How it looks from my perspective: If the decision to reuse container soils is an economic one, who can disagree with the decision? If you aren't concerned about expense and are willing to go to some minimal extra effort to build a good soil, that is what I would elect to do.

Here's why: Container gardeners need to direct primary focus to insuring that the soil they are using will remain able to provide adequate air to roots for the expected life of the planting. Along with the ability of the soil to hold ample air, comes good gas exchange and drainage for rootage. All the other major cultural variables affecting growth are easily manipulated. Water, nutrients, sunlight, and to some degree, temperature can all be controlled easily. Aeration cannot and its consideration is as important as water and light to plant vitality.

The breakdown or collapse of soil structure occurs at an exponential rate. If we imagine the usable life of a peat based container soil from a bag, it deteriorates from a reasonably good soil when fresh to unusable after 2 years. It's not unreasonable to expect the soil to lose at least 20-25% of its air holding ability by the end of the first growing season. In the first half of the second growing season, it will lose about another 25% of its air holding ability and in the second half of the second year, near total collapse is likely as the remaining 50% is lost. Though this is an example of an imaginary soil, it is very close to what actually occurs. Another way of saying it is: Even though a soil might be performing acceptably at the end of the first growing season, you should expect a rapidly accelerating collapse in the subsequent year.

So, if you use a soil for 1 year and mix it 50/50 with a fresh, similar soil, 50% of the soil will have totally collapsed by the end of the growing season, and the other 50% will have lost about 25% of its ability to hold air. If a soil is not holding enough air, it's holding too much water. Water and air retention vary inversely in soils and when one increases, the other decreases. When soils hold too much water, you have to hope that the plant will use enough water or that enough water evaporates to prevent root rot issues and even minor cases of over-potting can mean terminal misfortune.

At risk, I speak out often against the use of compost in container soils. It supplies nearly nothing in nutrients and clogs the soil macro-pores container gardeners should covet. Containers are not gardens and much of what works in the garden can cause difficulties in containers.

Additional possible considerations are carry-over of fungal spores, the possibility/probability of insects in various stages (eggs, larvae, etc) and carbonate precipitates from your watering water. A slow soil will also have accumulated fertilizer salts and possibly insecticides that you may have applied and forgotten about last season.

I suggest that container soils be turned into the compost pile or garden and fresh soil used in its place unless monetary considerations prohibit.

Al

al makes some great points. if are going to ruse potting soil its best to start a compost bin and cycle your soil thru that before going bck into your container with a proper mix to make it "looser"

BUT compost makes an excellent top dresser to potting soils and will also leach out some trace elements.

I grew tomato plants in containers last year and this year I just dumped the soil in a larger container and mixed in some fert. a little lime, and enough compost to be sure to fill the pot and put it back in the growing pot, and planted another tomato plant in it. and had another very good tomato. That is the third time for this soil.

last year I had a patio tomato growing next to large pot containing a brugmansia. I had to cut down the brug for the winter but before i could remove the stump in spring a very plump volunteer tomato was growin in one corner of the pot. ( I mulch my pots with semi finished compost). I did not have the heart to disturb this volunteer so I just planted narsturtium around it. It is still very robust and starting to flower.

I like to take out all the soil in the old pots, mix it with peat and compost and reuse it for annuals. Works for me.