It's natural to assume a bag of potting soil will provide the ideal growing media for your plants. Unfortunately, this isn't always true. Potting soils vary greatly among manufacturers and may not always be what they seem. While regulations require the manufacturer to list the ingredients in the potting soil by volume, in decreasing order, and state its intended use, this does not always equate to how well the potting soil will perform.
What's Not On the Label
Aeration is the most important physical property of potting soil, but it is not included on the label, notes Dennis R. Pittenger, Extension Urban Horticulture Specialist from the University of California, Riverside. Other important considerations not included on the label include drainage rate and water-holding capacity.
Dawn Pettinelli of the Soil Nutrient Analysis Laboratory at University of Connecticut explains that state and federal regulations do not govern the quality of potting soil. This means that even though your potting soil may list what appears to be quality ingredients, the proportions of those ingredients are up to the manufacturer and may affect how well your plants perform in the soil.
Potting soil typically contains peat moss, sphagnum moss, decomposed bark or wood products, compost, leaf mold, vermiculite and perlite, or Styrofoam. Potting soil may also contain sand or water-retention crystals. Up to 3 percent of the mixture can be labeled as "other" without labeling the specific ingredients. Low-quality potting soils may substitute polystyrene foam for perlite, which tends to compact over time, making it an inferior product.
Many potting soils are labeled for specific types of plants and reportedly contain the correct proportion of ingredients to support healthy growth. Some formulas are soilless, while others are soil-based. Soil-based formulas are a better choice for large plants potted in containers that may tip in the wind, while soilless formulas are lightweight and suitable for hanging containers or pots where weight is an issue.
Making Your Own Soil
Some people prefer to mix their own soil for pots and containers. Although there are many recipes available, University of Illinois Extension recommends a standard mixture of equal parts peat moss, loam or potting soil, and perlite or coarse builder's sand. This creates a lightweight soil with good aeration and drainage.