The Dirt on Soil in a Bag

Its as simple as 1-2-3!

• Open the bag and pour it into your pot.
  
• Stick in some plants and presto, you have an instant garden.

Are you sure about that? Did you happen to ask your new plants how they feel about this one-size-fits-all situation?

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If you’ve ever had problems with plants in containers, don’t just assume you got a bad plant. Most people assume that if its sold in a bag at the store the stuff is a miracle wrapped in plastic. Well to humans it sure is nifty, but we don’t have to live in it. Let’s take a closer look at what is in that clean bag of dirt.

You can buy three different soil products in a bag and none of them are the same.

Topsoil in a bag does not mean it is worth it’s weight in gold. All it means is that the soil came from the “top” – the surface of the earth. The mystery here is you that have no idea where on the Earth. You should be a little bit concerned as to what exactly this soil was on “top” of. It could be from anywhere there was construction going on. It may be shredded, screened and mostly free of weed seed, but you have no way of knowing anything about the quality of the topsoil in that bag. There is no such thing as the dirt police.

Some brands are labeled “Potting Soil” and others “Potting Mix” … this can be confusing. Actually, neither blend contains very little soil, if any. The proper name of this substance loosely known as “potting soil” is really a “soil-less media”. If you open a bag labled “potting soil” and it is real soil – do not put it in a pot! At least without some judicial amendments.

Soil-less media was created to assist container-growers to raise plants in an unnatural environment. Plants grown in plastic containers in regular soil are prone to many problems as there is no available airflow through the plastic. Drainage through a couple small holes at the bottom intensifies the problem when topsoil is used for potting.

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So, we have turned to natural, yet soil-less blends to assist in getting air to root systems and create quick drainage. Both these elements are needed to succeed in growing healthy plants in containers.

All brands of “potting mix” contain portions of fine pine bark, perlite or vermiculite with perhaps a pinch of topsoil, with a main ingredient is of sphagnum peat moss.  Peat moss is naturally very acidic with a pH level of 3.5 – 4.5. A high level of acidity means the same as a high pH level.

Some areas of the USA enjoy “perfect soil” that almost any plant does well in provided it likes the climate there. Other areas have highly acidic soil that unless amended to lower the pH levels and kept at the desired range by periodic amending, the plant will slowly decline as the acidity level rises.

Still other areas have very alkaline soil that offers little nutrients to plants but drains very freely. You may wonder what this all means and why it is important to worry about since you are only trying to grow a few patio containers. To really be successful at growing plants in pots and rewarded with lush beauty … it IS important!

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Soil-less media have very low pH levels. Manufacturers blend in dolomitic lime attempting to make it less acidic, but there are still problems when trying to grow plants that do not do well with acidity. The pH level cannot be measured accurately within the mix of the medium.

No manufacturer will state a level of pH; it is always listed as a range of 3.5 to 7.5 pH.  Most likely this is due to the loose, fluffy nature of sphagnum versus the weighty chunks of limestone. Some parts of the bag to be less acidic than others as the chunks of stone shift around in the fluffy moss, especially after shipment.

While many plants adapt to lower pH levels, some will become a sickly yellowish green and begin declining and others will simply die quite rapidly.

Commercial growers know that certain plants require amending the media they use for potting to get the plant to perform appropriately. They know to rectify the situation before they pot the plants. The plant’s requirements will decide how they will change the potting mix. Sometimes adding more lime is fine; other plants might do better with a little more nutritious soil added to the media.

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In my small nursery of ornamental plants I potted with my own mix based on 50/50 topsoil-Michigan peat mix with potting media added for drainage and airflow. Plants that I brought in from other growers required far more watering than those I potted. At times when I was short on the number of a given plant for a customer’s needs, I would bring the missing plants in from another perennial grower. I found that the very same plant potted in her bark-based mix was never the same color of green as those I grew. The difference in leaf color was so noticeable people swore they had been sold two different kinds of plants tagged the same.

The other grower’s plants were very healthy and full, and after a few months in the ground the noticeable color difference of the foliage disappeared. The more yellow coloring of the bark mix grown plants is directly due to pH. The level wasn’t off enough to hamper healthy growth, but it was definitely affecting good 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 is not enough iron available to the plant whether it is not present in the soil or something is causing the iron to not be useable by the plant.

Last summer I was faced with a few simple pots on the porch, having relocated to a rented house and no Michigan peat to be had in the southeast. I started testing out different bagged potting mixes. First, there was MiracleGro mix, which I found to have good moisture retention but was poor at remaining over roots when I watered. The light sphagnum peat moss floats around and settles higher at the rim of the pot as the heavier weight of the water pushes it away.

The next set of pots I planted I used Fafard potting mix and instantly regretted my switch. This one was even worse at floating away from roots and did not hold moisture as long as the MiracleGro brand. I knew I needed some real soil to stabilize the mix.

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My Gerbera Daisy’s leaves began to turn a sickly shade of yellowed-green and I contemplated what it’s ailment was as I picked off the really sad looking oldest leaves. It dawned on me one evening at dark that perhaps the acidic potting medium was the reason for the Gerber’s angst. I repotted it under porch light. Sure enough within a couple of weeks new leaves popped out in a far healthier shade of green.

The addition of 33% topsoil (1 part topsoil to 2 parts potting soil) to the MiracleGro medium and a couple of handfuls of pea gravel to assist in drainage were the cure. This is my “scientific recipe” for a 10” pot. The repotting recipe for the Fafard mix containers was 50% potting soil (1 part topsoil to 1 part potting mix). The Fafard mix gets one one handful of pea gravel as it has less moisture retention than the MiracleGro mix. The addition of topsoil corrects the pH imbalance while also stabilizes the mix to keep roots covered after watering.

If the plant you have in that container likes it dry, you will need to add a bit more gravel to sharpen the drainage. If the plant prefers a lot of moisture, then you would be wise to put most of the gravel for drainage at the bottom of the pot and a smaller portion within the mix to allow for air to the roots and some down flow motion for water to travel during periods of heavy rainfall.

The best way to determine what acidity or pH level a plant you want to grow will not just tolerate, but do really well at would be to do your research before you go through the task of potting it up. If it really adores a lower pH then it would be wise to add some aluminum sulfate to your potting mix when you prepare to plant that container.

It is pretty easy to research this kind of information in this day of Internet at your fingertips. Much of the commercial growing trades technical information is readily available to anyone if they know how to type the search terms in the right way. To use the Gerber Daisy as an example, type in “Gerbera Daisy – pH level”.

To accurately determine the actual pH level of the media you are going to use for potting, it would be best to test a few samples from different parts of the bag.

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Plants in distress will signal to you that there is a problem. Figuring out what they are trying to tell you is not always easy. Before you assume that you are over or under watering, do a little research. Start by investigating nutrient deficiencies caused by your soil. You will find an excellent, easy to understand fact sheet on this web page: Soil Nutrient Fact Sheet.

If the problem is related to a pest and not a soil issue you might find the information from one of the university agriculture departments or an Extension Service website.

The other area you need to consider is whether it is a disease that has attacked your plant. Try doing a search on the description of your plant’s symptoms. If you can’t figure it out, take the plant or a piece of it into your local Extension Service office. They’ll find out what is wrong even it they have to send it to the lab for testing.