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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

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    I posit that "timeline" is the the appropriate measure. The difference is apparently the amount of phosphorus that is soluble and can be absorbed by the plant.
    – davidgo
    Mar 27 at 0:48
  • @davidgo And over time the phosphorus compounds that were not immediately soluble become more so. I'm wondering where the line is drawn. Two weeks? Two months? Two years? aces.edu/blog/topics/crop-production/…
    – MackM
    Mar 27 at 14:26
  • To me it seems strange to put different "timeline". For agriculture: we have a list of fertilizers and often we prefer one for N, one for P, and one for K (and one for Mg, ev. also B). We decide which product depending on timeline (with so many fertilizers, it is often better to distribute one kind every year, so we want fertilizers which are released slowly. But on sandy soil or for short term solution we may choose differently. So if manufacturers add own interpretation we got chaos. PS: or maybe just hiding bad storage, so with decomposition. We need the ingredient list and googling. Mar 27 at 15:28
  • You can do a search for most ISO docs and find out "most" of the data or often more than what is in the standard. ncbi.nlm.nih.gov/pmc/articles/PMC8272235
    – jwilleke
    Mar 29 at 22:28

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