The proper grit will keep flowers fit

How complicated can potting soil be, anyway? Most of us obsess over the plants that go into the pots, not over what we're planting them in. But here's the deal: What's in the pot — what you hardly ever notice after plants go in — will affect how the plants grow. In a big way.

Perhaps surprisingly, potting soils contain no soil.

Soil is the precious result of disintegration of rock and plants that supports much of the Earth's agriculture. It's limited — there's not enough soil for manufacturers to bag it for our container plantings.

Most potting soils go by the awkward term "soil-less mix." Mixes act as anchors, preventing taller plants from falling over. They also hold water for plants to use.

That's tricky, but even trickier is the drainage, which lets excess water seep out of the holes in the bottom. A good mix balances both: the feats of holding and releasing water.

Read the bag

Reading the ingredients on the bag will help you know what you're getting. Sometimes you have to flip the bag over to find the list, requiring some bag wrestling to see the contents.

Like food labels, packages list ingredients in order of amount in the bag from most to least. You'll seldom see a percentage that tells you how much of the bag any one component is. Some enlightened nurseries show a sample of the contents to allow you to feel its texture, but this is rare.

The first and largest part of all the components will be "forest products," sometimes called "softwood bark and sawdust." Wood products, a logical result of the local lumber industry, will be composted and broken down before use. These help with plant anchoring and with holding water.

Occasionally you'll see a mix containing "sphagnum peat moss" or "coco fiber" as the first ingredients. Both of these organics provide stability and water-holding capacity. Peat moss originates in Canada, where it's harvested from large bogs; coco fiber is derived from coconuts.

Potting mixes need "true grit." If your pot contained only the organic component, roots would rot from sogginess. Porosity and air spaces result from two necessary common materials, pumice and perlite, used either separately or together.

Pumice — ground volcanic rock — doesn't break down and provides vital airiness in the midst of the organics. It's generally in light gray pieces about 3/8 inch in diameter.

Perlite, another common source of porosity, also starts out as volcanic rock. It is popped, just like corn, heated to 1,600 degrees Fahrenheit, at which time it puffs and expands into small, pure white silicon bits. Perlite does not disintegrate, and its light weight makes full pots easier to move.

Mix your own

If you want to make your own potting mix, combine two parts composted bark, one part pumice, one part perlite and one part homemade screened compost. This will produce a lightweight, versatile mix. If space and time are limited, relying on a bagged mix can be both economical and easy.

What's not needed, you ask? Fertilizer's often added to soilless mixes, and it ups the price. You can fertilize plants regularly with any moderate nitrogen type, such as Alaska Fish Fertilizer.

You'll also find some mixes with added microorganisms like "mycorrhizal fungi." These currently are in vogue with some gardeners, but they aren't necessary in your potting mix. Added polymers to "hold water" can leave mixes too soggy, especially in winter.

Whatever you use, be sure to get that gritty component to keep your plant roots breathing well.

Garden expert Mary Robson is a retired area horticulture agent for Washington State University/King County Cooperative Extension. Her e-mail is marysophia@olympus.net