Due to the very very sticky bonds that P almost instantly forms with the medium, you’re left with two choices. One, dump absurd amounts of P into your medium with the hope that you use so much, the plant gets some of it. This is where those finishers and bloom lines come in, and it’s also what synthetic farmers do. You hit the plant with so much P it can’t help but take some of it up, but 99% of it is lost.
Or, if you’re growing organically, you use myco and a very small amount of P, juuuust enough to keep things balanced. For example, I cooked up 48 gallons of soil for this run. In that 48 gallons I only used a 1/4 cup of soft rock phosphate. That comes out to around 1/4 tsp per gallon of soil.