Gardeners shopping for potting soil may find it harder to determine a product's content than they had expected. Without formal state or federal regulation, commercial potting-soil producers can combine a very wide range of ingredients to make potting soil. Knowing the properties of frequently-used ingredients can help you purchase wisely or create a soil to help your seeds and container plants grow their healthy best.
Potting soil needs to provide seeds and plants with a light, porous, moisture-holding medium that lets small roots develop without struggle. This usually means that 40 to 50 percent of the mixture is organic materials. Peat moss, sphagnum peat moss, coir fiber, shredded bark and leaf compost are among the most frequently-used organics. Ground bark or other sawdust, rice or other grain hulls and treated sewage sludge may also be part of the organic contents. Sludge, composted manure and sterilized or pasteurized soil appear in combination with other more fibrous materials. Exact amounts of single and combined ingredients can be difficult to determine from package labels, although those used in larger amounts usually lead the list, as they do in food-labeling. Organic matter creates a soillike medium containing the air and water plant roots need, while providing fairly low levels of nutrients needed for growth.
The other half of a potting soil mixture, inorganic matter, enhances the necessary flow of water and air through the soil. The two most common inorganic ingredients in potting soil are perlite and vermiculite. Perlite, which is heat-expanded volcanic obsidian, functions somewhat the way stones and pebbles do in garden soil, providing aeration and some stability as plants grow taller. Vermiculite, a heat-expanded mica by-product, is valued for its moisture-holding capacity. Chemically neutral, both perlite and vermiculite augment soil volume and porosity, while diluting the possible effects of excess organic nutrition. Sand, usually coarse-grade, is often added to increase soil density while maintaining drainage. Calcined clay, fine sand or ground limestone are sometimes used in small quantities.
Some manufacturers and home gardeners routinely add fertilizer to their potting soil mix. Others insist that good potting soil supports growth without it. If you use potting soil briefly as a seed-starter, fertilizer is seldom a concern. For long-season or year-round container plants, fertilizer needs vary greatly by plant variety. Some commercial labels clearly state the amount of nutrients provided and their source. One manufacturer, for example, provides the N-P-K -- the nitrogen-phosphorus-potassium content formula -- of its soil, citing poultry litter as its nitrogen source. Some manufacturers specify the kinds of plants best sustained by a soil mix without further detail. Without regulation, potting soil producers may provide information they believe useful or not. Potting soil mixtures containing bone, blood or feather meal, sewage or milorganite, fish or seaweed emulsions or chemicals like ferrous oxide offer enhanced nutrition to plants, but exact amounts do not need to be listed.
Many potting soil mixtures don't contain any soil at all. Soil-free, or soilless, mixtures are designed to keep soil-borne organisms and weeds from damaging or destroying plants. Mixtures labelled as containing sterilized soil hold no nutritional advantage over soilless mixtures; heating soil to the 212 degrees F needed for sterilization damages nutrients as well as destroying pathogens. Pasteurizing soil, which involves heating it to 180 F for 30 minutes, can destroy harmful fungi and bacteria while keeping nutrients intact. Pasteurizing is also adequate to destroy weed seeds. Look for soil-containing mixes that specify pasteurized soil, or bake soil at 180 F before adding it to a homemade mixture.
Organic vs. Organic
Even a potting soil high in organic, or formerly living, matter is not automatically organic in the regulatory sense. The strict certification standards of the United States Department of Agriculture's National Organic Program do not extend over planting-related products like potting soil. If you want a commercial product that meets organic growing standards, look for packages indicating that a mix is Organic Materials Review Institute-listed. OMRI is a nonprofit organization charged with approving or listing products eligible for use in certified organic farming.