I have a number of pots that I used last year for summer plants still have the compost in. do I have to fully replace the compost for this years plants
I do, but if you want to economise you can replace just the top half - but tip them out and go through the old compost to check for vine weevils and any other nasties first
I'm not sure what the right answer is but I have taken about half of last years out and out it on the garden then topped up with fresh compost bought this week. I have only done a couple yesterday though and also repotted my Christmas Tree
If you do decide to use last years compost you will need to add some slow release fertilizer as the nutrients will have been used up. I buy it from the pound shop and in a pot comparison I did last year it worked just as well as the expensive stuff from the garden centre!
I generally mix up old and new compost and if I remember chuck some feed in to it
I put new in on top too, but not as much as half the compost from my big pots, which have earth mixed in with the compost in the bottom half. I remove top 6 inches of compost and replace with new. When I plant out my bedding plants I add slow release fertiliser granules.
I do half and half, at the end of the summer I let the pots and tubs dry out. This makes it easy to tip it out, the roots from the bedding plants bind the soil together. Then I saw it in half. I throw the top half away and put the bottom half back in the pot. I throw in some hort grit as a drainage top up and fill to the top with fresh MPC. I let it get soaked in the rain for a few days to make sure the water can drain through OK before putting plants in, you can always top up with slow release feed.
I only do this every other year, it's best to start from scratch the following year.
I put the pot shape bottom half back in the pot, then I only have to fill half the pot back up with fresh MPC. Simples
In general I always fully replace old compost which I spread over the open soil each winter to improve its structure and drainage. This also improves all plant/soil interactions eg nutrient uptake and hence better root growth and plant health. I feel you can always continue to improve the soil quality and the results show up in the summer.