Potting Soil for Planters


I have big planters ( indoor and outdoors )
Should the entire planter be filled with soil ?
I’ve heard that a filler can be placed half filling the planter , what would be a light filler material before we put the soil . Thank you


Thank you very much for posing such an interesting question to the Toronto Master Gardeners website.

Filling very large planters all the way with potting soil can certainly be very expensive, but I think it is worth it.

  • Good soil can last for many years, with a top-up of some compost each year
  • If there is a division of totally different mediums (soil and then peat moss), the roots only go as far as the soil.
  • Plants such as tomatoes, like a lot of room for their roots. If the planter has a light filler material at the bottom, in effect, you have cut the size of the depth of your planter. Instead of having a three-foot deep planter, you now only have a two-feet deep planter — and your tomatoes will not be so happy.
  • You could put peat moss, sand, or even plastic pots at the bottom of your container to reduce the quantity of space you have.

Since you have big planters, it may be worth your while to mix your own potting soil. My recommendation is that you get the basic idea for a potting-soil recipe from the Square Foot Gardening Foundation: https://squarefootgardening.org/2020/05/what-is-peat-moss-sfg-mels-mix/

However, the Foundation recommends adding peat moss to your mix. Keep in mind that peat moss is now not considered good practice for environmental reasons. Peat moss comes from the sphagnum moss bogs of Canada, and depleting these moss bogs is detrimental for the environment. The Royal Horticulture Society in England is now strongly recommending to their gardeners to use well-rotted animal manures or composted plant remains along with wood chips, wood shavings, bark and other materials as a substitute for peat moss. I might add perlite to that list.https://www.rhs.org.uk/science/gardening-in-a-changing-world/peat-use-in-gardens/peat-alternatives

Happy gardening this weekend!