It may surprise you that when it comes to buying potting soil, not all potting soils are created equal – in fact, just because it says potting mix on the bag, doesn’t mean it’s meant to use it pots!
What is in Potting Soil?
Potting soil also does not necessarily contain actual soil. Many potting mixes are composed of ground bark, peat moss, sawdust, sand, sphagnum peat moss, and minerals such as vermiculite and perlite. If your potted plants aren’t doing well, it could be that the potting soil you bought has too many additives! Simply put; plants don’t grow in ground bark and sawdust. The more fillers, the worse it is for plant growth.
Potting soil can also include compost, cotton gin waste, coconut husks, mushroom compost, peanut or rice hulls and pecan shells. Some mixes add fertilizer, which may or may not be a good idea.
Ingredients and their function include:
Peat and sphagnum mosses: Harvested from peat bogs – which is a non-renewable source. They will retain moisture – unless allowed to dry out completely, in which case repel moisture. They contain little nutritional value.
Ground bark and sawdust: Cheap fillers. In small quantities, help to improve drainage. If in large quantities and larger pieces, will reduce moisture retention.
Perlite and vermiculite: Minerals added to improve drainage and lighten soil. Vermiculite helps moisture retention better than Perlite.
Types of Potting Soil
When choosing a potting soil, you want a mix with the ability to hold moisture, but not too much moisture. For example, while African Violets grow in a moist, tropical environment, they prefer a fast draining soil that has the moisture level akin to a damp sponge. Soggy soil means crown rot.
Potting mixes labeled as amendment are not intended for pots, but for in-ground plantings. Depending on what you are planting, soil amendments are rarely warranted, especially trees and shrubs. If you are planting a species native to your area, stick with native soil.
Special potting mixes for cactus and succulents are also available. While you certainly do not want to plant these in a potting mix full of soil retentive materials, quite often you will find peat moss in cactus potting mix, which is not advisable. It is easy to make your own cactus mix – something you might want to consider.
The best way to check your potting soil before you buy it is to open a bag and feel it with your hands. Observe the amount of additives, which is easy to do just by looking for large pieces of bark. Lots of bark usually means a poor growing medium.