I really enjoy the debates that gardeners have regarding this issue. For the frugal-minded, it’s a no-brainer. The purist, on the other hand, has no qualms discarding potting soil after a single use. So why the difference? And who is right? – if there is a right and wrong.
Here’s some background reading [link since removed] if you’ve never contemplated the question. In summary the arguments feature like this;
Advocates of recycling claim sphagnum peat moss, perlite, and vermiculite, the main components in soilless potting mixes, are mined or manufactured from non-renewable resources. The relatively high price of commercial potting mix is also cited. So, in view of these reasons, they say used potting soil should be rejuvenated and recycled.
Opponents of recycling contend that used potting soil could contain disease pathogens and usually loses its nutrients, porosity and much of its organic content. They also say nematodes may have invaded the container during the past growing season.
Whenever I contemplate this argument I’m amazed that no-one brings the question back to, “Do we reuse our garden soil?” The answer is obvious. Yet, if we considered the debate from this vantage point then we wouldn’t really have any discussion, would we?
Garden soil can’t be discarded after a single use even though it harbours the very same problems that proponents of single use potting soils argue. However, gardeners know that to continue growing similar plants and vegetables in the same spot year after year without doing anything to improve the soil will in fact reap a whirwind of plant failure. Instead we alternate crops, amend the soil and even leave fallow for a season.
The same logic needs to extend to potting soils. If you were to use the same potting soil for one plant family (for example Solanaceae – tomatoes, capsicums, eggplants etc.) every year without doing anything to the mix then indeed you would face many growing problems.
While I would argue that potting soils can be used time and time again, I would qualify my comment that unless the potting mix were to be rotated through different plant crops or amended after each use, you may as well discard it.
Soil is a medium for plants to grow in. It’s where they get most of their moisture and nutrients and it’s the glue that supports the growing plant. Each plant will suck out as much nutrients as it needs thereby rendering the soil inadequate again for that plant. Therefore the soil needs to be replenished so that it can accommodate other plants.
We understand this when it comes to our garden soil – so maybe we need to apply it to our potting soils as well.