Apparently urea or ammonium (NH4) is not good for indoor seedlings due to the cummulative effect that can occur. The ideal seedling fertilizer has a 2:1:2 (N:P:K) ratio. Nitrogen should be in the nitrate (NO3) form. Fertilizer should be applied once a week to seedlings that have their true leaves. I am still going to wait until they have their 2nd set of true leaves.-- and yes I watch too much Spongebob Squarepants!
Jeorgensen, Kenneth. (December 4th, 2003). Suite101.com: Choosing Seedling Fertilizers.
Janne, E. E., Roberts, R. E. (February 2001). Texas A&M University. Timely Tips on Starting Seedlings at Home.