In the formal language of economics, X and Y are substitutes if the demand for X increases when the price of Y increases, or a positive cross elasticity of demand.
Potting mix is the material that goes into a garden pot before you put seeds and plants into it. 1  It keeps the plant in place. 2  It holds water for the plantâs roots to soak up.  It contains nutrients that feed the plant.
Potting soil is a combination of several ingredients that are chosen to provide the best drainage, support and nutrition to plants. Commercial potting soils and homemade mixes contain similar ingredients. They are usually a mix of three substances.