No matter what shade your green thumb, chances are you’ve got (or once had) at least one plant in a container, most likely potted up in some blend of store-bought potting soil or mix. This soilless media was designed to grow plants in an unnatural environment. After all, plants grown in plastic containers in regular soil are prone to many problems, since plastic pots don’t allow air to flow through. Plus the few small holes at the bottom just aren’t adequate drainage for real soil. So professional growers and home gardeners have turned to natural, soilless blends to assist in getting air to root systems and create quick drainage. (Both are necessary to succeed in growing plants in containers.)
This
rose plant is very yellow, with the newest leaves definitely pointing toward a lack of iron in the potting mix. The problem can be fixed with an application of bonemeal or chelated iron.
Photo Credit: Tammy Clayton
This
Loropetalum is either suffering from not enough water or the potting mix has too high of a pH level (alkaline).
Photo Credit: Tammy Clayton
The yellow leaves on these Japanese cleyera show signs of drought and a lack of available nitrogen in the potting mix.
Photo Credit: Tammy Clayton
The Gerbera daisy in the front has gotten too dry, but it’s also suffering from a lack of iron or too low of a pH level in its own personal little potted world. The pH in potting mixes can drastically change from pot to pot – even if all the mix came from the same bag.
Photo Credit: Tammy Clayton
These plants were just delivered only a few days ago from the grower’s facility, and they’re already pretty dry. Better moisture retention in the mix would’ve helped this situation (a lot).
Photo Credit: Tammy Clayton
No matter what brand of potting mix you buy, it’s sure to contain varying portions of fine pine bark, perlite or vermiculite and perhaps a pinch of topsoil. But the common main ingredient among the different brands is sphagnum peat moss. Sphagnum is naturally very acidic, with a pH of 3.5-4.5. (High acidity means the same as low pH.) Overall, soilless media have very low pH levels. While manufacturers blend in dolomitic lime to help raise the pH, there can still be problems when trying to grow plants that don’t like acidic soil.
It’s also hard to get a good bead on the pH within the mix of the medium, so it’s typically stated as a pH range like 3.5-7.5. (Most likely this is from the loose, fluffy sphagnum versus the weighty chunks of limestone.) And some parts of the bag can be less acidic than others as the lime shifts around during shipment.
While many plants adapt to lower pH levels, others turn a sickly yellowish green and begin declining. And yes, some will die quite rapidly. But luckily you can do something about this unfortunate situation. If you know what pH your plants prefer, you can amend your potting soil before you plant. Sometimes adding more lime is fine, but other plants might do better with a little topsoil added into the mix.
In my own small nursery, I potted plants up with my own mix of 50 percent topsoil and 50 percent Michigan peat with a regular soilless mix and vermiculite. Incidentally, plants I brought in from another grower required far more watering than those I potted myself. The plants also had a different leaf color (mine were a deeper green, while the plants I brought in had a yellow tinge to the leaves). Customers swore they were buying two different plants when they bought one growing in my mix and one from another grower. But after a few weeks in the ground, that difference in foliage color disappeared.
The yellow tinge to the leaves of the bark-mix-grown plants was directly due to the potting media’s pH. While the level wasn’t off enough to hamper lush growth, it did affect the plant’s iron uptake. Iron is what allows chlorophyll to form in a plant’s leaves and stems. Chlorophyll is what makes plants green. Pale or somewhat yellow leaf coloring can be a sign that there’s not enough iron available to the plant – a sign that either the nutrient isn’t in the soil or something is preventing the plant from taking it in.
Sometimes the best way to approach soilless mixes is just to experiment with them along with your containerized plants. Last summer I started testing different packaged potting mixes. First, there was a Miracle-Gro® mix, which showed good moisture retention but often “floated” around after watering – exposing the roots and settling higher at the rim of the pot after the weight of the water pushed it away. Next I used a Fafard® potting mix, which floated around even more, and it didn’t retain moisture as well either – so I stabilized it with some real topsoil.
Later, when my Gerbera daisy’s leaves began to turn a sickly shade of yellow-green, it dawned on me that the acidic potting medium was likely the problem. Within a couple of weeks of repotting in an amended mix, new growth appeared in a healthier green.
My “scientific recipe” to cure the floating, as well as the acidity, was 1 part topsoil to 2 parts Miracle-Gro media with a couple of handfuls of pea gravel for better drainage. To “correct” the Fafard-mix containers, I used 1 part topsoil to 1 part potting mix and just half the pea gravel since the moisture retention wasn’t as good. (The addition of topsoil also helps to raise the pH level.)
If your container plant likes it dry, add a bit more gravel to sharpen the drainage. If the plant adores moisture, then put most of the gravel for drainage at the bottom of the pot and a smaller portion within the mix to allow for air and some down flow motion for water to travel during periods of heavy rainfall.
The point is, the idea that a “one-size-fits-all potting mix” doesn’t ring true for all plants. If you notice some of your prized potted possessions in distress, they’re telling you they’ve got a problem. Figuring out what’s wrong isn’t always easy. Before you assume that you’re just over-or under-watering, do a little investigative research. If it’s not a pest or disease issue, it could be a nutrient issue caused by the potting mix – and it just might be time to start mixing in your own amendments!
