Yes, your current understandings are pretty right on.
The problem here is that cheap kits that we use often measure total phosphorus content rather than specific type (Lab grade...expensive) and if your organic bound PO4 (Cannot be used by plants until broken down into inorganic forms which will also depend on your largely on how efficient your bio bacteria are.) content is very high, it will be picked up and folks would think that they do not need to dose any PO4.
You don't really need to get anal about PO4..[
] Just make sure there is enough for plants over the week should do the trick and inorganic forms are the best way to do the job (You dose it, you know how much it is available...leave it to the fishes or fish food and things get complicated.)
Plant Physiology by Taiz and Zeiger
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