Plants absorb their nitrogen first from nitrates, secondly from ammonicial nitrogen sources, and finally from urea. I know that urea is inexpensive but why use a source of nitrogen that plants cannot uptake efficiently?
75 Gallon, 2X55W AH Supply CF 8800K, 1XMarineland Doublebright, 2X Marineland 350 Magnum; 45 Gallon Tall, 96Watt AH Supply CF 6700K; 30 Gallon Long; 2X36W AH Supply CF 6700K; 20 Gallon, 1X26W AH Supply LED kit; all with Press. CO2; Heat Treated (Calcined) Montmorillonite Clay
Greater Seattle Aquarium Society (GSAS)