Identify KNO3 and KH2PO4 at home I am not expert in chemistry. I need to find a way to identify a $\ce{KNO3}$ and $\ce{KH2PO4}$ powder at home. I suspect the seller gave me the wrong powder. What is a simple way to test between these compounds at home?
In the absence of other chemicals you can utilize the fact that nitrates(V) are much stronger oxidizing agents than phosphates(V).
You can soak a piece of newspaper in a saturated solution of the salt, let it dry well in a ventilated place and try to ignite it.

If the salt was $\ce{KNO3}$, it will burn vigorously and fast (that's why kids are often using paper soaked in sodium nitrate as primitive fuse for home-made firecrackers and rockets).

$\ce{KH2PO4}$ doesn't promote combustion of paper, so it will burn normally or even slower.