A tidbit about potting mixes:

The stuff we are growing the poinsettias in is called Pro Mix. This is from the back of the package. Check out other brands of potting mixes – many have similar ingredients.

Canadian sphagnum peat moss comes to us from bogs, coniferous forests, and wet tundras of the north. Over eons it can build up thick in such places as mosses grow, live, then die. It is able to absorb a lot of water; hence in the olden times it was commonly used for diapers or for wound dressing. It is also useful as tinder, or to burn for fuel. These days it is mined to be used as potting mix. It is acidic in pH, in the 3.0 -4.5 range.

Perlite is the little white exploded expanded volcanic rock used to aid in drainage and aeration and keep the soil mix permeable. This ensures that the plant roots are happy and not sitting in wet goopiness.

Dolomitic and calcitric limestone are added to balance the acidic peat. Limestone is pH 7.5 -8.0 and basic. Most plants like a pH of about 5.5 – 6.0 – 7.0.

Some plants prefer an acidic soil: rhododendrons and ericas (pH 4.5 – 5.5), or the oblong leaf sundews (pH 3.7 – 5.3). There are also other plants that like it more basic and alkaline like the Pinguicula butterworts (pH 7.0-8.0). It is important to keep the pH of your soil within a certain range, otherwise nutrients can become unavailable to plant roots. You can test your soil’s pH with a slurry method, using a pH meter or paper test strips.

Wetting agents are chemicals added to help the water to infiltrate and evenly wet the soil mix. It does this by breaking apart the ‘stickiness’ of water (lowering its surface tension). A common wetting agent when we use when watering street trees is Dawn dish soap. If you do not use a wetting agent, you may see the water just sort of roll off the soil. This is because water is cohesive and prefers being bound to itself rather than breaking apart and going into the pores and cracks of the soil.

Mycorrhizae is fungus root. The species in this package is “Glomus intradices 1 active propagule/gram”, which is about 27,216 spores per 60 pound bag of potting mix. So this is a relatively new additive to potting mixes. What scientists found out is that in nature, plants are working with partners underground – fungi. Plants give the fungi some of the sugars they make from photosynthesis, and in return fungi bring the plants water and nutrients they collect from all around. A mutually beneficial symbiotic relationship. There are fungus that live on and around the plant roots, as well as fungi that live inside the roots. Glomus lives inside and in between the root cells.

The whole idea is that by inoculating the potting soil with these fungus spores, the spores will grow into a network of fungal threads around the roots. it will then help the plant improve its nutrient and water holding ability, and increase the intake of of nutrients like phosphorous. By the way, Glomus intradices was recently renamed to Rhizophagus irregularis. On the package, it says “do not sterilize or pasteurize”, likely because that would probably kill the fungal spores.