Starting plants indoors? Think about the soil
Seeds are arriving in mail and I’m thinking about planting them, but as they’ll be growing under lights, I’m only thinking. It’s still too early to start most. With exceptions, March or April is soon enough; however, this is a good time to think about the soil that you’re going to be starting your seeds in.
In the old days, gardeners prepared their own seedling soil using garden soil and leaf mould, or peat moss if it was available. Consequently, there was always the risk of disease or weed seeds in garden soil. Some avoided this by cooking the soil to sterilize it. Hence the popular song: Home, home on the range, Where I sterilize soil in a pot, And broil every bug in the soil that I dug, And hear “What’s that smell?” quite a lot.
Today, we can purchase potting soil, with some types especially formulated for seed starting. Like everything, there’s a wide range in quality and there’s no requirement to list the contents on the bag, although the better ones do. Some have a small amount of slow release fertilizer or even beneficial mycorrhizal fungi added.
This can be helpful though it’s not essential for seed starting. It is important that the medium is clean, and we often hear about using a sterile product; however, that’s only a descriptive term. Most commercial ones do not go through a process of sterilization as if they were destined for a laboratory.
Even if they did, it would be difficult to maintain once the bag is opened, and although we’d like to be sure there are no pathogens in the soil, it’s not usually an issue. Even if your potting soil is not absolutely sterile, the typical contents start out quite clean as they don’t contain any actual soil.
Most seeds grow best in a lightweight medium that provides a balance between moisture and air. Potting soil does not usually contain actual soil, but rather a blend of materials that may include peat moss, vermiculite, perlite, sand and lime, and is described as a soilless mix.
Peat moss is used as the main ingredient in most because of its ability to hold moisture while providing high air porosity. It’s the dead material that has decomposed and accumulated in the lower levels of a sphagnum bog and is essentially weed- and disease-free. Coir, from the husk of coconuts, is sometimes used as an alternative to peat moss.
Two of the other main ingredients are also lightweights when dry. Take a close look at a handful of potting soil and it should be easy to spot the perlite and the vermiculite.
Perlite resembles small, crystallized popcorn and is just as airy and light. A silicate mineral, the crude perlite is heated to more than 871 degrees Celsius (definitely sterile), expanding rapidly and creating countless tiny bubbles in the process.
Perlite loosens up any soil and used alone, it is an excellent medium in which to root cuttings.
Vermiculite, another volcanic silicate mineral, is easily seen in the potting soil as shiny, gold-coloured flecks. Like perlite, vermiculite is lightweight, but it’s spongy rather than crisp, and easily soaks up moisture.
Sand is sometimes added to potting soil to further improve drainage, and lime is included to counter the slight acidity of peat moss. Other ingredients may include wetting agents to promote absorption of moisture, slow-release fertilizers and charcoal, a natural filter that absorbs any odours.
When dry, the final product is light and fluffy and if you can hoist a bag easily, the stuff is probably just fine for starting seeds — but not yet.
To chat with local gardeners, share tips, pics, discuss potting soil, see Grand Gardeners on Facebook at www.facebook.com/groups/Grandgardeners/.