Burning from either feeding too much or from a soil mix that is too "hot".
You are feeding these small plant a lot of different things, baby plants need baby feeding!
Autoflowers can usually get by just fine with half strength nutrients too.
With soil growing, a schedule of plain water/plain water/feed/plain water/plain water/feed etc, so that they are only being fed every 12 days or so, which is plenty, as soil "holds onto" nutrients a lot more than other substrates.
Remember too, it is very eas to fix a slight under feeding, but very hard to rescue a burned, over fed plant.
Hope this helps, Organoman.