His Nitrogen source can be either a nitrate salt , urea, or ammonium salt (or any combination of these three). In fact he even mentions that he uses "carbamide" which is a very fancy rocket science term used for something as common urea - which is a component of urine. Have you ever notoced that the grass always grows greener in the areas of the lawn that the dog urinates on? Or have you noticed that cow pies in the field always seem to have fresh grass growing through and around it taller than the surrounding grass in the field? Well that's because urea and ammonia are powerful sources of Nitrogen, which in turn is a major component of Chlorophyl, which is green.