. This is the most simple growth medium, good for beginners. More advanced growers may wish to create a type of organic "living soil" via methods such as Bokashi
Additives
Coco Coir
Coco coir is a subtrate derrived from dried coconut husk. It can be used alone or mixed with soil, coco coir provides less of a pH buffer than soil however. Its also notable that coco coir is inert and provides no nutrients itself, meaning that nutrients must be added via additives. Coco coir also has excellent water rentention capabilities similar to vermiculite and can hold 10x its weight in water.
Vermiculite
Has very good water retention properties and provides Silica which has a variety of uses in cannabis[1]
Perlite
Perlite is a derivative of volcanic glass that can be added to soil or coco coir to increase aeration, by keeping the soil less compact. It also holds a lot of moisture. Perlite should be wet before use as it can easily spread dust