HOW TO PICK OUT THE RIGHT POTTING SOIL
I would hazard a guess that most gardeners are pretty comfortable with the term potting soil. Everyone who has grown any kind of plant in any kind of container knows that potting soil is essential for keeping the roots alive. However, many gardeners find it a bit more challenging to describe what constitutes a high quality potting soil.
The good news is that it’s really not that complicated — if you understand the basics.
A BIT OF SCIENCE
For those who garden in containers, it will come as no surprise that high quality potting soil is fundamental for growing great plants. Taking regular soil from the garden and dumping it straight into any container simply won’t provide a great root zone environment. Garden soils are far too dense and heavy, and if they don’t kill your potted plants they will certainly destroy your back.
True potting soils are an entirely different beast. The best ones have a high percentage of coarse-fibred peat moss, and thus great “bulk density” (not too heavy, nor too light textured). But not all peat moss is created equally. There are several types of peat moss used in potting soils — hypnum and sedge peat moss being the most common — but they are too dense, too decomposed and often much too acidic compared to sphagnum.
Sphagnum peat moss is the king of the mosses because of its superior water-, air- and nutrient-holding capacities. Still, even though sphagnum is the best peat moss for containers, it still needs a little help with respect to optimizing drainage. Perlite — a rock that is super heated and popped like popcorn into a lightweight, nearly pure white, coarse granule — is the partner of choice for helping out with drainage. Typically, good quality potting soils will contain roughly 80 per cent sphagnum peat moss and about 20 per cent coarse perlite.
However, while the physical properties of the potting soil are addressed with the combination of peat moss and perlite, there is a chemical problem that must be addressed. Sphagnum peat moss is acidic, so the addition of horticultural lime is necessary to sweeten the potting soil and steer the pH into the correct range for plants.
For potting soil, pH values less than about 5.8 or greater than 6.4 are outside the “sweet zone” for plants. If the potting soils are above or below this range, the majority of plant species have difficulty reaching their full potential. I’ve tested thousands of samples of potting soils over the years, and it’s amazing how often too little lime has been added to potting soils or, even worse, the lime has been left out entirely.
Beyond peat moss, perlite and lime, there is one additional tiny ( but critical) component that must be added to potting soils. It’s called a “wetting agent.” Wetting agents allow water to move readily throughout the entire potting soil mixture, and thus make the water “wetter” so to speak. A common household product that has wetting agent properties is soap, although it’s definitely not recommended for use in potting soils because it can also severely burn plants.
Keep in mind that it’s OK to add some compost or well-rotted manure to the potting soil to “beef it up” a bit without comprising the integrity of the mixture. I like to add some sea soil (composted fish waste and bark) to my potting soil because it adds some nutrients and beneficial microbes.
If you are amending your potting soil with manure be cautious. Never add more than about 20 per cent to your potting soil, and ensure that it is from a reliable source. Also, keep in mind that some manures have exceedingly high levels of injurious salts, along with weed seeds and plant diseases.
WHAT SHOULD YOU DO?
It’s a good idea to have your potting soil tested if you’re unsure about its quality. A basic soil test isn’t expensive and can save you a lot of money in the long run. Mixing your own potting soil is an option but it can be messy and dusty, so I would recommend testing out a small batch before attempting to mix a large quantity.
Finally, remember that potting soils are not only good for the plants but they are much safer for your back. Gardening is an enjoyable past time but, at times, it requires some strength and athleticism. Lightweight potting soils can help keep you off the injured reserve list and enjoying your garden to its fullest.