It's hard having little or no outdoor space. But I think for those containers, I'd just buy some bricks of coir, which take up almost no space until they are soaked. And a big bag of perlite and some slow-release fertilizer. Miracle-Gro Potting Mix is peat, perlite and plant food, with some wetting agent and "bark fines." You can buy bark fines, but I wouldn't bother. They're kind of expensive for what is essentially pine bark sawdust.
MG Moisture Control Potting Mix just adds palm coir. And you don't need peat if you use palm coir.
Let's run some numbers and see how Miracle-Gro Potting Mix stands up. To tell you the truth, I never compared the costs. Six cubic feet is about $60 at Walmart.
DIY:
Palm coir. Ten pounds $20 from Amazon. That's a bit more than two cubic feet expanded. Let's say twenty pounds, four square feet, $40.
Perlite, 4 cubic feet, $40. From Amazon. Walmart it too high.
Compost. Good compost. 40 pounds Timberline compost from Walmart, $2.50. By two, or three, suit yourself. say $5. 40 pounds is one square foot.
So now our DIY potting mix is up to six square feet of potting mix, less some slow release fertilizer, for $85.
BUT Miracle-Gro doesn't say how much of that bark fines is in there by proportion. A lot, I suspect, because they also add a wetting agent, and you only have to do that when a major component is difficult to get wet. And bark fines is still composting, which is not something I'm attracted to in a mix. But that's probably a fine point, and I've used a lot of MG Potting Mix over the years.
But economically, it appears to be close enough to a toss up. And it's really pretty much a one-time purchase. You can top up the fertilizer at intervals. Personally, I like to know what's in my mix so I can judge how well different recipes do. And I add vermiculite for water management. And because it's all custom mixed, I don't have to keep different dedicated bags for containers, raised beds, different plants, seed starting, rooting, etc.
