Potting Soil Basics

Flower & Garden Magazine, June, 1999 by Lois Trigg Chaplin

Besides water, a top-quality potting soil is the single best thing you can give to a plant in a container.

Have you ever grown a plant in a container on your deck or patio only to have the soil dry up as hard as brick? Most of us can answer with a resounding "Yes!" because it is quite easy to find inferior potting soils in the marketplace. In such poor potting soil, plants will never have a chance to grow well. Plants need a high-quality potting soil -- one that won't pack like a brick and will hold the water, nutrients and oxygen that their roots need to produce lush, healthy stems, leaves and flowers. The question is: How can we be sure of what we are buying?

The answer lies in the potting soil's ingredients. Unfortunately, because potting soils aren't something that you can "try on," you must rely on the bag's listing of contents, and you also must know how to judge them. Here is where mistakes are easily made.

As coddled consumers, most of us are used to seeing a list of ingredients on food labels, fertilizer labels and many other products that we buy. But that is not necessarily so with potting soils, as they are not required by law to list ingredients. In truth, just about anything could be called a potting soil -- low-grade compost, dirt and even ground up crates! A few states, such as Georgia, do require detailed labels on all soil products, but in most states we gardeners are on our own. So when the contents of a potting soil bag aren't listed, beware that maybe there's nothing in the bag worth mentioning!

YOU GET WHAT YOU PAY FOR

When shopping for potting soil, you will find prices vary dramatically. Today, many of the cheaper soils have been driven down in price to the point where manufacturers cannot afford to include the best quality ingredients. Instead, these products are commodities. So, that 99-cent bag of advertised potting soil may be nothing more than a poor-quality compost or the very black, reed sedge peat that compacts easily. Take a second look at products that contain the word "compost." This is a very loose term that can include yard waste, sewage sludge or other unknown composted items. Make sure the kind of compost is stated on the bag; generally, compost made from aged forest products is considered to be top quality.

I've opened bags containing what looked like scrapings from the woods: bits of pine needles, dirt, pebbles and something decomposed (leaves?). Such a product was fine for use as an amendment in my flower beds, but it was not what I wanted for my prized houseplants or outdoor flowers whose roots are limited to containers. In a container, I want an airy "professional" or "premium" potting soil that will provide good drainage and air exchange, and has the ability to hold water and nutrients for my plant to grow. Although more expensive, it is definitely a better value for my containers. Here are some reasons why.

SURPRISE! GOOD POTTING SOIL DOESN'T CONTAIN SOIL

In one of the ironies of nomenclature, gardeners will find that a good quality potting soil is not really soil at all, but a mix of several ingredients which include sphagnum peat moss, vermiculite, perlite, aged composted forest products or sand (see sidebar). These are ingredients which must be gathered, purchased, shipped and finally mixed by a potting soil manufacturer in the proper proportions. A proper mix often also includes lime (to balance the pH, if needed) so as not to be too acid or alkaline.

These mixes are often referred to by horticultural professionals as "soilless mixes," which means they don't contain dirt from the back yard. Originally developed for the nursery industry for their cleanliness (free from soil-borne plant diseases) and their ability to encourage fast root growth, the many versions of these professional soilless mixes are what we see today in the better potting soils used and sold at garden centers. Although more expensive, quality soils will encourage plants in your containers to grow, grow, grow just as they do in greenhouses and at nurseries.

POLYMERS AND WETTING AGENTS

One of the drawbacks to soilless professional mixes containing peat moss is that they can be difficult to rewet if they get too dry. If allowed to dry out the mix practically sheds water. So some mixes also contain water-retaining polymers or wetting agents to help prevent this problem. For example, Sta-Green Smart Soil is a brand that contains Aquaguard, a combination of a wetting agent (to help the mix absorb water evenly and easily throughout) and a water-retaining polymer. Polymers look like little bits of gelatin that absorb excess moisture and release it back into the mix as it begins to dry. You can also purchase polymers such as Agrosoke, Water Mizer or other brands, and incorporate them into the soil yourself. Mixes containing wetting agents and polymers are especially useful to plants in hot, outdoor locations and to plants in window boxes and hanging baskets, which can dry quickly.

FERTILIZER

You will find that occasionally some potting mixes also contain a small amount of fertilizer. The small amount contained is barely enough to help your plant get started, so you will need to add more fertilizer at planting time. For potting soil manufacturers to include enough fertilizer to get your plants through the season, the bag would then have to meet all kinds of federal fertilizer requirements. At that point, the manufacturers would rather leave the fertilizing to gardeners.