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.
Ready mix concrete is a mixture of paste and aggregates: the paste, composed of Portland cement and water, coats the surface of the fine and coarse aggregates. A chemical reaction, called hydration, causes this paste to harden and gain substantial strength - forming the rock-solid mass commonly known as concrete.