Nick,
I did a quick look at Plantone fertilizer. They are all organic, I have used them in the past and liked them.
There are two versions that you might like.
#1. The original Plantone is a general garden fertilizer (comes in a brown bag). The NKP rating is 5-3-3. I have found this is a general all-round fertilizer, is easy to start with and generally covers a wide range of fertilization needs. It is hard to go wrong with this one.
#2). The second one I would recommend is Garden tone. This comes in a green bag. It is slightly optimized for fruit settings (by fruit, think tomatoes). It’s NPK rating is 3-4-4. It has not so much Nitrogen but a bit more Potassium and Phosphorus. Considering that Potassium deficiency is a possible culprit, this might be a good option.
Realistically, either one is going to be a very balanced, well rounded fertilizer and
should rapidly clear up any nutrient deficiency quickly. Better yet, most of the nutrients are in a long release version so an application should last.
Food for thought,
Eric