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.
New potting mix contains nutrients your plants feed on. Over time those nutrients can be drained from the soil by healthy plants, leaving you with less than an ideal mix of soil to be used again. By adding a few amendments to used soil, you can have it ready to use again.