OK, here's my two cents, since this has come up for me before...
First, let's assume that most people growing the ganj are NOT self-proclaimed "old-school" "experts." Let's also assume that they ARE capable of reading and following the instructions printed on most brands of plant food, whether it's Wal Mart tomato food (what up, UB!) or some high-tech hoo-hah hydro solutions.
Let us also assume that the individuals that compose this vast majority of small-scale growers have jobs, lives, families...in other words, the ganja in their closets is not the focus of their existence, nor is it their primary source of income. This means that they are unlikely to engage in taking detailed notes about their long-term experimentation with different concentrations of nutrients, brands, and mediums (media?), simply because they have more immediate concerns, like walking their dogs or feeding their (human) children. So:
We have a picture of a person who aims to grow a little in his closet, while otherwise engaged in the rest of his life. He plants his plants and proceeds to pour the manufacturer's recommended dosage of nutes on them before going back to his job/wife and family/taxes/whatever. He is not wearing a lab coat or carrying a clipboard. He is not an arborist, he's a regular dude. If the plants do not keel over and die, he (quite reasonably) assumes that the nutes are doing their job, i.e. making the plant stay green and grow taller.
He is probably not aware that the manufacturer's recommended dosage of most of the nutes on the market (in my experience, anyway) is exceedingly high, inevitably resulting in salt buildup on the roots and in the medium, and why would he be? This is not his primary breadwinner, just a side project. Those salts need to be flushed out of the medium and off the roots, or nutrient lockout will result. This is not debatable. Therefore, flushing regularly during flowering, as well as pre-harvest, is advisable for EVERYONE WHO DOES NOT CONSIDER THEMSELVES TO BE AN "OLD SCHOOL" "EXPERT."
Personally, I also like to flush right before flip, and I will do it at least once during veg, especially an extended one. Again, that excess salt buildup...
Some of you have undoubtedly perfected your feeding regimen. (Some of you are undoubtedly kidding yourselves about the perfection of your feeding regimen as well, but that's for you to find out.) In any case, advising the people described above to feed their plants precisely what they need in terms of water and nutes, no more, no less, and give up their noob-y flushing is fatuous and manifestly un-helpful. Regular people have more to do than rip bong hits and bookmark forum threads, not to mention wading through hundreds of pages of sneering, snarky pissiness for the answers to fundamental questions. These sorts of sneering replies have kept me from posting questions here; I'm no old school expert myself, and I need answers, not to be sneered and pissed at.
For the record, I'm using Cutting Edge nutrients (although next round I'm gonna try out UB's tomato food on a couple for a side by side...). The medium I'm using atm is Sunshine Mix; I don't remember if it's #2 or #4. Probably some of both. My well water is real high in dissoved solids (ppms range from 300-450), so I dial back the manufacturer's recommended dosage by 50%, and my ppms wind up hovering around 1000. I used to run it full strength in a flood and drain hydro setup, with city water, and had many troubles with extreme saltiness. I used to spend unholy amounts of time scrubbing reservoirs.
So to sum up: if, like me, you dont consider yourself to be GG2G (god's gift to ganja), for the safety of your darling little girls, flush regularly. Many people advise doing so every third watering during flowering, and after the yellowing that occured halfway thru my current round when I slacked on flushing, I agree. Do it at least once during any veg that lasts more than 3 weeks. Do it right when you flip, to avoid any nute confusion during the time when the needs of the plants are changing. And yes, do it at the end, pre-harvest. My argument for this: at harvest time, the plants are done, finished, finito, geschlotten, DONE I say with using the nutes for growth or anything else. They are dying, whether you give em nutes or not. So not only is it nice to sorta clean them out, but they may keep on sucking water and packing on weight. Weight=love. And yes, two weeks seems like an awfully long time. I plan to flush pre-harvest for a week, or three waterings, whichever comes first.
'Nuff respect to rollitup and its denizens. I've learned plenty here, and hope to continue doing so for a long time to come; I sincerely hope that my comment is useful to someone, and that I'm not just strokin it here.
