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I definitely don't have phycho's expertise and knowledge, and sorry if I sound like a fool I'm very stoned atm... but I have the same exact thing you do when using just plain water and have experienced this as well with the Hydroponics of Holland 3-part. I've dealt with this issue during all 3 of my grows, two of which were flood and drain like yours, the other was hempy (which is a bit like a manual flood and drain.)
There is nothing wrong with my water. LOTS of growers in my area...I live in the 'Dam and I talk shop with the guys from the Dampkring pretty frequently and we've talked about the water here in the municipality... Magash might know those Kiwis... anyway these PH slides aren't unusual here.
From my understanding, this isn't a bad thing necessarily..... it's advantageous in fact if you control it carefully.
As you can see from the chart, there is no PH level for hydroponics where all nutrients are available, but there is a significantly large interval in which cannabis can uptake most of the nutrients it needs. So in my opinion, a PH slide of some type in hydroponic applications is very beneficial. Why would it not be?
What I always do when I fill up a fresh 100ltr tank, I bring the PH down pretty close to 5.5, with the expectation it will become 10x more alkaline within the next 12 hours. I readjust 3-4 times in a 72 hour period, and then it will become stable. This process of recalibrating will bring your EC up slightly.
With the hempys, I would do the same thing, except I would collect the runoff and recalibrate it and then re-water the plant until the runoff was the same PH as the water I was pouring in. This was done every other day - granted with fresh water each time, which is why this was a recursive process from beginning to end.
There are certain nutrient brands out there that have some added ph buffering qualities. At the moment I'm using PH-Perfect 3-Part from Advanced Nutrients. Using this, my PH stays very stable from the first time I calibrate it... however, when I use overdrive with the stack, it stays stable for a week and then I get PH slides down! This is easy to deal with as I just add a few jugs of fresh unph'd water and then every other day I throw in a jug with nutes mixed into it.
So if I were you, I'd take advantage of these slides - I believe if you recalibrate several times over a day or two, and your water will become stable.
As I side note... I've created some functions that can, now, predict fairly accurately the rate of change, and how much buffer is needed to adjust it. It may pay to do something similar. I might be able to help.
-------------------- andyistic said: Ok so let me bring you idiots up to speed.
The admins are tired of this shitfest being made the joke of the weed community on the Internet.