Hey guys. Look at what I got for $25 today.
It’s from a guy’s garden. So technically maybe not a yamadori per se. But hey we dug it out and now I’m pot training him. So for me it is a yamadori. 
I think this one will be an awesome bonsai one day.
Should I do anything specific to help him get used to his pot?
Any possibility to put it into a grow box with pumice? The foliage is the strength so keep all the foliage you can and mist to reduce desiccation. Interested if you had to cut tap root. If there were few roots the care must be to get roots to form, so that is why I recommend pumice.
If you are not comfortable with aftercare, I can take care of it for you until I am too old to do so!
Nice find! 
Hey bob. Nice to see you again.
I had to cut off some stability roots but sealed them with cut paste. As for the smaller roots. I let them in their original soil, but I surrounded the unused soil with my own pumice, lava stone, akadame mix. Hope this will help him to grow roots.
What do you think about fertilization?
Haha. I live in Berlin/Germany. I guess you don’t wanna travel that far 
I sometimes use a transplant fertilizer that is supposed to promote root growth. Plant Start Root Stimulant is a common brand in USA . package instructions include: Use the same product that our landscape specialists use. Designed for newly planted trees, shrubs, flowers and vegetables. Plant Start develops a strong root system while reducing the shock of transplant. Contains 3-10-3 fertilizer, Vitamin B-1, a rooting hormone and a soil penetrate. Apply to the soil prior to sodding new lawn. Use at planting time and repeat again in two to three weeks.
I do not have studies that document this is scientific method, but I have had good results.
Do not know if you have anything similar in EU.
I am concerned about the container. Not very large, and will not allow much air exchange into the soil. I recommended a grow box or any container that has a mesh bottom to allow air exchange into the soil and roots. Anderson flats are a great choice in US but I bet some growers in EU have some similar product. To build a wooden box with many drainage and air holes is what most collectors use. If you have the ability to drill or poke holes into the bottom and lower part of the container that can help as well. 
“Yardadori” or “Urbandori” is the term our landscape collected trees have been called 
Just wondering about the root system, any fine feeder roots?
Like @Bonsai_bob said, grow box, or root bag with pumice with stabilization.
Did you remove any foliage?
Junipers strength comes from their foliage and it is going to need all that energy to produce roots. Expect to see some foliage die off.
Winter protection?
Cut paste is not needed for cut roots
Agreed that a shallower, wider box might be better. Would also allow you to place a brace between the box edge and trunk to help with stabilization.
Nice yardadori!
Definitely shallower, but does it need to be wider? In the yamadori stream Ryan and crew built a box that was just big enough for the rootball. They even place pieces of wood inside of the box to help reduce the amount of space in the box. Reason being that you want to eliminate “cold zones” in the box.
https://live.bonsaimirai.com/library/video/post-collection-potting
Speaking as someone that has recently killed a piece of collected material 
definitely get it into better soil…just get pumice. I used native soil because I was naive and it created an anaerobic environment. Poor tree had no chance. 
This is the best deal I’ve found on pumice so far for this type of thing. https://smile.amazon.com/gp/product/B008YSVPHW/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Great find, has a lot of potential. As others have said I recommend considering improving the soil. At this stage it may be to risky to try and get it in pumice. My recomendation would be to make sure there are plenty of holes in the bottom of the current container and then bury the entire container in the ground for 1 full year depending on how it does.
Taking it out of the pot with garden soil I believe is the better idea. Keep the garden soil at the core and surround with a good bonsai mix, and replace the whole in a caulinder type container. This will encourage new root growth and also allow you to water as the tree really needs. The garden soil will rot the roots if you water too much and the bonsai soil will allow you to establish a water and oxygen balance that you really need for recovery and growth. Repotting in one to three seasons of health will then allow you to clear out the rest of the garden soil ( leaving total bonsai mix). IMHO
