The main difference between hydro and soil fertilizers (Miracle grow) is the nitrogen source. Ideally, only calcium and potassium nitrate are used in hydro, where soil fertilizer often uses urea and ammonium based nitrogen. The urea requires a soil organism that does not exist in a hydro system to convert to a usable product. The ammonium nitrogen can cause root damage if too much is used. It seems to work OK with up to 15% of the nitrogen source. Ammonia nitrogen can be used in moderate amounts to bring a high PH down.