The Scoop on Potting Soil
Good gardens start with the soil.
The bagged product that you buy is frequently called potting soil, but that's an incorrect description. Ideally there is no actual soil in the potting mix. Plants grow best in a medium that remains lightweight and loose through daily watering. The seemingly contradictory potting mix advice is that it needs to be moist but well drained. It must retain enough water to supply the plant with all that it needs, yet it should release the excess moisture so it will drain out of the pot. This allows air to fill the
pore spaces so roots can have the oxygen they need for growth.
Why Garden Soil Won't Work
So why not just use soil from the garden? Garden soil, no matter how good it is, will compact in a container and hinder plant growth.
Southern soils vary widely from pure sand to pure clay, and most are a mixture, along with silt and organic matter. The component most easily lost is the organic matter, which is what keeps the soil loose and fertile. The South's hot summers hasten its breakdown, so compost, mulch, or both are frequently added to garden beds to maintain a suitable soil mix.
The same process happens in a pot, only when the organic matter is gone all that's left is an impenetrable block of the mineral components, which doesn't make plants' roots as happy as they would be in soil-free potting mix. Bagged potting mixes offer container gardeners the best chance of success.
Note: Bags sold at garden centers that are labeled "garden soil" may or may not contain actual soil. However, they are largely organic materials intended to enrich the soil in your garden, as opposed to supporting plant growth in a container.
What's in Potting Mix?
If there is no soil in potting mix, what's in it? The answer varies widely with brands. Generally, the staples of potting mix are Canadian sphagnum peat moss for holding moisture, aged pine bark and perlite (or vermiculite) to keep the mix fluffy, lime to raise the pH of the more acidic sphagnum peat moss, and fertilizer to nourish growing plants.
Usually a surfactant is included to make watering your pots more effective. Without it, the water runs through the dry mix and isn't readily absorbed. You may find other ingredients in the mix, such as sand, compost, Michigan peat, and polystyrene pellets, which are not always the best choice. In recent years, moisture-holding polymers have been added to potting mix, allowing it to hold more moisture without sacrificing its aeration.
While the fibrous, spongelike components in the mix physically anchor the plant and even hold nutrients to prevent them from washing out with the excess water, new additions may be biologically active as well. These include mycorrhizal fungi that occur naturally in undisturbed soils and boost the health of growing plants.
Can You Reuse Potting Mix?
Reusing potting mix is always tempting. But whether you reuse it or not is a judgment call. If a plant dies and the problem appears to be that the roots rotted (this is rare if the pot has drainage), then you definitely should not reuse the potting mix. When you remove plants from your pot, look at the remaining mix. Can you dig into it with your gloved fingers, or is it too compacted? Look at some of it in your hand. Is it crumbly with both small and large particles, or is it a dark homogeneous mass? If potting mix is old enough to be a compacted mass of muck, it has composted beyond its usefulness. Dump it into a garden bed, or use it to amend a planting hole in the garden. If the potting mix still looks like it did originally, mix it with some fresh potting mix to make it fluffy, and replant.
What's the White Stuff?
The white pellets in potting mix are perlite, polystyrene, or both. You can tell the difference between the two because perlite is an expanded volcanic mineral that crunches when squeezed between your fingers; styrene bounces back. Perlite is added to potting mix to keep the mix fluffy and help with drainage. Styrene is a poor substitute because it floats, covering the surface of the potting mix with white dots. Ultimately, it can blow or wash out of the pot into nearby streams and keeps going. Mixes with or without perlite are preferable to mixes with polystyrene.
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Sinking Potting Mix
What do you do if the potting mix sinks and the plant is growing in only half of the pot? It's a common problem. You might think the soil is being lost through the drainage hole, and that could be part of the problem. Admittedly, potting mix is also made of decaying parts of formerly living plants: aged bark and sphagnum peat moss are primary ingredients. As plants grow, they build living tissue from the dead organic matter in the pot. So as plants grow and as potting mix ages and composts, these organic components disappear, making the mix in the pot shrink. If you want the plant to remain in the same pot, the first instinct is to add more potting mix to the top. Some plants will survive that, but it's best to remove the plant from the pot, shake as much old soil from the roots as possible, and then repot with fresh potting mix, raising the plant to a level just below the rim.