What do you think of when you hear the words light and airy? If you're like most people, it probably isn't potting soil. But light and airy is a good description of the texture of high-quality potting soil.

I know people who spend a lot of money on plants and containers, only to then fill their pots with the cheapest potting mix they can find. Then they wonder why their flowers and vegetables don't grow well. The sad truth is that most commercial planting mixes are unsuitable for use in containers straight from the bag. And the less expensive your mixture is, the more problems you will probably have with it.

Successful container gardening requires excellent soil. Use the wrong planting mixture and you will likely be disappointed with your results. Mother Nature intended for plants to grow directly in the ground where, among other things, they will have good drainage, adequate air circulation, natural aeration due to worms, access to rain water, room to spread their roots and access to naturally occurring nutrients in the soil. Planting in containers creates an artificial environment. This means that you have to compensate for the things your plants will be missing by not being planted in the ground.

You may be tempted to go out into your garden and dig up soil to fill your pots. If plants grow so well planted directly into the ground, then why won’t they grow in the same soil in your containers, right? Because garden soil is too heavy and dense to be used successfully in pots. The particles in garden soil are tightly clumped together. This results in not enough oxygen around the roots of your plants. It also makes it much more likely that water will collect around the roots of your plants, causing root rot. So don’t put straight garden soil in your containers.

Commercial Potting Mixes

Most commercial potting mixtures contain generous amounts of peat and perlite. Some of them don't contain any soil at all. The more expensive the mixture, the finer it will be milled. Common ingredients are finely milled sphagnum, perlite or vermiculite, ground bark and fertilizers. Some add gels that absorb water and release it slowly to the roots of your plants.

So why do these mixtures work better than garden soil? Because they are light and less dense. A good potting medium, whether mixed at home or purchased in a bag, is going to be light and airy. It will allow the roots of your plants to absorb oxygen, water and other nutrients. 'Light' is the operative word here. When purchasing a potting mixture, pay attention to the weight of the bag. You will likely see many bags of similar size. But one may weigh 40 pounds, while a similarly-sized bag may weigh only half or a third of that. Given this option, always choose the lighter-weight bag.

If the only thing you can find is heavy and dense — some cheap mixtures are nearly indistinguishable from topsoil — then you will also need to buy peat and perlite or vermiculite and lighten the mixture before you put it into your containers. Use four parts heavy potting mix or garden soil to one part sphagnum peat and one half part perlite or vermiculite. Most commercial mixtures use perlite. Perlite and vermiculite work similarly in your mixture, but are not chemically identical. Both will provide space between the other particles in your mixture, but the perlite will not absorb moisture. Many growers prefer it for this reason. I personally use vermiculite, but that is probably Hillbilly contrariness.

Make it Yourself

I plan to make my own mixtures for my containers and raised beds. First, in spite of my grey hair I'm a kid at heart and enjoy playing in the dirt. Second, I am going to need a lot of soil this spring to top off the huge vegetable planters I made this year. Buying enough bags of a good-quality mix would cost a fortune. I can save at least fifty percent by making my own.

There are many, many recipes for making potting mixtures. The one I am going to give you works well for me and is relatively inexpensive. Plus, it does not contain sand, which is a common ingredient in many mixtures. I hate lugging bags of sand around. Too hard on the old back. Measurements are by volume, not weight. Cut the top off of a gallon/4-liter container to use for a measure.

Hypatia's Potting Mix

• ½ bushel (4 gallons) horticultural grade vermiculite
• ½ bushel (4 gallons) shredded sphagnum peat moss
• 2 gallons compost
• 1 cup granular slow release fertilizer — 5-10-5
• 4 Tablespoons 20% powdered superphosphate
• 8 Tablespoons ground limestone or calcium carbonate

This mixture works well for most plants. It is too alkaline for acid-loving plants such as blueberries, azaleas, rhododendrons, gardenias or camellias. For these plants, you will need to add one tablespoon of potassium nitrate and three tablespoons of dolomite.

I'm going to end this series on container gardening by discussing briefly what size containers you will need for your plants. Unfortunately, there isn't a magic formula that will supply this information. I am going to rely on your common sense. One marigold or petunia will happily live in a six-inch pot. But many people prefer to create gardens of flowers or herbs in larger containers, planting close together to achieve a quick effect. Most annual flowers have relatively shallow root systems and so you can get away with crowding them. It depends on what you have in mind and what you are trying to achieve.

If you are planning to grow edibles in containers, you will have to give your plants enough room to mature and set fruit. Most vegetables require at least a five-gallon container and most small fruits require at least a ten-gallon container. There are so many hybrid, dwarf varieties of fruits and vegetables these days that it is now possible to find varieties specifically bred for container gardening. I wouldn't plant a tomato in a container smaller than ten gallons. But that same container would grow three bell peppers.

If any of you have questions about container size for specific plants, please ask. There are just too many possibilities to list them all here.

Happy planting.

The Cranky Gardener Archive

Hypatia

10.11.05 Front Page

Back Issue Page