Sammy, soil is a precious commodity around here too so I'll be interested in the answers you'll receive to this question.

I live in a location that happens to receive less rain than the little that the surrounding area gets so that, along with the VERY thin soil over limestone rock, makes for fast drainage even in raised beds or holes we manage to make in the rocks; therefore roses do better in large pots rather than planted in the ground. Not all my roses are in pots, but many of them are, especially the Austins and other newer hybridized cultivars.

I change the soil in the pots about every three or four years. What I've been doing with the old soil is to refresh it by mixing it whatever soil amendments I have on hand at the moment like organic fertilizer, green sand, peat moss, manure, compost, maybe some bagged high quality potting soil, etc. and use it over again. So far it seems to be working pretty well.

I intended to say that I'm doing similar to what you suggested: "What if I mixed in that potting soil, and then added either compost or top soil or both to fill the rose holes".

Sammy, certainly you can re-use the potting soil - my whole home garden is based on containers and there is a regular shuffling around of old topsoil from the veg patch on the allotment, the many, many bags of John Innes loam I get through and all the multi-purpose composts I use for seed starting and so on. Essentially, the actual soil is often fairly inert - what matters is the microbial life contained within and the resulting structure of the mix to allow exchange of gases,pasage of roots, movement of micro-life. What you can do, to encourage a healthy bioclimate, is to add a little mycchorizal granules to the old potting mix. If you have any garden compost or leaf mould, this is also good and will certainly boost the micro-climate of your mix. Don't worry, your plants will always tell you what they need and you can adjust or amend when necessary. If you are going to replant directly into the tomato mix, you could also bung in a bit of bonemeal - good for phosphorous and therefore, root growth. Soil is not an infinite resource so we do need to re-use and recycle. It helps the planet and helps our purses.

sorry, must clarify - mycchorhizae need to be in contact with roots - just adding to a mix will not make any difference but, if trickled or patted onto exposed root systems during the planting, it really does encourage a healthy micro-climate.

Sammy,

I've often dumped used soil in pots into a garden bed. The ingredients in the potting soil are "probably" perlite, bark mulch, peat moss, sand and maybe some other things. All of those are useful garden amendments, and there's nothing in there that would hurt your roses. I've grown roses and tomatoes in the same bed and they like the same things, water and food.

I would mix the used soil well though with the existing soil and the other amendments you're using. I would guess your soil is a heavy clay? I'd just be sure not to dump the used mix into the rose holes, if the holes are heavy clay that form a bowl that you're just adding potting soil into. Make sure drainage is good and the soils are mixed. I'd add manure and rotted leaf mulch/compost and mix it all together, too. Roses love manure.

A soil test would be good after that to see what you need to add, nutrient wise, for growth next year. Or like campanula says, watch your roses to see what they need, whichever method you like.

I'm for sure not a soil scientist, but that's just what I'd do.

The best thing to do would be to mix it in with the native soil as best you can.

Composted organic material is organic material, whether it is named "potting soil" or "soil amendment". Roots that are in one type of soil--when they encounter another type, they often stop right there and don't go farther.

Also drainage can potentially be reluctant when one type of soil meets another due to differences in surface tension.

I would mix the potting soil with a bag or two of compost and a bag of peat moss.