Yellow leves, is it lack of N?

hullo,
Out of my 9 plants growing indoors 2 or 3 have bottom leaves, the first that grew that are yellow or turning yellow.

I read on here that lack of N can cause that, but the plants were only planted 2 or 3 weeks ago and my compost should be good for at least a month.

what else could it be?

I have started adding tiny amounts of 2.1.1 and chicken shit compost that is 4.1.1. to try and rectfiy it, but is that the right thing to do?

Iam scared!
thanks in advance for your advice

Now is the right time to slowly start addin nutes @ 1/4 of the recommended dosage. Those bottom leaves are supposed to do that. Yer Nitrogen will take care of it! Here's a link fer further info:
http://www.1stmarijuanagrowerspage.com/growfaq/marijuana-fertilizing-and-feeding/sick-plants-changes-in-colour-of-bottom-leaves/