I don't want to completely disagree with you fatman, because I respect your opinions and how well you grow. But I'm going to have to... I guess what I was trying to say is that it's ok for pH to fluctuate, and in doing so some nutrients "may" and actually "should" be better absorbed. Though the chart I am placing in this reply shows 5.5 as the low end of the range, I still think that 5.3 to 5.8 is optimum. But that's my opinion. If you're able to hit a pH of 5.5 and have it stay there ALWAYS, well then you're a better grower then I. My pH NEVER stays completely solid (nor should it. and nor would I want it to). But, I am always able to keep it between 5.3 and 5.8 (my personal optimum values where I see my plants do their best). According to the chart I've placed here, certain nutes like certain pH levels. My assumption is that when the pH level hits an exact optimal number for each different type of nutrient (when fluctuating), it will be better absorbed by the plant. Having said that, as long as you're within the correct ranges, all essential nutrients will always be absorbed by the plant. But, each different type of nutrient definitely likes a different pH level for "perfect" absorption (e.g. according to the chart for hydro, phosphorous starts best absorption at 5.8, so there for when my pH level is lower than that, my plants aren't getting the optimum amount of P they want. In this case, I don't mind that my pH level will swing higher than the 5.3 that i generally set it at when I do a nute change)... To be completely honest, my pH seems to sit at 5.5 more often than not. But it definitely swings higher and lower from day to day, and to me that's a good thing!
And besides, it's NOT possible to actually keep your pH level at one set number at all times!
Not true! It is very easy to keep the pH within an optimal range of 5.6 to 5.8. if you have a problem with doing so by just adjusting just once a day at approximattelty the same time of day every day, then adjust twice a day or simply automate it to allow a swing in those ranges. If adjustements are needed once per day or twent or 100 times a day a controller can handle that with ease. With a grow of say 100 one foot SOG plants the stability of the EC and the pH provided by a couple controllers an a couple pinch valve from a a few rersiors of Ph up, pH down and some fertilzer concenrtae plus an auto watertop off sytems as simple as a gravity fed float will easily be recouped with value of the increased yield and quailty of just one grow if buying your equipment on eBay or LabX.
say In fact, when your plants are absorbing nutes, the pH level MUST change.
Not necessarily but quiteoften as fomulations are c based for average grower needs not the specific needs of the individual. If your pH level is constant day to day, then your plants are not absorbing some of the nutrients available to them...
But as you just said the pH is not constant. They are allowed to constantly swing within the optimal range. The pH goes up nad down constantly all day and to a lesser extent at night. So if as desired the pH is kept within the optimal range then the chance of the roots to have the majority of the nutrients vailable for the majortity of time will happen if the pH stays in the opimal range. It is like rolling the dice. There is a proprotionality formula that shows the odds of say for example: if you roll 6 dice 100 times how often you will likely come up with a 3, 4 and a 5 with six rolls of the six dice. The same math (statistics) formula shows 5.6 to 5.8 to provide the best odds that the the three controlling nutrients will most often be available. Or there are other statistical ways of showing probability methods odf statistacal analyis that will provide basically the same answer.
Ie with a pH controller you set your high at a pH of 5.8 and a low of a pH of 5.6. You then set just how far outside that range is exceptable such as 0.01. The controller will then if the pH reachs 5.81 will lower the ph to only 5.79. Then if the on on its own swings back and forth betwwen 5.59 and 5.81 it will do nothing. However if the pH drops to 5.59 it wil raise the pH to 5.61.
eh, but what do I know???
-S
PS
After studying this chart a bit more, I think that I will start letting my pH fluctuate to as high as 6.0. It looks to me that 5.3 to 6.0 is optimum. Again, that's my personal opinion....But that's what the graph seems to show.
Poor interpretaion. 
is optimal. You have declared this as fact with no back up or support. You have stated years or research and yet havent named a source. I have researched tons of articles, mags, and books and they all seem top vary on whats "optimal". I could just pick out three and say these are "facts". Just like the government has stated that MJ is bad for you, that was a "fact" for a very long time. That is why I beleive my own eyes over other people's "facts".
