Most soil tests do not test Nitrogen in the soil because it is so volatile. Almost any lab soil test will recommend adding Nitrogen.
I do not garden in containers but I am side dressing with feather meal this year and it is 12-0-0. It is the first time I have used it so I can't say how well it will work.
Anyone who adds fertilizers on a regular basis, even compost, will most likely see high levels of P and K in their soil which is why I do not add additional fertilizer.
However, remember I am gardening in ground so my nutrients do not leach out like I think that they do in containers.
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