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.
Add perlite to the mixture in small amounts, such as a 1/2-cup at a time, to improve the drainage and crumbly texture of the rejuvenated potting soil if desired. Small bits of pine bark, coarse sand or grit can also lighten the organic matter component already mixed in.sing the old mix as a foundation, organic matter can be added to bring new nutrients, texture and use to the soil concoction. Just avoid using topsoil as one of the additives to the old potting soil mix.