Leaves are yellowing. Advice please.

[boikola]

This morning I found out that half of the plant from the bottom up, the leaves are yellowing.
Could it be lack of nitrogen?

 

[Muddy]

It's hard to say without more information.

What is your soil?

What are you feeding them, and at what strength?

What is the pH of your feeds?

What is the pH of your run off?

What is your room temperature and relative humidity?