gradual pH drop/ regulating pH

Since the beginning of the grow 9 weeks ago the pH of my nute reservoir has always gradually fell instead of rise. This is opposite of what most knowledgeable growers say you should experience in a normal hydro grow op. I understand there is an interplay among nute uptake rate by plants, reservoir size, temperature, water source, effect of nute additions, etc.

Setup: reservoir capacity is approximately 3 times (42 gallons) the capacity of the grow tubes/pipes used; cycling nutes every 2 hours with a 25 minute fill/drain time. Nutes are maintained between 65 and 72 degrees. Rainwater (50-100 ppm) is only source for reservoir; it is slightly acidic at about 6.9; during bloom I am now maintaining 1300 ppm (.5 scale) nute concentration. I am not regularly changing nutes out completely. On transition to bloom the reservoir was drained and the plants were flushed before refilling w/ fresh nutes.

Plants are in excellent health health and vigorous. I have adjusted pH periodically as it falls from a pH of 5.8 down to 5.6 over 3-4 days; using wood ash filtered through cotton cloth. Agitating the nute solution will give a slight boost to the pH and I can hold off on actually adding more ash by stirring nutes but eventually it does become necessary to boost pH again.

Would dropping EC to around 2 or less help stabilize pH? Can I just let the pH fall to below 5.6 and see if it levels off somewhere close on it's own? Some growers have indicated 5.2 to 5.6 is still suitable for efficient nute utilization. I have not tried substituting well water (limestone geologically) instead of rainwater to this point but that is still an option. Would aerating the nute reservoir contribute to the drop in pH?

The plants are robust and growing aggressively so I am not very concerned at the moment, it's just that I expected to see the pH work opposite from what it has thus far.

Try dropping nute levels perhaps. Near the end of flowering I refill res every 5 days. My PH will start to drop at the end of 4 days and I cannot keep it from dropping unless i add up every day so i change it. Im a real newb so perhaps Im doing it all wrong lol. I never run more than 700 ppm tds and seems that my plants are happy at that. Also if i add a bud booster (overdrive) it will knock ph down as well.

My PH drops as well but no worries here. I am using RO water which is around 7. When I add my GH micro/bloom to bring the PPM up to around 1325 @.7 (947 @.5) the ph drops to 5.0. Adding just a tiny bit, 1/4ML (not tsp), of PH up brings the PH up to around 5.8. Then, because more water is used than the nutes it contains because of evaporation the PPM will rise as the water drops making the PH drop.

I am not an expert but 1300 @.5 (1820@ .7 or 2.6 EC) would toast my plants. As WD said try dropping your PPM... and maybe stop chasing the PH and just keep it as close to 5.2 - 5.8 range as possible.

MuckyDucky said:

My PH drops as well but no worries here. I am using RO water which is around 7. When I add my GH micro/bloom to bring the PPM up to around 1325 @.7 (947 @.5) the ph drops to 5.0. Adding just a tiny bit, 1/4ML (not tsp), of PH up brings the PH up to around 5.8. Then, because more water is used than the nutes it contains because of evaporation the PPM will rise as the water drops making the PH drop.

I am not an expert but 1300 @.5 (1820@ .7 or 2.6 EC) would toast my plants. As WD said try dropping your PPM... and maybe stop chasing the PH and just keep it as close to 5.2 - 5.8 range as possible.

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That makes sense about the pH drop as more water is removed (plant uptake/evaporation) and the nutes slightly increasing in concentration. So you're OK with levels below pH 5.5? I've heard others say things work just fine as low as 5.2. Someone else commented to me that the pH will always drop if you have an active/robust aeration of the reservoir. They made it sound like the aeration was the biggest contributor to pH drop. ???

The 1200-1300 ppm (.5 scale) is what the nute manufacturer recommends for the bloom phase. My plants show no signs of toxicity and seem happier than a pig in shite. I may let it drop off to around 1000 since all that will do is make my nute supply last that much longer. Some successful growers commonly use an E.C. of only 1.6/1.7 so maybe it is overkill.

I never go over 1.4ish and in flower i start the refilled res at 6.1ish by refill its typically around 6.7-8.

My setup, ph drifts up in veg and down in flower.

Different brands of nutes will react differently.. Nute uptake and evaporation rates also change ph. All I can suggest is you check the ph everyday if you're concerned. A little variance is actually a good thing, so don't get paranoid about it.

Why is variance a good thing? I thought keeping fairly constant was a good thing, Im so confused lol.

just look at the ph chart everyone posts around here and it makes sense. See how some elements are absorbed at low ph and some are only available at higher ph? This is why it's beneficial to have a slow drift through 5.5 - 6.2 more or less. Notice the ca/mg is absorbed at different ph? drift will ensure they are able to absorb sufficient amounts of both. When the ph is off, think of it like the roots are trying to suck a thick milkshake through a straw. During rooting and veg you can have ph in mid 5s, during flower I like it to be mostly around 5.9 or 6.

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Great answer Hydroburn. And I also don't trust my PH readings to be .1 % correct.