Sources of nitrogen for plants:
Ammonia (NH4) - most plants, but not all, will accept ammonia, but it is toxic to fish/inverts. Generated by fish waste, converted to Nitrate by biofilter
Nitrate (NO3) - all plants I know of will intake nitrate for nitrogen
If you need more nitrate, KNO3 is probably the simplest, fastest way. That said, i would test your nitrate levels before assuming you need more. No sense adding more if you have lots from your bioload.
Running it through the calculator on calc.petalphile.com, your current 3/32nd's dose in 10 gallons should be about 8 ppm of NO3 once a week.
The calculator's EI low-light calls for 10ppm once a week (1/8th tsp KNO3 would provide 10.5ppm), but it isn't like you are hugely short (unless you are doing high-light and CO2).