This is a most interesting thread, for sure.
I went to a rose growing "lecture" once (for lack of a better word...it was a two hour, one day gig) which was geared to growing roses in my area and in the type of soil common to this part of the state.
The person giving the talk said to never use ammendments with redwood in them for roses...something about the tanins in it that roses don't like when it's incorporated into the sandy clay soil we have here. Prior to learning that I had used redwood conditioner, but then stopped using it, switching instead to compost. It did make a big difference in my roses...that plus the fact that redwood soil ammendment is scare anymore.
Here's what I don't get: "Potting Soil" is usually predominantly made up of assorted "barks" as the main ingredient anyway,(along with peat and some sand, perlite, etc.) and technically isn't "soil" per se. Lighter weight for use in containers (pots) and will get depleted of any nutritional value after a while, once the plant uses what's available and then that which is flushed out from watering the container. These potting "soils" seem to become inert after X amount of time. It seems to me that compost would be a better (and cheaper) soil additive.
If you have a rose growing in a band with potting soil and then transplant it into the ground, how can that small amount of soil from the band affect the ground soil that much ? The root system will (hopefully) take off and extend well beyond that small amount of potting soil into the surrounding "native" soil. IN TIME that original potting soil will just decompose and be replaced by roots.
Years later when you dig up a plant from the ground that was once planted in nursery potting soil, that original bit of potting soil it originally came in is pretty much broken down and decomposed and the plants roots have taken over that space. Granted, some of does remain but how can it be that much of a problem ?
I guess my main question is why would you want to add potting soil to the ground soil vs. compost ?
Otto and Sons (rose growers in Filmore, Ca.) has a very unique soil mix that they use. I've tried to decipher what it's made out of, but never could.
It's a very heavy potting medium...way heavier than any medium I've ever seen from any grower, yet their container roses thrive in it. The first time I went there and actually went to pick up a rose in a 5g container I was in shock at how heavy it was, yet the plant was thick and massive and about 4' tall. Their roses are also able to live in those containers for much longer than roses I've bought from other nurseries that are planted in very lightweight mediums and look like #%$@ after about 6 months. I tried to find out what they're mix is made of and they wouldn't reveal that. Can't blame them I guess.
