@Rexx they look really great mate I would probably agree it looks like a Nitrogen your readings are perfect so I would recommend maybe a grow nute or stand alone Nitrogen definitely if you are growing organically hope this helps mate.
Yes it progressed into top leave but i think i found my problem lacking nitrogen i wasn’t following my nutes mixing guide it uses calcium to up take nutrients according to them its more efficient so the N P K levels are lower and i was using them half strength so they were not getting fed so i gave them full strength and we will see what happens . Only problem is I forgot to ph first gallon and put on plant when i tested 2nd gallon ph was 5.6 i ph balanced that one.
That was my problem the manufacturer explained to use full strength because of the way calcium based nutrients have much smaller % of nutrients but are delivered more efficiently .The moral to this story is read the manufacturers instructions .
There are a few companies that are selling nutrient lines and are wanting to run HIGH values. Remember that they are in the business of SELLING nutes so they want you to use those values to sell more nutes!
This is where it pays to pay attention to what is really going on and use common sense to develop your own schedule. Of course, experience is needed to make sure it’s done right.
IMO all who grow in soil should trust that soil for the first month at least.
How old is the yellowing plant, what percentage is yellow, are you in flower and if so how long?