Yellow leaves & brown leaf tips, is this a magnesium deficiency?

My G-1 plant appears to be developing yellow leaves at the bottom level of the plant. It just started but it is progressing fast. Is this a magnesium deficiency? I haven't given it any nutes in about 2 weeks and the plant is over 7 weeks old. Just starting to flower at the the top of the plant. Any help as to what is going on would be greatly appreciated.

it seems like its a nitrogen deficiency. that happens normally during flowering, but not usually that much.

So should I add some nitrogen when I water next time?