I'd say it would be safe to use, but I would absolutely not use it at the concentration they recommend! It is primarily a fertilizer, and that application rate will give you 550 ppm N!!! I would stick to less than 1 tablespoon per gallon, as 1 Tbsp/gal is about 160 ppm N.
Out of curiosity, what is the ideal PPM range people shoot for on a weekly basis? Currently my plants get about 150 per week.