First time grower

hi,im a first time grower and i have 4 litle young babies..they are on vegetative stage at the moment and they are 3 weeks old. if anybody can take a look at these pics and tell me why are the tips of the leaves becoming yellow,and what can i do i would really appreciate it.thanks i will post the pics when i know how.meanwhyle please helpppp

add those pics...

I love the look at these pic threads. Then there are no pics.

hmm im not so sure dude. check pH, aim for 6.5. check temp, aim for 75. make sure you arent nute burning

Duece.

i didnt use any nutrients yet. but the soil im using has between 5.5 and 6.5 ph, humidity 50-60% ,n - 150-250 mg/l ,p - 150-250ml /l , k 300 -500 ml /l . where im from i cant find neither the miracle grow or the peters fertilizer ,so i really dont know what to do.

i would put them in separate containers. does your soil have good drainage? 5.5 pH sounds very low for soil (I should tell you i grow hydro so my help will be limited)
- what kind of light you using and how close is it?

here is a page about plant problems w/pictures:
Marijuana Plant Abuse

you can order whatever you need online - that way you're not stuck with whatever a hardware store carries.