blocked on nitrogen

I'm convinced my aquarium's plants' growth is blocked on nitrogen. My
nitrate concentration reads way below .25ppm N (< 1.1ppm NO3) on a
LaMotte test kit, and also reads unmeasurably low on another test kit.
My CO2 concentration, carbonate hardness, and other water values
(including iron and phosphorus) are within normal levels. Yet my
plants' growth is not spectacular, and two species of green hair algae
are able to thrive.

From this I infer that my plants are blocking on nitrogen, which is
somewhat of a pleasant surprise, as I figure this has got to be easily
remediable. I have a nutrient-poor gravel substrate. Short of a
complete teardown and substrate change - which might be in the works
soon anyway, for room-redecorating reasons - what are the optimal ways
to add nitrogen? It seems the aquarium fertilizer products that have
nitrate in them generally have phosphorus as well, which is
unacceptable as I seem to have enough phosphate.

Is safe dosing with an ammonia fertilizer possible? I've seen blood
meal used with terrestrial plants; its main fertilizer component is
nitrate. I suppose this or something similar could be added to the

Any ideas are appreciated. I'm especially interested in possible
ammonia fertilization solutions, as they seem more elegant somehow.