"Nitrate can be incorporated into organic compounds in both root and leaf tissues whereasNH4+is only synthesized into amino acids in the root tissues near the site of uptake to avoid toxic accumulation (Engels& Marschner 1995)."

It helps explain why "root tabs" with urea sometimes work (if the rate of absorbed ammonia by the plant exceeds the rate at which is leeches into the water) but otherwise when used in the water column are a dansgrous (because of the toxicity of ammonia) and foolish practice that promote algae more than anything else you can do really. With stem plants and weak feeders they can't keep up, things like Lilies, Sswords, Aponogetons, large Crypts, Barclaya and the like, they generally can. Usually growth slows or is reduced in size when it needs one, it may be unwise to feed it if it is not showing a need, given urea/ammonia is a potentially lethal chemical in aquaria, erring on the safe side seems prudent here.