I have a large planter
In general, it is advisable to empty, clean thoroughly (if it’s not too large), and ensure drainage holes are adequate before filling your planter box with new container mix in early spring.
While it is possible to ‘pep up’ your existing soil in the various containers you mention, we would strongly advise starting the new season of vegetable growing with new potting or container mix.
I pulled some flax out
While flax is not an excessive robber of soil nutrients, it would be a good idea to give your soil a boost, especially if you are struggling to grow new plants in the same location.
My question is how the
This is a great question as there are many different types of growing media available, which can be a little overwhelming.