I have used my compost as my potting mix for a number of years and have noticed that the containers were heavier then with just a commercial potting mix. Did you by chance mix some soil with that compost, or was it maybe a commercial compost that might contain soil?

thank-you for your response Kimmsr. My compost is home grown and it didn't seem to have much soil in it. I may have worded my post wrong, perhaps 'heavy' was not the correct word to describe the soil after watering, but 'denser' moreso than just commercial potting mix. Is yours this way and do your annuals do well in it?

Compost is not highly recommended as a major component of potting soils, despite the 'success' some might have experienced with it. It will continue to break down and decompose, collapsing available pore space, and it tends to be overly moisture retentive -- this combination of issues tends to impact good drainage and hinders oxygen exchange. This is what provides the "heavy" appearance or qualities - the stuff just compacts too easily.

For best results in a window box or any other container planting situation, a good quality potting soil (often referred to as a soil-less mix) is advised. You may want to refer to the Container Gardening forum for details.

My standard container mix is 1/2 sphagnum peat moss, 1/2 screened compost by volume. Perlite is added until the mixture looks "professional." The compost does contain soil and it settles over the season but the roots find their way throughout the containers.

For me it works and it's relatively cheap. I don't see any advantage to using the Pro Mix over straight sphagnum peat moss. A bale of Pro Mix is 3 to 4 time the cost of the same bale of peat moss.

At the end of the season, the container mix is removed, screened and used to mulch for winter protection. Much of that can be salvaged for the next years containers.

I usually mix in equal amounts of compost or vermicompost with potting mix to rejuvenate it so that it can be reused a couple of times. It's cheaper for me that way.

What Pam (Gardengal) said.

I always suggest you focus on a well-aerated soil that will remain well-aerated for the intended life of the planting. Compost is simply not a material that holds enough air when it's saturated. If you don't saturate it and flush the soil when you water, salts accumulate and make it difficult for the plant to absorb water and the nutrients dissolved in water. There is no getting around the idea that to use compost as the primary fraction of a container is going to leave you on the horns of a (probably more than one) dilemma and in a catch 22 situation.

Al

The containers on my front porch get dumped into a wheelbarrow every spring, I use a shovel to break up the soil as by that time it is pretty hard. I then add about the same amount of compost from my tumbler, give it a good stir to mix it all up well, then fill the pots again. I have done this for years. I find the plants do so much better in this mix than in straight potting soil. I think it is because it does hold water better which is a major plus as I hate to water twice a day. Yes by the end of the year it has shrunk down a bit, however that is when winter hits and the plants are dead anyway.
I also top dress other pots with compost. Those are to big of plants for me to take out of the pots, knock off the soil to replace it again.
Sandy
tree huggin' soil worshippin' trash pickin' dog lovin' recyclin' woman
please control the pet population, spay or neuter your pet
Mystic(German shepherd), Ginger ( Vizsla boxer (?)mix) and Pixie (JRT Chihuahua mix)

Hmmm - how about if we draw some delineation between what's good (or better) for the plant and what's good for the grower, because more often than not the two are mutually exclusive. If we set the measure of a soil's suitability by its ability to extend watering intervals ("I think it is [better] because it does hold water better which is a major plus as I hate to water twice a day."), it should follow then, that pudding would be close to ideal, but we know it's not. Well aerated soils that need frequent watering are going to be decidedly more productive for (practically speaking) all the plants we commonly grow and discuss on forums like this. The question is "Are you willing to make the small amount of extra effort/commitment required to take advantage of them?" Some people are and some aren't - different strokes .....

Top-dressing with compost is much different than growing in compost, but FWIW, and since compost adds so little in the way of nutrients, it seems rather redundant to top-dress a medium that is likely 95%+ organic already and going through the composting process .... with compost, unless it's a water conservation measure, and then almost anything that shades the soil and inhibits air movement across it would be equally suitable.

Al

Many people mix a lot of various kinds of vegetative waste, peat moss, coir, bark fines, etc. together to make a potting mix, pretty much the same stuff that would go into a compost pile to make compost, pretty much compost except those materials are dead.
I have never had a problem with my compost getting water logged although I have had the amount in the various containers require replenishing during the growing season, because compost is a living organism, unlike peat moss which is dead.

I don't see why container gardening HAS to be a hydroponic operation - with sterile soil-less mixes, correctly applied macro- and micro-nutrients, etc. Sure, the plants might be most productive that way, but where's the fun in that? It's completely different from gardening in the ground - no room for flexibility, experimentation, etc. It's boring. It can also be expensive, and being on top of everything, including watering, can be a nightmare for most gardeners working full time. Of course, that's just my opinion.

Hey - I have no problem with someone who wants to make their adventures into container culture a purely organic thing. ;o) Different strokes for different folks...... There is no right way or wrong way, but there are methods that are easier and more productive than others. I understand if someone wants to master the challenge of trying to perfect their organic approach. It reminds me of the kind of satisfaction someone might get if they walked or rode a bicycle from coast to coast rather than driving or flying - a true accomplishment; but some people would rather skip that sort of challenge and take their satisfaction from the end result instead of the journey. We could probably draw another very close parallel between the fact that we all don't hunt for the meat we consume or grow our own food. We buy it instead.

It's only my view, but it seems like the organic growers and those that choose to grow in fast soils with soluble fertilizers would be better served if they were to show a mutual respect for each other's choices that is even deeper than that between hunters and gatherers and grocery shoppers. YMMV

Al

I find that mixing worm castings, bone meal, and a little greensand into potting soil adds enough slow-release, organic nutrients to last a season. Adding the same mix in successive years, together with potting soil to replace the lost volume, keeps it growing for five years or more.

There is a huge difference in the quality of purchased potting soil mixes, though. Some seem to be 95% peat and are next to impossible to wet if allowed to dry out. Others, like Hyponex, seem to be mostly sand, are very heavy, and become very compacted; I have found it to be pretty useless. My favorite is Ocean Forest soil from FoxFarm.

Also, if you let soil sit outside in closed bags for a year, or in a closed trash can, or in an unused planter, it can "sour" and develop a musty smell. If this happens, chuck it, hose out and scrub the planter, and use new soil.

Whether garden compost will work as an additive depends on the quality of your compost. I'd try it, but a third of your mix is a bit much. Top-dress one of the planters with a few inches of your compost and compare the results for the season to others that use products from the farmer's market or nursery.

I guess doing an experiment with different pots of different things would be fun for some, but I know what I have done for pretty much all of my adult life-learned from watching my parents and grandparents. I know the plants on my front porch are huge in no time. I plant shade lovers in there (mostly impatients), and they really fill the planters and are non stop blooms. I have people stop to look at my front yard and all of the blooms. If I am out front working on the beds I have had more than once have had people out for their walks say they now add that part of the street because they love looking at the flowers.
Then the pots that I do top dress with compost( might be considered worm castings as I do dump it out of the tumbler into a trashcan with holes in the bottom and let sit for a while for the worms to enjoy) those plants flourish all summer long. I don't have the need to use fertilizers, do use cold coffee water when I have coffee left over in the pot. But that isn't even much. I can just say I know from what I have done for ever how well it has done for me.
Sandy
tree huggin' soil worshippin' trash pickin' dog lovin' recyclin' woman
please control the pet population, spay or neuter your pet
Mystic(German shepherd), Ginger ( Vizsla boxer (?)mix) and Pixie (JRT Chihuahua mix)

I can't agree that "worm castings, bone meal, and a little greensand" is enough to sustain container plantings. The bone meal breaks down so slowly in containers that it is virtually a nonsource of nutrients. Worm castings add almost nothing in NPK, while the greensand might add a little Fe and K. The medium breaks down so slowly it cannot provide the nutrients the plants need - so where does the nutrition come from?

You are correct in the observation that there is a large difference in commercially prepared potting soils, but one similarity is that all of them that are based on peat (or compost) are very water retentive, I don't care for the FoxFarm products for that reason ..... but that's just me. If I had to choose a commercially prepared potting soil, I would choose either Fafard's 51L or their Nursery Mix.

We can talk about whether the quality of compost makes a difference or not, but in the end it is not the nutritional value of compost that makes it valuable or a detriment to a container medium, it is it's structure. 'Finished' compost is extremely fine textured and to go along with that fine texture - extremely water retentive. Unfinished compost, while it might have some larger pieces in it it prone to N immobilization, so it's a catch 22 again.

To those wanting to make compost work as the primary fraction of their container media, I'll just say 'good luck' and wish you a good experience; but from multiple perspectives, there are much better ways to approach container culture; this, for the benefit of those that have not imposed limits on what they can choose as an approach to growing in containers.

Al