We don't replace soil in raised beds because we have the earth below creating movement of water and air in the soil. Raised beds are earth based gardens. Containers are completely disconnected from the earth by virtue of being containerized. The concept that I want you to understand is this: Water doesn't move unless air does. A simple example is a drinking straw in a glass of water. Dip it in and place a finger over the top of the straw. Withdraw the straw and the water doesn't move out of the straw. Remove the finger and the water runs out because now air can enter. In the earth water is constantly moving through the soil and therefore so is air (the earth has no drainage hole where the water exits the system). In a container the water runs down and out the drainage hole and then water movement stops and therefore so does air movement. This is why containers and the earth are so different. In a container we need a coarse mix that allows the free movement of air and water (drains well). In the earth we can get by with tighter soils because water still moves and thus air still moves. A potting mix is only as good as it's particle size. If you purchase a store bought mix it will probably be just fine in the first year, but the organic part of the mix gets broken down by biological activity and weathering. As it is broken down it breaks into smaller and smaller particle sizes and before long the smallest particles which wash to the bottom of the container are getting closer and closer to having clay soil in the container. This holds a lot of water and very little air which plants need. Miracle Grow soil is peat based and peat based soils are generally good for one season and sometimes 2 if they aren't exposed to the elements any longer than necessary. After that they are so collapsed/broken down that they will have an adverse affect on plant performance and should be dumped out into the earth based garden and replaced. For more info, please visit the container gardening forum.

The problem with reusing old soil is that it breaks down. The ideal potting mix should have good aeration created by solids in the soil(wood chips, perlite, ect), it will usually have peat for volume and water retention and then some composted nutrients(manure, ect). The organic soils I have seen at home depot have all been terrible at producing those ideal traits. They are usually very sandy and when wet become muddy. If you created your own then that's fine. The way that I reuse soil is to remove a few inches off the top(get rid of pests, pathogens) and remove the stuff at the bottom and sides where the roots have created a hard shell when tipping the pot over. The stuff in the middle can be reincorporated with new amendments or new potting soil, about half.

Dave (digdirt) and I use very similar methods of reusing mix. I put all my mix from all the pots into a very large tarp and add some dolomite lime and some 10-10-10 slow release fertilizer. After mixing well, I roll it all up and let it spend the winter working on decomposition. This will allow the calcium in the lime to get closer to useable compounds over time. Also, the slow release will have rejuvenated some of the nutrients lost to watering washout the previous season. Then, in the spring, I add some new mix to the old as I fill my pots, buckets, and tubs(roughly 25% new mix). I also add some composted manure (usually Black Kow). With another handful of dolomite lime, some Tomato-Tone (from Espoma - about a cup per container), and another small handful of 10-10-10, I fill the tubs and set in the tomato and pepper plants. I add some Miracle Grow for Tomatoes fertilizer to the watering every two or three weeks during the growing season. When all the containers are properly filled and plants are growing, I use any leftover mix from the last year in flower beds or hanging planters or whatever. So, nothing wasted. Hope this helps.

Linked a number of previous discussions about this below. As you will see there are several different approchaes folks use. As I have posted in some of those discussions I dump all my containers out onto a large tarp and spread it out to dry well in the sun - that alone takes care of some of the resident bacteria. Then remove all the root clumps and other debris, mix in approx. 1/3 new quality container mix with it and also add a good dose of Osmocote or other slow release fertilizer and refill the containers. dave Here is a link that might be useful: Reusing container soil discussions