[BKKG]

If I had to guess I'd say that your problem comes from the potting soil. Potting soil often has nutes in it, which wreak havoc on poor little plants. Big plants can handle it better, but eventually they'll succumb too. Sprouts barely have a chance; the burns you're noticing are probably nute burns from the potting soil. What kind of potting soil was it, and does it say anything like "Feeds plants for up to X months" on it?

To fix it, you'll need to transplant to new soil and flush out the root ball of each plant during the transfer.. I'd recommend buying topsoil (even from walmart or home depot, dirt is usually dirt), and avoid potting soil like the plague. If you need to keep using the potting soil you've got for some reason, you can try flushing the new pots incredibly thoroughly before moving your plants into them.