Hi, PB ;o)
Well, your mix initially has some volume or loft because the small particulates don't fit well together yet. I don't think it's so much a problem with soil collapse because of decomposing. Rather, I think that watering, over time, continually rearranges the particles so they mesh together & this eliminates the loft or decreases aeration and causes what you observe to be shrinkage. Increasing particle size, more uniform particle size (not always practical to try to achieve for the average grower) and using a bark-based soil can help to greatly reduce shrinkage and insure aeration.

You'll need to address the possibility of not enough water retention in whatever mix you come up with, but I'm thinking that if it becomes an issue for you with a more aerated mix than you're using, the water retaining polymer crystals might be a solution.

I am finding it difficult to get affordable perlite. A 2-3 cup bag is around $3.50.

Keep looking. I promise it's available where you are in 3-4 cu ft bags (huge) and they'll be less than $20, probably around $15. Ask at nurseries or greenhouses, they'll have a source if they don't have it.

What is your opinion on using rabbit manure as the 'manure' part of your mix?

The original thread you read is very old. I first posted it in spring of '03. It hung around quite awhile & then I posted it again in Spring of '05 and then JD reposted it this year. Since I wrote it, I've stopped using any manures. Too many weeds in some & it breaks down too quickly to please me. The rabbit manure would fall into this category & is probably best saved for the garden. There are lots of micro-nutrient additives out there, but I'm pretty confident you'll not find household ingredients to supply them. I can help if you write off forum, or you can search out fertilizers that contain them and use those in your supplement program. I don't use coffee grounds either. They are weak in nutrient value and only add a little N to soils over time. Not a good trade, as they tend to fill air spaces in soils as they quickly break down.

I am thinking of switching to a time-release, but not sure how much to throw into the batches of your mix.

I don't remember the exact formula right now, but it's around 19-6-12 that I use initially. I probably use around 1/4 - 1/3 cup per cu ft of soil, but that is only insurance & won't last the whole season. I start a supplemental program almost immediately after building the planting.

I began composting, and plan to use that in place of the cheap bagged potting soil ingredient.

Of course, it's your soil, but compost adds little in the way of nutrients & tends to compact and settle into soil macro-pores. I never use it in containers. I've had many spirited discussions about the advisability of using it in containers. Most that argue for its use draw no delineation between garden culture and container culture. They figure that because it's good for gardens, it must be good for containers. You can see I disagree. The other common argument is "It works for me", which doesn't really convince me I should try to make it work too. You'll have to decide this for yourself.

I wasn't too sure if I was running on a parallel track with you, or if I went astray somehow. If you have other questions, please voice them & I or another will help further, I'm sure.

Good luck - take care

Al

PB - please contact me off forum. I have a question for you.

Al

glad I ran into this thread because I was just coming to the computer to hunt you up Al. Now there's a grammatically correct sentence!

I used new commercial Pro-Mix this year in every container. In past years I've mixed previous yrs soil with bark fines, new soil etc to make a more friable mix but kept attracting skunks and coons that tore up my containers. So this year I started all over again.

I find that it doesn't settle much so that is not a problem but it does really hold moisture. I've barely had to water. For some things like dahlias I cannot understand why I don't get better results by this time of year. I've fertilized adequately with Mighty Plant and tomato fertilizer, and using Messenger. My containers are generally quite large - up to 62 gallons. I have mixes of herbs, dahlias, annuals and lily bulbs. I just did not have a good container year and I'm not happy with a single one of them. And I don't know why they didn't perform. I grew Shiso Perilla, a very dark purple foliage Japanese salad herb that is a beautiful foliage plant - its doing great. So for a lettuce family to do so well, I think I have too much moisture.

In the past I always piled in bottles and cans to raise the soil level in those large containers and to reduce the weight. This year per your discussion about water tables I took them all out and filled the containers with soil.

I'm frustrated! I don't have full sun but still have been quite able to grow sun plants like dahlias.

I've watered a couple times with an H2O2 solution that supposedly increases O2 supply to the roots. I got some interesting good results from that in my seedlings early in the spring. So I did it again in the containers. Maybe it needs that again.

Any thoughts?

Thoughts?
I'm not sure why you'd be attracting critters or how the soil would/could be the problem. If you're using fish/seaweed emulsion, blood or bone meal, other organic stuff, therein lies the problem (most likely).

The filler in the bottom doesn't help promote drainage, it only reduces the soil volume, which can make a difference in some cases. Since soil volume is reduced, water volume is reduced commensurately and the plant can use soil water more quickly. This allows O2 to return to the soil faster, which is essential to root metabolism, and overall plant vitality, not to mention its beneficial effect on keeping root rot issues at bay. You're still better off using a soil that drains well, homogeneous from top to bottom in the container, but I seems that your soil might just be retaining too much water - period. Probably too late to remedy it now, this late in the growing season, but you could try inserting a wick into the drain hole(s) and allowing water to drain into soil below. The wick must dangle below the container, or contact an absorbent material (soil) below the container to be effective.

If you're interested in something I wrote about H2O2, let me know & I'll forward to you. If there's additional forum interest, I'll post, but it's pretty boring, though short. ;o) Besides, H2O2 isn't a solution (well, technically, it is A solution, but it's not a remedy), it only temporarily relieves the cause of the symptoms.

Really sorry you're having problems. I feel your pain (sincere here). I had several of my containers blown over & damaged by wind several times this year (hail a few years back, too) and it's a real disappointment to see your efforts diminished by circumstance. Good luck in whatever you decide to do, Alyrics.

Take care.

Al