It's pretty obvious that you have some sort of ideology that prevents you from accepting that the biological processes that occur outdoors in a forest or field cannot happen indoors, where the environment can be more controlled. I find that odd, but I accept that you have a very different point of view.A plant's nutritional needs are not static over the course of a grow.
If you believe that substrate K or NO4 at 2 weeks should be the same or lower than a 6 week old or flowering plant then there is good news, you have a huge opportunity to improve your flowers.
But I do take exception to your statement that crop stubble is harmful. Agriculture has changed in my country in recent decades. The holy grail among commercial operations is to have ample organic carbon continually cycling in their soil. Around here most big dairy farmers don't rough-till their corn stubble in the late fall anymore. Everything goes back on top of the soil except for the corn grain itself. Even smaller operations are starting to do this now as the equipment to do it gets a bit cheaper. Farm equipment stuff is expensive, so a change over to support less or no tillage has to pay for itself at some point. The fact is, it does and it's capitalism in action.
I'm not going to argue about the laws of conservation of quantity concerning phosphorus or any other substance because it would not be worth my time or yours.
But I quoted the part of your post above that actually interested me, that the "plants nutritional requirements are not static over the course of the grow". This is obviously true. However plants are fully capable of summoning certain bacteria in vaste armies that can actually modify their genetics to produce what the plant wants when they want it. Plants release certain "messenger" proteins that will mobilize bacteria, and the bacteria in-turn modify their genetic offspring through a mechanism called quorum sensing. Since bacteria multiply fast, if a cannabis plant is in need of extra nitrogen for example, it can have its availability increased within a couple of hours.
Allowing my plants to get whatever nutrients it wants whenever it wants them is one of the reasons I went organic, because the nutritional requirements of the plants change over the course of the grow, and even during the time of day. Letting the plant decide is way better than someone deciding for it while thinking "well hmmm, should I use these flowering nutes now that stretch is almost ended, or wait a bit more? Let's try this PPM concentration then!". Good grief.