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Index : Container Plants & Patio Gardening
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- 6. Potting Soils Make the Difference - Top
- There are two keys to success when it comes to growing beautiful potted plants: healthy plants and good potting soil. Examining plant roots, leaves and stems will give you a good idea about the health of the plants your purchasing. The potting soil you choose to plant them in can be more difficult to evaluate.
You can’t open the bag to examine the potting soil prior to purchase, but you can examine the label for ingredients. Most potting soils will list the composition of their product. Many are comprised of mostly peat moss. The quality of this moss can vary. Sphagnum is a type of peat moss that is only partially decomposed. It’s lighter and looser than plain peat or peat moss and is preferred for potting soils. In addition, some potting mixes contain coconut husk fibers called ‘coir’. These fibers are very fine and are used as a ‘renewable resource’ substitute for peat but do not necessarily improve the quality of the potting mix.
One of the biggest draw-backs of many potting soils is that they’re just too dense and heavy. They don’t allow for adequate drainage and air movement through the soil, both extremely important for healthy plant growth in pots. Some mixes contain composted bark, which keeps the soil looser and lighter. Others contain white, crusty particles called perlite. Made by super-heating crushed volcanic rock, perlite is an inert material that can also loosen the soil and improve drainage and aeration. The problem is that most potting soils don’t contain enough perlite to be beneficial. That brings us back to the need to physically examine the potting soil.
If the labeled ingredients seem satisfactory, and the weight of the bag isn’t too heavy, then give it a try! However, just buy one bag at first. Open the bag after purchase and feel the soil. Does it seem light and fluffy, or heavy and dense? Does it contain course materials like sphagnum moss and bark slivers or are all the particles extremely fine that could hold too much water? If you like the soil following your inspection, then purchase more. If not, try another brand.
One way to control the quality of your potting soil is to mix your own. An excellent potting soil can be made by mixing one part sphagnum moss with one part perlite and one part vermiculite. Vermiculite is a heat expanded clay with great nutrient holding properties. All of these ingredients can be purchased separately in small or large bag quantities at most garden stores. An easier option is to buy an inexpensive potting soil of mostly heavy peat and lighten it by adding a generous amount of perlite. One part perlite mixed with four parts of a heavy peat potting soil will also make an excellent mix for growing your potted plants.
Some potting soils come with water absorbing polymers and advertise special water-holding properties. These polymers have not been found to significantly increase water-holding capacity or make more water available to plants. In general, the problem with most potting soils is that they hold too much water, so selecting mixes with special water-holding ingredients is not advised.
Finally, many potting soils now incorporate fertilizer into their mix. Most are slow release nutrients including nitrogen, phosphorous and potassium. Although beneficial, the amounts used are small and should not be depended on solely to provide the necessary nutrients for your plants. I prefer adding a timed-release fertilizer to the potting soil at the time of, or immediately following planting. Follow label directions on the amount of timed-release fertilizer to use based on the amount of soil and the pot size.
Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona,
520-626-5161 - Updated: November 12, 2006
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