Charcoal is a porous form of organic matter that has many uses in horticulture. As an excellent soil conditioner, especially for heavy soils, it improves the soil's aeration, drainage and texture. It also has the ability to absorb odors that may appear in your pots. In terrariums and pots that lack appropriate drainage, it improves root oxygen levels. Put at the bottom of a pot without drainage holes, it has the ability to provide a place for excess water to stay. With its purification properties, charcoal helps keep the water fresh until the plant roots need it.