Potting soil needs to drain well but still hold moisture
Photo/Illustration: Melissa Lucas
One of the most important things a potting soil needs to do is provide roots access to air by letting water drain away from them. In the ground, the soil is usually deep enough to let excess water drain beyond root zones. In pots, however, water tends to accumulate at the bottom, despite drainage holes. The smaller the pore spaces of the soil in the pot, the higher that water layer will reach. Larger pores, formed by adding mineral aggregates to potting soils, readily admit water into the soil, then carry it through the medium and out the bottom. Then, all those large, empty spaces can fill with air.
Perlite, vermiculite, calcined clay (kitty litter), and sand are the mineral aggregates most commonly used in potting soils. Perlite and vermiculite are lightweight volcanic rocks naturally filled with air. I prefer perlite over the others because it does not decompose with time nor lose its aerating ability if the potting mix is compressed. Vermiculite is a valuable additive because it prevents some nutrients from leaching away, and it even provides a bit of potassium and magnesium.
A potting mix also must have ingredients that help it retain moisture. This is where organic materials—usually peat moss, sphagnum moss, or coir—come in. They cling to some of the water that the aggregates are helping to drain. Organic materials also hold on to nutrients that might otherwise wash away.
In addition to peat moss, vermiculite, and perlite, commercial mixes often contain sawdust or various grades of shredded bark. Lime may be added to help balance the acidity of the peat moss, and a small dose of fertilizer can often make up for the lack of nutrients.