What's the real deal with yellow leaves?

[tadaa]

After doing some searches on here I haven't been able to distinguish what can cause yellow leaves. I've read it could be not enough N, too much water, not enough water, too much fertilizer, etc...

2 out of 3 of my plants have some yellow leaves at the bottom, they are about 4 weeks old. I'm using soil that has fert built in, but my one plant is perfectly green so I doubt it's burn from the ferts. The biggest plant has a few small yellow leaves at the bottom that are completely yellow. What do you think?

[Herbus]

Are you using Miracle Grow soil or some other kind?

Herbus

[J DOG 6000]

Those leaves might not be getting enough light so the plant uses the built up nutes in them and they turn yellow and die.

[tadaa]

Are you using Miracle Grow soil or some other kind?

Herbus

another kind. Also, what causes the plants leaves to point upward? My light is really close so I know they aren't stretching, the stems aren't too long-the are nice short and bushy but the leaves sometimes point upwards a day or two after watering.

[Herbus]

It's hard to say what's wrong when you use a soil mix which contains slow release fertilizer. Do you have a means of checking the PH. Upturned leaves can be caused by high PH.

Herbus