Hi guys, the rainwater I feed my plants is 12ppm, I add a base nutrient NPK 1-9-1 and a additive NPK 1-12-13 to the directions on the bottle which brings the food to about 280ppm which seems really low when most seem to talk bout being over 1000. Does this mean I can add more of the nutrients to the food solution? I’m also feeding it the food every 2nd day, should I be alternating between feed and water?
I would say yes but we will bring @latewood to fully answer that
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soil ppm is way different than hydro since the ppm builds up over time when you here higher ppms it is usually an older plant and the soil has built up salts. In hydro the higher ppm is seen mainly during flower or after a plant has substantial root mass
So ppm’ over 1000 are probably either hydro, or runoff ppm rather the food solution?
yes Because that is what most feeding schedules class as aggressive growth when I flush my ladies in soil my run off may hit 1500 on first rinse after 6 weeks of feeding around 800 ppm also depends on product line and additives my dwc buckets get 10ml micro 15ml bloom 15ml terpinator per gal my starting water is 160-180 after adding nutes and ph correction my ppm is right around 1000. During early veg it’s 5ml grow 5ml micro 5ml bloom so closer to 500 also have to factor that ph adjusting changes ppm and my starting water is nearly 200 ppm. During aggressive veg I hit around 800 but this is on a plant with 6-8 tops and lots of root mass 
If your ppm is only 280; Yes you can add more nutrients to the solution. My issue with what you posted was that you are overwhelming the plant with P, but, not adding much N at all. Nutrient solutions should be balanced when starting out.
If you want to kill the N in flower, that is OK but, plant needs N in the vegetative phase!
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In vegetative stage I use NPK 12-2-7. What ppm would be safe to step up to or is it a real and error thing?
aim for the 500-800 range start lower watch for any signs of stress or burn if none and plant responds how you want slowly increase as plant gets bigger
Thanks, and should I be giving food every watering, or should alternate between food and just water?
If in coco should be mild dose of food every water if in soil alternate