You're (eventually) going to find that any combination of compost and topsoil is going to be very difficult to grow in and maintain good plant vitality in, simply because of the amount of water (and insufficient air) it holds. Some will tell you they are satisfied growing in compost as opposed to soils with better aeration. When it comes to container media, 'structure/aeration/and durability' are the watchwords. What works in the garden is very often foreign to container culture, and that includes a soil comprised primarily of fine particulates, like compost, sand, topsoil, peat, coir .....

See below.

Al

Here is a link that might be useful: Click me for a clearer perspective re container soils

The best soil to grow anything in is loam, a soil type that is usually a mix of 45 percent sand, 25 percent silt, 25 percent clay, and about 5 percent organic matter. I have found that most people think this is what "topsoil" is when "topsoil" is merely the top 4 to 6 inches of any soil.
Potting soil sold in the stores is peat moss, or coir, or finely ground bark, with some vermiculite or perlite added to promote drainage, there is not any of the mineral portion of soil, sand, clay, silt, in that mix.
I have used my compost as my potting soil for a number of years, even though a number of people here insist that I could not possibly have done that since plants will not grow in just compost although they seem to grow with no problem in peat moss.
Because of drainage issues no one shouild suggests using the soil from your garden as potting soil.

For the record, I think that is quite an exaggeration - I've NEVER seen anyone, including me, EVER say that you can't grow plants in compost with no problems. But the FACT is, compost is so water retentive that to avoid issues associated with excessive water retention, you MUST go to great lengths to avoid perched water, OR you leave potential growth and vitality lying on the table. Another fact is, the more perched water there is in a soil, the more difficult it is to grow in, to maintain your plant's vitality in, and the narrower the margin for error.

You have said/admitted on more than one occasion that your plants "might not be the biggest/best & yields not as good as they could be" with regard to your containerized plants, but you're satisfied with the results, and I've provided links to that quote in the past, so there is no question about whether I'm imagining things. All that's important is that YOU are satisfied with YOUR results, but satisfaction and the perception of what constitutes a quality growing experience vary widely by individual. The number of people I have helped AVOID the ill-effects of their practice of growing in media based on small particulates (like peat/compost/coir/sand/topsoil) runs into the several thousands just here at GW, which is pretty convincing testimony that ANY soil made primarily of small particulates is fraught with potentially negative issues centering around the effects of excessive water retention and accompanying limitations on the grower's ability to water properly.

Al

rmontcal, I have had pretty good luck with cheap potting mix blended with my own free sifted compost. I recycle my potting mix every year by adding some fresh compost, and use this in the bottom of larger pots for tomatoes, etc. (to seal it off, since I tend to get blight) and use fresh potting soil on top.

I think the take-home message of the little debate going on above is that soil doesn't work very well as potting mix. ;-]

If there's a message to be taken from anything I said, it's that container media are more about their physical structure and durability than they are about what material they are made of; and that soils comprised of fine particles (any mix that has peat, compost, coir, sand, topsoil, or other fine particulates in any combination as the primary fraction) are inherently more water retentive and much more difficult to grow in than their counterparts comprised primarily of coarser materials.

Gardening and container culture are two distinctly different approaches to growing, with container culture being much closer to hydroponics than it is to growing in the earth. Water behaves in containers very differently than it does in the ground; and the most successful and proficient container gardeners recognize and allow for the differences, leaving the practices that work so well in the garden, but prove to be limiting in containers, in the garden.

Compost is a wonderful addition to our gardens and beds, but a poor choice as a significant fraction of container media by virtue of the fact it's excessively water retentive and supports significant volumes of perched water.

Al

I would agree that some materials like sand or soil would have a 'fine' particle size by definition (although the term is relative). In my experience the particle size of compost is related to how fine your sieve is. I use chicken wire or 1/2" hardware cloth, and my potting mix of 1/3 compost to 2/3 commercial potting mix certainly doesn't stay soggy in the pots in the hot summer. Quite the contrary. YMMV.

'Stay soggy' is also a relative term, so let's clarify. If it 'stays' soggy for more than a couple of hours, and I can assure readers that unless the planting has to be watered more than once per day that it does, root function/metabolism is being impaired and fine roots (the almost microscopic hair roots that do all the heavy lifting for the plant) are being killed off in the anoxic part of the root zone. When the plant uses the excess water or it evaporates and air finally returns to the entire root mass, the plant then has to expend energy that would have gone toward growth - foliage, fruit, blooms, or just an increase in biomass via additional cambial growth, to replace the lost rootage. This cyclic death and regeneration of roots occurs in all soils that hold perched water, with there being a direct relationship between the severity of the problem and the ht/volume of the perched water in the soil.

We can say with certainty that soils with a high % of fine particulates hold more perched water, so they present more difficulty and a much narrower margin for error.

I think it's important to understand that a few larger particles (perlite or bark pieces, eg) mixed into a water retentive combination of the fine ingredients listed above (peat, compost, coir, sand, topsoil) won't improve drainage or the reduce the ht of the perched water table. The very best you might hope for is a reduction in the o/a volume of water retention, and then only if the ingredient you're amending with isn't internally porous.

To illustrate what I just offered, ask yourself how much perlite it would take to make a quart of pudding drain well & improve it's drainage (flow through rate) and aeration? Do you think 50% perlite would do it? No? 60%? 70%? Now we're getting close. But wait. If we add 75-80% of something like pine bark or perlite to pudding or compost to get it to drain and improve aeration, aren't we then growing in pine bark or perlite AMENDED with compost?

Compost in soils can be made to work well if it's a small fraction of the whole (under 15%) and the rest of the particulates are coarse. From the plant's perspective, grower convenience aside, growing in a soil with any combination of peat, compost, coir, sand, topsoil, or other fine particulates as its primary fraction is going to impose inherent limitations not associated with soils comprised primarily of larger particulates.

It doesn't matter to me what people grow in, and I'm not trying to change the way anyone who doesn't care to change grows; but there are many aspects of soil science so well defined that they can't be misinterpreted, allowing us to draw some very specific conclusions about the cause/effect relationships of certain practices. Hopefully, clarifying some of these cause/effect relationships will help others make the decisions that make their growing experience more rewarding.

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Al

Just remove the bottom of the tin, place it on the dirt somewhere, and fill them with your soil mixed with whatever compost you have. They would be just like mini raised beds.

If you wanted to keep them as containers I think that the cheapest way to fill them would be to go to a nursery of landscape supply place that sells potting mix in bulk.

You can also mix your own potting soil - there are a number of threads about "Al's mix" (tapla from above) in the Container Gardening forum - it might not be cheaper than bulk mixes but using the recipe and suggestions I think I can make a much better potting soil than I can buy locally premixed.

Your post reminded me of how people grow carrots for shows (I assume mostly in the UK) and guiness records. The link below is a nice blog about one gardener's season. You could also google 'growing carrots in tubes' or 'growing carrots in pipes' for more ideas.

Best of luck!

Here is a link that might be useful: http://www.allotment-diary.co.uk/Exhibition-long-show-carrot.html

CC - you can actually make a very productive soil with 5 parts of pine bark fines, 1 part perlite, one part of either sphagnum peat or finished compost, and a little dolomite, and it ends up costing less than half of what the peat-based commercially prepared mixes.

The ratio listed above can be adjusted to fit your water retention needs by varying the ingredients, but the key to the soil's productivity and to how easy it is to grow in lies in the fact that it is well-aerated and holds little or no perched water, yet still has good water retention ..... it just doesn't hold water in lakes at the bottom of the container like the peat or compost-based soils.

Good point about the raised bed thing. Partially burying your containers or sometimes just setting them on the ground so there is a bridge of soil between the soil in the pot and the ground, turns your container into a mini-raised bed (hydrologically speaking), and employs the earth as a giant wick to help drain the excess (perched) water from the soil in the container. This allows you to grow in soils that would be too water retentive to offer best results if used conventionally.

Don't get me wrong .... I'm not arguing against what someone WANTS to do, do whatever you want - I have no dog in the fight; but perceived results are very subjective. What suits one person fine may not be good enough for the next. I've helped enough people (thousands) rid themselves of the problems associated with water retentive soils to know that a well-aerated and durable soil stands head & shoulders above the soils based on fine particulates.

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Al

Al, I remember when the Tupperware Tuppercraft containers came out that I was shocked at the "rocky" coarseness of the planting medium, but the plants really thrived in it.

I grew my container gardens all summer in nothing but compost and garden soil. The growth was just nuts. Watering is definitely something to focus on. It's hard to keep up with the watering due to the small containers but I achieved fantastic results. Go for it! Lots of compost of course to keep them fed. :)

Here is a link that might be useful: Strange and Wonderful results

For every one person that claims good results in a mix of compost and topsoil, there are hundreds that fail due to compaction and accompanying excessive water retention. I've never said it can't be done well enough to satisfy the grower using such heavy mixes, only that it is much more difficult, the margin for error greatly reduced, and potential for plants to grow to their genetic potential virtually nonexistent.

That may seem like a radical statement, but it's entirely true. All plants have a given degree of genetic potential to grow and produce under perfect conditions. Our job as growers is to eliminate the limiting factors or reduce then to the greatest degree possible. It is only by making conditions perfect that we can optimize growth and vitality. Soils that support significant volumes of perched water (and the soil you described assuredly does - probably 3-4" or more) are limiting if you water profusely (for reasons stated upthread) and they are limiting if you try to water in sips to avoid the effects of soil saturation; this, because of the build-up of dissolved solids from fertilizer salts and tap water that aren't being flushed from the pot when you water + the lack of adequate aeration due to compaction.

As I said, I have no quarrel with whatever someone wishes to grow in, but there are limiting factors assigned by science to soils comprised of fine particulates as their primary fraction.

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Al