Well im not going to say if synthetic nutes are safe, thats beyond my knowledge. I do know most store bought leafy greans are grown hydroponically with synthetic ferts, and they do not practice flushing.
The question should be, are synthetic nutes sustainable? Can we continue to farm cannabis over the next 50 years using bottled nutes? How many gallons of used nutrient solution is being flushed down the drain or into our water stream? We flush the nutes out of the cannabis into our water supply and just end up taking it in in a different form.
I have a large background in sustainability and have worked on permaculture farms. My method of farming is to create a living ecosystem where the plants choose what they want to uptake as food, and all waste is put back into the soil in the form of compost. In this case there are no nutrient buildup to flush, the plant creates this beautiful fade as it eats the remaining nutrients stored in its leaves and naturally finishes its life cycle before harvest.
In spain we use ammended soil that is composted and cooked to be resued several cycles down the road. No reason to flush nutrients out of the soil when we can try and reuse them down the line.
Again this is just my opinion, I am loyal to the soil as some say. I don't grow with salt based nutrients so I will not comment on if flushing is necessary, but in a organic style of growing flushing is not necessary imo. If your interested in learning more you can do some research on permaculture, no till, living soil, knf, regenerative farming.
Jeff lowenfells books on microbes, fungi, and nutrient have helped me tremendously understand how plants regulate and absorb nutrients. The kis organic podcast has also been a major influnce on me.
Sent from my SM-N950U1 using Tapatalk