The advice about starting in cups is valid and key. When it come to transplanting, you simply turn the cup upsidedown and take the whole dirtball roots and all and put it into the larger pot hole without the roots ever seeing light or falling apart and backfill. It`s more difficult to transplant small saplings out of large pots because the disturbances cause shock and you can cut roots.
The sagging plants have no blood pressure anymore, something is restrictign the flow of liquids. I`ve seen some recover but you have to find out why. I would pull them and put them into cups of water, like cloneing, see if they get pumping again !i
One thing you can do to improve your soil is to pull up wild plants and cut the stems off leaveing just the roots. Then let them dry out (a week) and chop them into little tiny bits. (put them through a food proccesor) Just the roots, the stems and leaves are too acidic and take a long time to break down naturally. Get lots of them make a big pile turn into a little fine pile. Fracture some stone to release minerals with a hammer, make close to dust. Find a bombed out tank or artillery piece and scrape off a couple cups of rust. (iron oxide) mix all three ingreediants into your soil well. Then mix in a sidedish of stuffed animal fillings and another of potash.
You`ll be basically doing what nature does to break down vegitation and create soil for re-growth, but you will be doing it much quicker than nature will. That`s why just roots, no stem or leafs. Transplant into this soil very carefully. They`ll eat better and tip you well later on !i
You have very little natural deposits of soil to find in your location, you have to make them. This of course is not good for farmming cuz that`s large scale, but for potting plants it don`t take long to do !i
It`s best to do this in the fall but it can be done immediately too !i