The organic components of your container soil continue to reduce to a mush that will cause the soil to contain less and less oxygen the roots need to take up nutrients. It is a gradual collapse of the soil structure that makes replacing it a good idea. Al

There are 2 reasons for crop rotation: Disease/pest avoidance and avoiding nutrient depletion.

If your plants weren't diseased and weren't bothered by overwintering type pests then there is no need to rotate.

Nutrient depletion is a non issue in containers as they are fertilized regularly.

How long you use the same mix is up to you, but as noted above the ingredients break down into smaller and smaller particles over time. Many here would replace mixes with a high percentage of organic matter each season, others would use them for years.

In my own experience you have more latitude with most veggies in moist, dense soils than you do with shrubs/trees/succulents so it's up to you how long you want to use a mix for. If you start noticing a decline in overall health/production definitely don't reuse again.

A slightly different perspective and why I hold it:
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.

Additional considerations are possible carry-over of fungal spores, the possibility/probability of insects in various stages (eggs, larvae, etc) and build-up of 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 used container soils be turned into the compost pile or garden and fresh soil used in its place unless monetary considerations prohibit.

Al (tapla)

Thanks for your advice. This was my first ever container vegetable garden adventure, and it was very successful. That's because I asked questions first.

So, I will be stocking up on more Miracle Gro Organic Soil for my next projects. The old soil will go into a compost pile in a huge container in my back concrete back yard.

Al, is there a difference as far as how long your soil mix lasts compared to MG potting mix. It seems like yours would last longer. Could peat and maybe perlite be added back each year and still do ok as far as an economical view? Its one thing to buy a new bag of MG for one pot every year and another to buy 20 bags for 20 pots every year. Just wondered if peat was the primary ingredient of your mix (or most mixes) that broke down. Not sure how long perlite lasts. I liked your explanation and think even I can understand that. But economics is a part of life and I like the fact you acknowledge that. So I was curious to a happy medium, lol

Under the same conditions, a conifer bark-based soil that has from 10-15% peat should retain its structural stability and suitable aeration for about 4 times as long as a soil that has peat as the primary ingredient. In the long run, it is also much more economical to make your own bark-based soil than to purchase one that is commercially prepared.

Al

Bark based potting mix is available in bulk from landscape supply firms who make it for the propagating nurseries. Our local supply makes 10 different mixes for nurseries growing different crops from cactus to Japanese maples. Cost per yard is around $45, but they will sell a quarter yard if you want less. Al