Need help, Browning of leaves.

18 to 20 in is a bit far away. Put your hand at the plant top, and lower the light until your hand feels hot but not too hot to stay there. Also, ease up on the whole analyzing every variable thing. You are in soil, not hydroponics. The only factors you need to remotely concern yourself with is soil pH and NPK amounts. (NPK = Nitrogen, Phosphorus, Potassium)

MJ is a plant, and this is simple botany and horticulture. Relax and let this be fun, not stressful.

ok so that is probably why one of them isn't growing, my intake is in a room that sits about 65-71.... so its usually pretty cold in my house, im wearing a sweater as we speak lol.. i can also turn that light down if you recommend running it at w/e 50% would be. i think its either 25 50 100 or 50 75 100 cant remember but the ballasts are dimmable.

lol ok, im not too stressed but i know i stress bout stupid crap in everyday life... but hopefully these turn out good. like i said its only the two leaves i touched that are turning brown... like they are burnin or w/e and the other ones look fine. the other plants look great, a lil stretchy but i think they look great. just that one plant that is kinda curling up and the bottom two leaves turnin brown. the temp changes quite a bit which probably causes some stress right? id say between 75 and 90. i turned the one light off and moved the plants but when the do get bigger what can i do about that heat? or will it make that much a diff when they are bigger?

Lets examine. We eliminated (pretty much) deficiency, your temps are ok, that tells me you have one pheno that's a bit sensitive to the amount of nutes in the soil. You say it's just the one plant, right?

Ya just the one plant i have another post on here that has a picture of each plant if u wanna take a look at the other ones and let me know what u think about them. Heres the link https://www.rollitup.org/newbie-central/474951-first-time-grow.html

Thanks for all the help guys, i the room is stayin about 75-79 right now. i dont know whats up with the one browning plant still. It has some brown spots on new leaves now. I havnt added any nutes and one for a couple more weeks. it is starting on its next set of leaves though so its still growing just dont know why the leaves are browning like they are. None of the other plants are having any other problems except for the one that still hasnt grown. I was told it was just caused by the heat and it will shoot off soon.