At first glance, potting soil may look very much like the soil that you dig out of your back yard or garden. If they are, in fact, similar, you have outstanding soil in your yard. However, most potting soil is a combination of ingredients that help make ideal rooting and growing conditions for plants. Understanding the components of potting soil will help you choose the right commercial soil for your plants or help you make your own potting soil from base components.
Sterilized Loam Soil
The base of most potting soil is sterilized loam. Sterilized loam is available commercially, but if you have good soil in your yard, you can sterilize it using your oven. To sterilize soil, place the soil in a metal or glass baking dish and place it in a 180- to 200-degree oven. When an oven thermometer measures the internal temperature of the soil as 180 degrees, allow the soil to remain in the oven for 30 minutes. Soil needs to be sterilized to prevent contamination of potted plants with bacteria, fungus and other diseases. Sterilization also kills any weed seeds that may be in the soil.
Sphagnum Peat Moss
Plant roots need air to remain healthy. They also need a soil that can hold a certain amount of water without becoming waterlogged. Waterlogged soils do not allow proper air flow through the soil to the roots. Sphagnum peat moss helps increase aeration of the roots and helps potting soils retain water. However, too much peat in a potting soil can reduce soil drainage and encourage the soil to become waterlogged.
Sand improves soil drainage and aeration. It is often a primary component of potting soil. Although sand increases drainage, it does not retain water. Too much sand can cause soils to dry out too quickly. Sand should not be mixed with heavy clay soils.
Perlite and Vermiculite
Perlite and vermiculite are similar. Both are made by heating hydrated rock to high temperatures. The water in the rock expands, creating a light, fluffy rock that holds air and helps improve drainage. It can also help decrease the overall weight of potting soils. There are a few disadvantages to perlite and vermiculite: a tendency to float in water, inability to hold water and their cost. Perlite and vermiculite can also compact, reducing their ability to improve drainage and soil aeration.