Fertilizers' NPK values tell me how much nitrogen, phosphorus pentoxide, and potassium oxide equivalent they will have 'available' for my plants. The fertilizer may also contain nutrients are not 'available' now and not reflected in the NPK value.
Another question about fertilizer asks how to compare colloidal soft rock phosphate mined in Montana is labeled 0-7-0 when that mined from Florida is labeled 0-3-0, while they both contain about 20% total phosphate. The company's blog post about the transition says that this this is because the rock mined from Montana has more 'available' phosphorus than the rock mined from Florida.
How can I compare the availability of these two fertilizers over time? What timeline are the reported NPK values based on?
I think what I want is behind this paywall: ISO 18644:2016 Fertilizers and soil conditioners Controlled-release fertilizer General requirements
