How to find the right potting mix

Sunset, June, 1996 by Lauren Bonar Swezey

Potting mixes, widely sold by the bag at nurseries and home improvement centers, may seem pretty much the same, whatever the brand. But they're not all alike. Some brands are lightweight and dry out too quickly. Others are overly heavy with sand and tend to compact.

On the bag labels, manufacturers are required to list only the ingredients that make up the bulk of the potting mix but not all of the ingredients or the exact amount of each. They "can put almost anything in a bag, slap a label on it, and call it potting soil," says Peggy Campbell of Molbak's Nursery in Woodinville, Washington.

Products in a mix may or may not be well defined on the label; they're generally listed in descending order according to quantity. These might include Canadian sphagnum peat moss or something called "peat," compost, fir bark or "forest products," sand, perlite, pumice, dolomite limestone, and a wetting agent.

How do you know what you're getting in a bag of potting mix? You don't.

How do you know what combination of ingredients is good for your plants? If you learn how to read the label, you can glean some information from the limited details it provides. Ultimately, though, finding the right mix is largely a matter of trial and error.

WHY NOT USE GARDEN SOIL?

Besides nutrients, plants need air and water to grow. The soil also has to drain well. When plants grow in the ground, gravity pulls water down through the soil to drain away (assuming other factors, such as hardpan, aren't limiting the soil's drainage). Containers are too shallow for gravity to affect drainage, so you have to create good drainage by combining ingredients of the right particle size - the kinds that exist in good potting mixes.

PEAT MOSS AND GROUND BARK

Canadian sphagnum peat moss and fir or redwood bark are widely used in potting mixes. (If the label says just "peat moss," be wary; the bag could contain Michigan or Delta peat, neither of which has good air-holding capacity.) Which is better, bark or sphagnum peat moss?

Sphagnum peat moss "is consistent, so you know what to expect," says Virginia Walter, horticulture professor at Cal Poly San Luis Obispo. She prefers it to forest products such as ground bark. "It's durable (doesn't break down as fast as forest products), and it has a high water-holding capacity plus very good aeration." That means that even if the planting medium is wet, air is still available to roots. On the downside, it's more expensive and, if it dries out, difficult to rewet.

Forest products are less expensive, so the potting mix they're in will be less expensive. But because these wood products break down faster than peat moss, the mix eventually compacts, limiting aeration. That's why mixes should also contain perlite or pumice.

The best bark for potting mixes is composted. If the product is too fresh, it could be high in tannins (toxic substances). And since wood products use up nitrogen as they break down, this element will be depleted from the mix. Some manufacturers try to compensate for the poorly composted wood product by adding too much nitrogen, but the excess nitrogen results in a high salt content, which can burn roots of sensitive plants.

COMPOST

Many labels do not list the kind of compost the bags contain. The compost could be made from almost anything. Other labels do spell out the kind of compost, such as "composted fir bark and sawdust," for example. But the kind of compost used may not be the best. Mushroom compost, for example, is often high in salts and potentially harmful to sensitive plants.

PUMICE, PERLITE, AND SAND

These ingredients add airspace and improve drainage in potting mixes. Perlite and pumice are lightweight materials derived from volcanic glass. Both hold water and, unlike sand, are sterile. Perlite is more expensive than pumice or sand. The best mixes contain 10 to 15 percent perlite or pumice.

Sand is widely used in some mixes, mainly because it's cheap. It doesn't hold water, it's heavy, and, if it's rounded and too fine-grained (like table salt or fine sugar), it can clog airspace and hinder drainage. Mined sand, which is coarser and sharper-edged, is preferable.

LIMESTONE, PH, AND FERTILIZER

Potting mixes that contain peat moss are acidic, usually with a pH of 4 (the optimum range for potting mixes is 5.5 to 6.5), so manufacturers add limestone (dolomite is preferable) to the soil to raise the pH. Some bags list this ingredient; others don't. Salt content and pH can vary.

Most manufacturers don't list nutrient analyses on labels because doing so would bring them under fertilizer label laws. Some do, but nutrient amounts may be insignificant.

MISCELLANEOUS ADDITIVES

Organic potting soils sometimes include earthworm castings, which add nutrients to the mix. When combined with ingredients such as peat moss, they're beneficial. But too many castings can turn a mix into a block of concrete when water is added.

Because soil mixes containing peat moss can be very difficult to moisten, especially if they dry out, some manufacturers add wetting agents to help the peat moss absorb moisture. If a wetting agent is listed on the label, that's a good sign.