What a great question. Conversely I have never
understood why anything grows in a pot long
term. Hope someone smarter than me can
add some insight 'cause I've got no idea at all.

Perhaps Rhizo will see this and respond as it is my understanding she works with soil systems professionally.

I really don't understand it well enough to explain it other than to say the earth is a large, working system and containers are their own contained system and this means that ground based approaches which are dependent upon the earth's system to work well often do not work well in containers.

One example would be using compost and manures. These are pretty much universally viewed as desirable to incorporate into the ground, but nobody growing professionally in containers uses either as they tend to clog up drainage and impede the flow of oxygen through the container media.

So, I can't explain it, but I have come to understand via 'book learnin' and observation with my own containers that most plants in containers perform best in soilless mixes that have no 'dirt', compost, manure or anything else of a very small particle size or that will break down into a very small particle size during the growing season.

Organic fertilizers are another thing that sometimes doesn't work out as well in containers as in soils. Some organic ferts like fish emulsion work just fine, but those which require a significant amount of soil critters to process them before the nutrients are made available to plants tend to work less well simply because the constant fluctuations between wet/dry, cool/hot in containers do not favor the development of a large population of consistently active microbes and such. The organic ferts then do not release sufficient nutrients to meet the plant's demands in containers and to the extent they do release nutrients the frequent waterings tend to flush them out of the container.

I can only suggest that doing some extensive reading in the container forum will enlighten you in many of the physics and dynamics involved with soil and container grow plants and how they differ in provide the aeriation that is the single lacking non-controllable factor after potting up a plant in a container. I spent hours reading thru the discussions and gleaned more than I had hoped about various factors that I had often seen but did not fully understand. I don't think I could explain enough of it here to give you more than a glimpse as to the internal works of container culture so come on over and sit a spell and soak in some truly interesting reading for the container growing points of view
Happy Growing David

Thanks you guys. My questioning has me wading into soil physics now. Oh where these plants have taken me!! lol

I did not know there was a container forum -next stop. Thank you.

I have been reading more about it in my botony book too, gravity works differently in soil than in a pot. Gravity does cannot drain a pot as efficiently as it does in ground. (surface tension, saturation zones, etc.)

"Just a guy"-yes agreed, but would you believe, one of the all-time great books on Plumerias tells of his favorite mix in India which uses 50% well rotted cow manure. Of course, if it is not cut enough with leaf mold, or perlite, you are some trouble which I have learned(his climate is also perfect for plumerias which gives him much more leeway than the rest of us.) I agree with your comments about the microrhizomes being unstable in the pot environment. Better to use manure teas probably.

Thank you all for helping me out.

"One example would be using compost and manures. These are pretty much universally viewed as desirable to incorporate into the ground, but nobody growing professionally in containers uses either as they tend to clog up drainage and impede the flow of oxygen through the container media."

Funny you should mention that. Last year I mixed up some pots with lots of compost and aged horse manure in them, grew bell peppers in them very successfully. One I brought into the house over the winter, and then put outside a month ago. It has a lot of peppers on it already.

I have been mixing up lots of pot soil recently. One project was planting marigold seeds and small flowers with some Girl Scouts I volunteer with at a homeless shelter. The marigolds were up in every one of their pots in 7 days, I was quite impressed. The other pots are for dahlias and smaller flowers that will go around my yard in various places. My mix is about 4 parts finished compost, 1 part aged, crumbled horse manure, and 1 part clay soil. At the very bottom of each pot I put a small layer of leaves, or of shredded leaves and branches. I figure that adds a bit of fluffiness and air and keeps the soil from coming out the holes in the pot. I sprinkle some horse manure on top of that (also very light and fluffy) and then put the soil mix on top and around the plants in question.

The physics I taught didn't have much to do with soil! I don't really know what I am doing re: pots, am just too frugal to buy dirt, but it seems to work well for MY potted plants. The main problem I see in outside potted plants is that they dry out and the plants wither, so I would think that retaining moisture would be a good thing? I know that the plants in pots with my mix do better than the ones that come in the very light peat/vermiculite type stuff that flower plants are sold in. More nutrition and better water retention is my theory. I do know not to start seeds in this sort of thing normally, although as I said the marigolds did great.

Marcia