Potting soil is easily to find and is usually inexpensive to purchase for your house-bound or at least pot-bound plants. It is composed of specific ingredients combined to make a mix that it lightweight, porous and yet nutritious for your plants. The packaging that will usually list exactly what is in the potting soil, but unless you have been educated in types of soil, you may not know what these common components are.
Sphagnum Peat Moss
This is a decomposed wetland plant harvested from plants in Michigan, Canada and Europe as well as the Soviet Union. Since it absorbs water slower than other materials in soil, it also dries out slower, keeping the moisture in the soil longer. Peat is slow to decompose and when mixed with soil, it will keep it aerated and enriched.
This is a term used for a form of compost that occurs when organic materials such as leaves, grass clippings and other plant parts are composted until they are broken down into particles that no longer have any resemblance to the original materials. Sometimes it is called humus. Organic compost is controlled to eliminate any exposure to pesticides.
This is a natural glassy volcanic rock mined in the U.S. from the Sierra Nevada Mountains. It is mined as a rock, but can be crushed or expanded as is done for horticultural uses. When the rock is heated quickly to 1600 degrees, it expands 4 to twenty times its size, like popcorn, when the moisture in the rock is heated. The result is a finely-bubbled, super lightweight material that can hold many times its weight in water and is chemically inert so it will not decompose.
A derivative of yucca is used in organic potting soil mixes to work as a wetting agent in the soil. This basically means that the saponins found in yucca act like a soap, lowering the water resistance of the soil. This is especially useful in mixes containing dry peat moss, since it tends to resist water.
Lime is added to the mix as a chemical soil amendment. Hydrogen in the soil makes it acid and since plants have a hard time getting the minerals from the soil when there are too many hydrogen ions, the lime acts as a neutralizing agent. It is actually calcium carbonate and when combined with the hydrogen ions, it forms water and carbon dioxide as a byproduct and leaves the soil more alkaline than acid.