im using the same all purpose MG nutes! lol
but ya, the plant is pretty small for 19 days. but like dude above said, depending on where U live, U might not get a full 18 hrs of sunlight. that would cause slow growth.
and also some sativa's start off a lot slower than indica's. Im assuming sativa bcuz of the germination time (3 days) and the fact I had a plant that looked just like that (you lucky dog! what strain is that BTW??) my plant I had that looked like that was a stretcher too!
its all about the light intensity tho. the brighter the light, the less stretching.
that plant looks to be about a week old, and if that indeed were the case, that plant would be perfect. just the right amount of stretching. but for 19 days its not growing too fast at all, especially if its in miracle grow.
speaking of which, dont feed it any nuts till the bottom leaves start yellowing (like UF said above) feed the plant straight water once every 3-4 days for about 3 weeks, then do as UF said and give her a lower strength feeding.
Ive tried spraying the MG nutes on the plants only to find a powdery reside left behind when it dried. I wouldnt recommend it, especially since its not necessary. if U wanna spray, go get some super thrive and spray that on the plants every once in a while. that'll keep em nice and healthy