Potting soil serves important functions for container-grown plants. Not only does it anchor plants, it provides oxygen for root respiration and acts as a reservoir for water and nutrients.
Before the 1950s, people used garden soil in containers and for starting seeds. I still recall the awful smell from my mom sterilizing garden soil in the oven — to prevent disease and to kill any weed seeds. When the greenhouse industry began expanding, that practice fell by the wayside. Now nurseries and gardeners almost universally use potting mixes for seed starting and to grow plants in containers.
What is in these mixes? Everything was originally built around peat moss, but that is changing. Peat moss is made of mosses that have been decomposing for millennia. Because the mosses tend to collect in boggy areas, the wetness makes for slow decomposition. One of the most favorable characteristics of peat moss is its ability to hold water.
Because the amount of peat moss available for mining is limited, scientists have looked for a renewable replacement for a long time, and they seem to have found it in coir, the outer, fibrous husk of coconuts.
Coir has water-holding capacity similar to peat moss. Water-holding is important for seed starts and potted plants, but the planting medium also needs to drain well so plants don't stand in water. A variety of materials can be used to help water drain away and at the same time let roots get some oxygen.
Some of the early mixes used vermiculite or perlite, but the trend now is to use renewable, recycled, organic materials. In the Pacific Northwest, fine bark, sawdust, waste paper components, wood chips, manures, sand, pumice and biosolids from sewage sludge are often used. Don't worry — the sludge from water-treatment plants is rated by the EPA for pathogens and heavy metals.
Ingredients in potting soils are for the most part sterile, which means that food for the plants must be added to the mix. Some manufacturers add chemical or organic fertilizers, moisture-control agents and even mycorrhizae to promote plant health.
As with food, it is important to read the label on your bag of potting soil so you know what you're getting. It is not difficult to add your own bone meal, kelp, alfalfa meal, and so on — there really is no magic formula. But if you are a novice, you might want to start with a high-quality, ready-made formula, as the cheaper mixes contain more wood products, which lowers the quality and requires more additives.
I recommend that if you use the super-fine textured mixes developed for indoor seed starting. Also, the specialty mixes for cacti, African violets and orchids will give you much better results for those plants than you'll get with standard potting soil.
The day before you use your potting soil, add plenty of warm water and mix or knead it well. Although the ingredients hold water well once they are dampened, it takes a surprising amount of water to get them that way.
Coming up: "Ready-Set-Grow," a day of learning for beginning gardeners, will be held Saturday, March 22, at the Southern Oregon Research and Extension Center, 569 Hanley Road, Central Point. Four consecutive classes start at 8:30 a.m. and run to 4:15 p.m. The classes cover soil and water, vegetable gardening, annuals and perennials, and seed starting, all geared for the novice. The cost is $30. Call 541-776-7371 to register.
Carol Oneal is a past president of the OSU Jackson County Master Gardeners Association. Email her at email@example.com.