Purple Thai

1500PPM is not nothing, the land is not empty, you did not wash out all the fertilizers. Also when growing we tend to call the "land" or the substance its growing in inside the pot its medium/potting mix, and the nutrients/fertilizers tend to be measured in EC or PPM. 1500PPM is 1.5EC on a 500PPM scale, its a bit high for a seedling. I'd also get your PH a bit higher then 5.5, I would aim for aorund 6.0-6.2 Good Luck!