I have been doing a lot of thinking and reading about potting mix recently. For one thing, it is only four months till we start seedlings for 2016! (And only a month or so before we winter plant seed for perennials.) For another thing, I am thinking about using lots of wicking planters for greens in the upcoming year to improve their survival in our dry climate. Finally, a movable greenhouse has many benefits. I have been considering making our hoop house on the Littleton farm moveable. That way, we could start hardy spring greens, and then, once they were going and the weather was a bit more moderate, I could slide it away and start summer seedlings on the new site. Right now, spring plants languish in the increasing heat in the house, while delaying the summer tomatoes and peppers, and the tomatoes keep out the Fall kale and lettuce. However, a movable greenhouse is a big investment, and it might be better to just plant everything in containers, and then move and swap those. An unproven idea that we might try.
Anyway, there are lots of reasons for me to be considering potting mix strategies right now. For the past ten years or so, I have been using compost/ peat/ perlite type potting mixes. They are a logical extension to my organic gardening philosophy of feeding the soil, not the plant. However, I just now did some research and found a forum thread where a member was advocating a bark/ peat/ perlite mix, with the ratios being 5 1 1, very light and airy. They then add some commercial slow release inorganic fertilizer. The reasoning behind this is that a pot of fine grained potting mix contains a perched water table, where the natural attraction of the water to the soil particles is greater then gravitational pull. This zone becomes airless and is off limits to roots. So, some advocate separating the two functions of potting mix; structural support, and the provisions of nutrients. Of course, this is just what conventional farmers do; they view the soil as an inert medium for holding plant roots, and then douse the soil with chemical fertilizer. However, since the pot is already an artificial growing area with different conditions then the soil, it makes sense that a different approach might be warranted. As an organic gardener, I would use organic slow release fertilizer, or a liquid fish/ kelp product if I switch over to a coarse grained medium.
I may do some experiments to compare the two approaches.
I also may include a small pot or bag of worm castings in larger, long term pots, which would provide fertilization without gumming up the whole pot.
Here is a link to the forum thread that got me interested in this: http://forums2.gardenweb.com/discussions/2842847/container-soils-water-movement-and-retention-xxii?n=329