Or just give it a drink of Epsom Salts, about a tablespoon to a gallon and see if that helps. Can't hurt. I do give all my Ee's a dose of Epsom Salts at least once a month along with regular fert.
Most soil tests just give you pH and general levels of soluble salts, which may or may not indicate Magnesium. You'd have to send your soil sample to UF plant lab and pay $25 for the in-depth test if you want specific salts identified. Unless you know a chemist personally. In the rainy season here in FL, you seldom get much soluble salts showing up in a soil test. They've all washed away with the first deluge.
Jf, you definitely need to give that plant a little more sun protection, morning sun is ok but from about 11am, it's pretty intense at this time of year for Alocasias.
