Choosing container gardening soil is one of the first and most basic decisions container gardeners face. New container gardeners will most likely go outside and fill the pots with garden soil. Big mistake! Most yard dirt contains clay which is heavy and prevents soil from draining well. When clay is dry it resists water and yard soil tends to pull away from the sides of the containers. This makes watering a messy and difficult process. Clay will never support healthy growth in plants and should never, ever be considered when choosing container gardening soil.
Sandy soil needs a lot of conditioning to hold enough moisture for container plants. By the time you add compost, peat, vermiculite or other additives to your sandy soil, you still may not have a mixture that your container plants will thrive in.
In heavy garden soil, there is little space between soil particles. Because of these small spaces, when the plant is watered, the water forces the air from the soil by filling up the small spaces. The amount of air left in the soil after water drainage is mainly what determines the plants growth as roots need sufficient air for respiration, survival and growth. As you can see, by using garden soil you will find at the least, you’re not getting optimum results from your plantings. Worst case senario, your plants could wither and die from lack of air.
All this may sound complicated, but choosing container gardening soil need not be a huge decision. The soil properties for optimum growth should be:
- Fast drainage of water through the soil
- Sufficent air left in the soil after drainage
- A reserve of water left in the soil after drainage
Considerations when Choosing Container Gardening Soil
If you are a beginner, choosing a commercial potting mix could be your best option. These mixes usually consists of two parts, organic and mineral. The organic ingredients may be peat moss, bark of hardwoods, shavings, fir bark, or pine bark. The minerals are either perlite or vermiculite. Sand is also often used as an aerator and as an aid for rooting cuttings.
Vermiculite (Terralite) resembles mica and when crushed and treated under heat the mineral flakes expand to 20 times their original volume. Perlite (sponge rock) is a volcanic material. When heat treated it pops like popcorn and like vermiculite, expands to 20 times it’s orginal volume. The differance between vermiculite and perlite is in the way the minerals retain water. Water is retained within the flakes of vermiculite while perlite retains water around the granules; consequently the perlite tends to dry out faster than vermiculite.
The standard “soilless” mixes are free of diseases, weeds, and insects and are ready to use. Many gardeners purchase commercial mixes, then go home and add garden soil to “stretch it out.” This totally defeats the purpose of using commercial mixes. When you add soil, you lose all the advantages of a sterilized mix. Do you really want to start out fighting weeds, insects and diseases in your pots? Think about this before adding yard soil to your mix.
Along with these advantages, commercial mixes have all the nutrients needed for inital plant growth included in the mix. Therefore these mixes will be ready for immediate growth. Mixes such as Jiffy Mix, made of peat moss and vermiculite, has the further advantage of lighter weight. This could be a big advantage if you plan on moving your containers around or want to plant window boxes and hanging containers. If your garden is a rooftop, balcony or window box, this could make a differance in the way you plan your container garden.
Using Homemade Mixes
Once people have been gardening for a while, many gardeners prefer to make their own soil mixes when choosing container gardening soil. There are as many recipes as there are gardeners available for mixing your own. For more info and recipes on how to mix your own potting soil, check out the following resources:
A Long Post at The Garden Web forum This post contains a huge amount of info about soil, soil drainage, and recipes for potting soil. This is probably more info than you want or need, especially if you are a beginnner, but be sure to bookmark it for future reference. There are many gold nuggets to be dug out of this post.
How to Mix Your Own Potting Soil This is a good recipe for beginners interested in making thier own potting soil.
How Much Commercial Potting Soil is Enough?
If you purchase a 2-cubic-foot bag of commercial potting soil you can plant the following:
20-22 gallon sized containers
35-40 pots 6″ deep
If you purchase a 4-cubic-foot bag you can fill a planter box 24″ x 36″ x 8″ deep.