Urea should work but it's about twice as strong as Ammonium Sulfate so you might want to be careful with the application and I'd definitely not try it as a foliar fertilizer since the chance of burning your plants is too great. Also, I don't know the pH of Urea so you might want to check and compare to your soil as a precaution since I'm pretty sure that Ammonium Sulfate acidifies soil. If you want to go a tamer route, you can use 5-1-1 fish emulsion sprayed every week or two and get good results.