1) Yes, you can use a soil you make yourself, with small pine bark chips as the main ingredient. There is a recipe on the link I provided at the end of the post.

2) Depends on what the plant material is. The asphalt will absorb sunlight & buffer ambient temperatures somewhat, but you should still subtract a zone & use plants hardy to zone 5 to be safe.

3) May not be necessary. There is nothing in soil to harm asphalt, other than water, which could seep into cracks, freeze & expand - damaging the paving material, but the same conditions exist over the entire paved surface. The difference is a more nearly continuous source of moisture under the planter, but the planter will offer insulation & probably keep the water under the planter mostly in liquid state. Use your judgment here. Expect a color contrast in pavement colors when you remove the planter, however.

Here is a link that might be useful: C'mon! I'll take you to where you can find a good soil recipe.

Very informative link, Tapla. Thank you! Now all I have to do is find a place with pine bark chips (not so easy to do without a car in the middle of the city!) and I'm rearing to go! In the event that I can't find the pine locally, how 'soupy' does peat become? I've found one recipe for soil which consists of equal parts peat, compost and sand but I'm concerned that I'd be wasting my time and money going that route if the drainage isn't stellar.

Thanks again!
P.

Many people are consistently able to grow very healthy plants in peat based soils, so it's certainly possible, even if a little more difficult. The original post & many of the follow-ups in the link I provided were written by me, & I use my own soils for the reasons outlined therein. Having done lots of experimenting with various soil components, I'm familiar with their effect on soil structure & chemistry. I use sand in some soils, but it's not the material that most consider sand to be. I use sand composed of particles that are somewhere near the size of BBs. Some a little larger & some a little smaller, but the particles are large. I don't recommend you use beach sand, play sand, or builders sand because though it is effective in promoting drainage, it's small size is death on aeration & it clogs all-important macro-pores or eliminates them.

I regularly catch flack for speaking out against the use of compost in container soils. Already in an advanced state of decay, it breaks down quickly in soils, reducing aeration and retaining water. The additional retention of water as a result of its use further hastens not only the breakdown of of the compost, but the other soil components as well. Its one advantage, a possible source of minor elements, is outweighed by its deleterious effects on soil structure and can be compensated for by simply adding the minors in chemical or organic form. Some tout its use, citing the microbial activity it provides. All that's needed for all the microbes a 100% organic soil like most of us grow in can use, is a little moisture & some nitrogen. It's good to remember that too much microbial activity in container plantings has a negative effect on soil structure as a result of the rapid cleaving of hydrocarbon chains in any organic molecules - accelerated soil collapse.

Though not ideal, I think you'd be better served by using a good variety of commercial soil with some perlite & pine bark added than to try a mix based on peat/compost/sand.

Good luck in your growing endeavors.

Al

"Too much microbial activity" is something that in nature can't happen as once the population reaches a certain size based on available nutrients, it will either level out in response assuming a continual supply is made available or crash if the nutrients are depleted, afterwhich they would slowly build up again upon further addition of those nutrients via whatever source. Ponders and other aquarists see this happen frequently with organisms like algae when they are just starting out with a new pond or aquarium. The goal being to come up with that proper balance of nutrient load vs microorganism.

The only way to know for sure what works for you is to try some things yourself and eventually tweak based on the amount of time and effort you are willing to expend on it.

There are some concrete planter boxes with similar dimensions all around downtown Philly and it seems that they have regular soil in them. They have japanese hollies in them and some rotation of annuals planted around that. I don't know if I would have chosen to use that but apparently the plants are doing just fine for that style container.

"Too much microbial activity" is something that in nature can't happen as once the population reaches a certain size based on available nutrients, it will either level out in response assuming a continual supply is made available or crash if the nutrients are depleted ...

What you set forth would kind of fall apart if the original premise was not correct? Too much microbial activity - what is too much? Is an infection evidence of too much? Surely the limiting factor in treating infection is exposure to something pathogenic to the cause (of the infection), the immune system, or medication, not nutrient supply. If too much microbial activity was of no concern, we could grow in chopped celery. It retains its shape and the irregular shape would be good for drainage - right? So why don't we grow in chopped celery or shredded wheat for that matter? Because microbial activity is so easily able to break the hydrocarbon chains, destroying cell structure, and creating an explosion in their numbers, which also impacts other soil component particulates, turning soil to soup.

Based on the statement that the microbial population would eventually level out, this should be no problem - let nature balance it, until we add additional celery? What if the plants in the container are dead?

Compost is just another candidate for a component of container soils. It's unnecessary for the "microbial activity" some think it promotes (celery does it better), and in my opinion the deleterious effects it has on drainage and aeration outweigh the potential micro-nutrients it might contribute. Plus, there are more efficient ways to deliver micro-nutrients - fish/seaweed emulsions, frits, and other organic or chemical products.

Interesting anecdote that Philly has plants growing in containers with topsoil in them, but I bet that in spite of their successes you won't be changing over anytime soon. ;o)

Al

Thank you for sharing your experience and expertise! I'm sure I'll be referring to this post and the one you recommended continually as my own level of experience increases over the next few seasons. In the meantime, I'll try the peat based container mix amended with pine bark and perlite for this year and next year, tackle the recipe together with a few of my neighbors (with vehicles) so we can share a nice big batch.

Thanks again,
P.