Firstly, here is a link that can help explain it in a direct way: https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Principles_of_Modern_Chemistry_(Oxtoby_et_al.)/Unit_4%3A_Equilibrium_in_Chemical_Reactions/16%3A_Solubility_and_Precipitation_Equilibria/16.4%3A_The_Effects_of_pH_on_Solubility
Secondly, you should never be near 8.5 pH anyway, so no worries if a lack of necessary foundational chemistry knowledge makes that link look like mumbo jumbo.
there are different molecules that are used to deliver P, et al, in fertilizers. Some can't even enter the plant until microbes break it down further - which this extra step could definitely be impacted by pH too. If that was the narrow focus of a ph-solubility chart, it'd look different than a chart focusing on a different molecule delivering P. So each one could have some variation in exactly what results as far as a 'safe' pH range regarding solubility and delivering nutrients to a plant.
Thirdly, i've seen numerous solubility charts that shift this stuff around. For the reason just stated, it should only be used as a generalized concept and not too literally unless you know the chart is dealing with the exact molecule you are focusing on.
you need more than hs chemistry knowledge. Can probably extraplote after 3-4 consecutive semesters of undergrad chemistry, but it's been 25 years and minutia fades from memory. Maybe a chemistry teacher can chime in.
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Just to clear up some falshoods below:
Magnesium Sulphate - does not contain any Ca. That must be a type-o.
Ca++(aq) is a +2 positive charge ion (cation). CO3-- is a minus 2 charge ("anion") when dissolved/disassociated in water. Maybe, another type-o?
pH isn't about 'excited molecules' either. It;s about H+ ions... the resulting ratio of hydronium (H3O+) to hydroxie (OH-) that results in the water is how you calculate pH. Acids are proton (H+) donators and bases are proton acceptors. they impact the ratio of h3o:oh in water which results in a measurable pH. it is a log scale with "1:1" ratio at a pH of 7. Neutral doesn't necessarily mean it is absent of acid or base. It most often means there are equal amounts of it -- zero of either is possible but that wouldn't be a common natural occurence.
chemisty is about probability - things just bumping into each other that have the propensity/affinity to react with each other or not. Something bumps into argon, an inert gas (valence shell is full), will just on its merry way. Multiple variables like temperature, pressure and pH will shift what those probabilities are at any one moment in time.
Everything is in motion at all times... what seems like a constant is more often a resulting equilibrium that will shift when variables change in a predictable manner.