rocketman
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potassium phosphite
Why is it that potassium phosphate is readily available but potassium phosphite is not?
Phosphites of potassium are used as fungicides (in a loose sense) in agriculture to combat water mold infection. Confusingly, they have also been
marketed as fertilizers to avoid a regulatory burden. 
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KoiosPhoebus
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Demand for potassium phosphates is much higher than demand for potassium
phosphonates/phosphites. Monopotassium phosphate is a very common fertiliser which supplies both potassium and phosphorus (in the phosphate form), and
its crystals have useful optical properties.
Additionally, both monopotassium phosphate (KH2PO4) and dipotassium phosphate (K2HPO4) are also used as
buffering agents and sequestering agents in several industries (just off the top of my head - research (phosphate buffers), medicine, and food
preparation). Tripotassium phosphate is less commonly used but is sometimes used in place of trisodium phosphate as a cleaning agent or emulsifier.
Their wide range of uses means that they're widely produced (lowering price per unit mass) and reasonably available.
On the other hand, there aren't really any uses for the potassium phosphonates outside of agriculture/horticulture. And even in agriculture, their use
is limited - as per the article you quoted, phosphonates* are pretty much only used for combating Oomycete (water mold) infections**. There's
quite a range of fungicides which combat other fungal pathogens in addition to water molds; and many of them have curative and/or eradicant action, as
opposed to phosphonates which must be applied preventatively. Hence potassium phosphonate isn't used widely enough to be a readily available compound.
Having said all of that though, a solution of monopotassium phosphonate and dipotassium phosphonate is available at my local gardening centre as
"Systemic Fungicide", so there's probably some variation by location.
*: This also includes ethyl phosphonates, which are found as aluminium tris(ethyl phosphonate) or fosetyl-Al. Fosetyl-Al is more commonly used in
agriculture (especially in the solid form), which is another contributing factor to potassium phosphonate not being commonly available.
**: Though there's some evidence that phosphonates can induce plant defences against other pathogens, they're not commonly used for this
purpose.
[Edited on 23-6-2023 by KoiosPhoebus]
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