first time flood and drain project, major PH problem.

Hi all.
I have switched from using coco to flood and drain, and this is my first run.
I'm running 18 x 14 gallon tubs with clay pebbles, they are fed from an 80 gal reservoir.
When fully pumped out the nutrient level comes up to about an inch below the surface of the pebbles. It pumps up in 8 mins and drains in 30 mins and repeats every hour.
The temp in the grow space is 27C, the res is situated outside the building and is cooled by a heat exchanger (it's summer here) so it sits around 20C in the day and 16-18C at night. the cooling water is 14C, so can't bring the res down lower than 20C in the day. The res is bubbled with the biggest aquarium pump I could find (4 outlets, 4 air stones, tons of air.)
I'm using 50 milliliters of 50% H202 every 4 days in the res to prevent root rot. I think I got that calculation right, but I could be wrong.
Nutrients are an Australian brand called Nutrifield, I run the concentrations stated on the bottle. Currently 19 days from germination so I am feeding at EC 1.0. Another week and I should be up to 1.2
My problem is these seedlings look sick, real sick. leaves are pale and growth is slow. I pulled a couple out and no sign of root rot, just poor root development. Both roots and shoots should be 3-4 times this size compared to previous grows.
The water supply is PH 6.3 and doesn't even register an EC on my bluelab truncheon, the water is from a very deep bore and I never had a problem with it in coco and once nutes are in the solution it's about 6.5ish. I then lowered it to 5.8 with phosphoric acid but within a few hours it was back up to 6.7-6.8. lowered it a few more times and the solution turned cloudy.
Some googling indicated it could be calcium rich water, and the phosphoric acid was possibly causing the calcium to precipitate and neutralizing the acid, and I do not have access to nitric acid as it's heavily watched here and hard to obtain. I was going through something like 60ml of 80% phosphoric per day trying to lower this PH and it just wasn't working so I drained and refilled gave hydrochloric acid a go. Hydrochloric is doing a better job, but I'm still putting in 40ml of 35% hydrochloric per day and the PH still rises from 5.8 to 6.5-6.7 over night and these plants are still tiny, I'm not convinced it's the plants lowering the PH.
Sorry no photos of the plants, but the best way to describe them is that they are yellow on the bottom leaves graduating to lime green on the new growth. No signs of micronutrient deficiencies.
What am I doing wrong?

Thanks in advance
TGS

There are several different things that can cause drastic PH increases.

1) Biological action. It could be from algae, or other bacterial growths in your rez, or in your case, also in the flood tables. As far as I know the only solution for this is to use IN-organic nutes with absolutely NO ingredients derived from anything organic or plant based. Something like Jacks 321 hydro will work great, but be careful if you use any cal/mag, or other additives that have ANYTHING derived from plant material.

I had this exact problem when I was using Advanced Nutes PH perfect line, Mega Crop, and Fox Farm nutes. Once I switched to Jacks321, that all went away and I was able to reduce the chlorine I add to next to nothing. My rez stays fresh for up to 9 days between refills. I am drain to waste in an AAA setup, so no root exudates to worry about.

2) It could also be from the roots interacting with the solution, but that is dependent on your EC level to some degree, but if you have bad bacterial growths, your screwed.

Bottom line - do not ever use anything organic in hydro.

3) The calcium/alkalinity issue you mentioned is the next one. Every time you PH down, the dissolved C02 level increases, which lowers the PH by the formation of carbonic acid. Unfortunately, when you aerate, the excess C02 in the water leaves the solution, which lowers the concentration of carbonic acid, which raises the PH. The more you aerate, the faster the PH goes up. The worse your calcium problem is, the higher it will go. Bsically, your water has what I think of as a 'natural' or preferred PH level of just under 7. When you PH down, then aerate, it wants to go back to that PH. How bad this is depends on your water, but if its well water, there is an excellent chance you have this problem to some degree.

4) The other thing is temperature. I just confirmed this recently in my grow. I try to keep my rez water around 65F as a max, but that water feeds down to a small holding tank that is about 1 liter. That small tank is used to adjust the water level for the AAA nozzles to siphon from, so it has to be near the root chamber. That area where the tank sits varies from the hi 70's to the hi 80's temperature wise because I have to heat my root chamber or it stays too cold. So that water in the tank varies from the upper 70's to mid 80's, while my rez stays at no more than 65F and often down to 55F when I do a rez refill or add ice to keep it cool.

Here is where the problem arises. The dissolved C02 level goes DOWN when temperature increases, and goes UP when temperature decreases.

Remember the dissolved C02 forms carbonic acid which is what lowers the PH. So, any time there is a temperature change, the carbinic acid level changes which changes your PH. In my case, when the rez temp is near 65F and the siphon tank is down near 75 F, the PH goes up by about 0.1. When the rez is cold at 55F and the siphon tank is at 85F, the PH difference will be 0.3 to 0.4. So when I was setting my rez to 5.8, my roots were being sprayed at 5.9 to 6.2 PH.

Your situation is much like mine - your rez is kept cool, but the water in the flood table will be much warmer. That temperature increase will drive the PH up to at least some degree. How much depends on the temperature difference and the max temp the water reaches, so it might be different at different times of day - just like mine is.

I have been fighting all these issues for two years trying to sort out why my PH was impossible to control. It turns out I have all of them. My solutions were as follows.

1) Never ever ever use anything organic in the rez - ever. Bacteria and algae just love that crap and they grow like mad when its available. By the way - this very much includes any so called "beneficial bacteria". Dont use it. I also am absolutely fanatical about light discipline as far as light getting into the rez, any fitting, and all tubing. Algae wont grow if there is zero light, but you much check all fittings, and pipes, buckets, lids etc.

2) I dont have to worry about my roots messing up the PH because they never touch the rez water or the siphon tank water. Once the roots get sprayed, the runnoff get tossed out. In your case, as long as the EC is close, that should be easily manageable.

3)My alkalinity/calcium problem isnt too extreme, but it is there. My water likes to be at around 6.4-6.5 PH. I handle that by doing the minimum amount of aeration I can get by with in the rez. I use an aquarium pump in my rez to keep it stirred up and it flumes the water surface. It also has an air line that sucks in air and puts out small bubbles when the pump runs. I only run the pump 1 minute out of every 15. Just enough to be sure no anerobic bacteria can get started. I get all the aeration I need by the air atomization from the spray nozzles, and the micron sized droplets.

In your case, you could probably easily cut way back on the aeration in your rez too. The flood table will provide enough aeration when ever the water drains out. Thats the whole point of flood/drain is the extra aeration the roots get on the drain cycle when they are feeding on a thin layer of nute water. Tha this layer will come up to 100% DO very rapidly - much like my super small droplets. You might want to increase the dry time a bit, but I doubt that will really be necessary.

4) Im still messing with how to address the temperature caused PH changes. I have decided to try just PHing about 0.2 lower in the rez than my target PH. Thats about the average PH increase. I like some PH drift, so instead of letting the rez go from 5.7 to 6.2 over a few days, Im now starting at 5.5, but never going above 5.8-5.9. I dont like not knowing the exact PH my roots are seeing, but its too much trouble trying to constantly chack temps and PH in both places and doing constant adjustments. This seems to be working much better so far. My plants were also look pretty pale, but after 2 days of the new routine, they already look better.

Hope that helps, and good luck!!!

TheGreatSouthern said:

.......snip....
When fully pumped out the nutrient level comes up to about an inch below the surface of the pebbles. It pumps up in 8 mins and drains in 30 mins and repeats every hour.

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Caveat, I don't grow Ebb and Flow. I grow Coco DTW. I'd try increasing time between fill cycles so possibly once every 2 or possibly 3 hours?

Outstanding.
That's a lot of information. Thank you for taking the time to make it so detailed.
First thing I will do then is cut the aeration down. I'll probably cut it to zero actually as I also have two powerful aquarium pumps agitating the res. There is a huge amount of surface area in the clay pebbles, about 800 liters of them, so hopefully that will be enough.
My vegetative additive has kelp in it, so that will have to come out. My base nutes smell like they might have something organic in them also so I may have to change, however they are a hydro specific product so you'd think they would be right.
It's worth noting I also have two UV sterilizers running now in the res as of this morning, so between that and the H2O2 the aeration may no longer be necessary, I'll just have to keep a close eye on iron deficiency.
So.... Ditch the *possibly* organic nutes and ditch the aeration and hope for the best.

Thanks again
TGS

To some degree adding CO2 can effect the level of your pH.
At least, that is what people who use aquariums say.
With airstones, this is what you do.

In E&F no airstones are needed as Larry3215 correctly stated.
Plenty of DO is the water anyway.
And plenty of O2 in gaseous state around the wet roots.

After 3 days without air stones running and no organic veg booster the seedlings are looking a lot better. They pretty much perked up and started growing normally after 24 hours and are now about 90% back on track. I lost about a week's worth of growth, and being autos I won't be getting that back unfortunately but it is what it is, they're in a SOG anyway so hopefully the losses can be clawed back to some degree by letting them branch out a little more than I would have. I'm growing OG Candy Dawg Kush Auto, Auto Critical Kush, a strain I bred by crossing Auto Critical Kush with Amphetamine Auto which I call Critical Meth and a strain I bred by crossing Auto Mendo Mass with Auto White Widow which I call White Walker. They have all recovered equally well.

I am, however, still using quite a bit of PH down. Not as much as before but because I am using hydrochloric acid it seems to be increasing the EC of the solution and I would assume that increase is chloride, so it's going to hit a point soon where I start having problems. EC is increasing by about 0.2 per day.
My options for PH down are limited, as my res is currently minimum 300L (3 tables, I'm currently putting 500L in for buffering) and will soon be increasing to 1000L when I bring another 6 tables online so I'm going to need a lot of acid.
I still live in a prohibition country and Nitric and Phosphoric acids are notified, which means if you order them you get a phone call from the coppers asking what exactly you want it for. I got one for something a while back and it scared the shit out of me.
I can get sulphuric acid easily in 20 litre containers as it's used for automotive batteries and it's cheap, can anyone tell me what the long term effect of using say 20-40ml per day of 98% in a 300L - 500L res is going to be? what does it break down into?

I doubt its the acid thats changing the EC. I have been using plain old hydrocloric battery acid in my system for over three years with no side effects, and it does not change the EC at all.

The key difference is my system is drain to waste, so no roots ever touch the water in my rez.

Im betting its your EC level that needs adjusting or there is something else going on. You might check the roots and see how they look.

I dont know how good this chart is, and I cant even remember where I found it, but it might help. Ive seen it posted several times for people doing DWC, etc. When I was recirculating, it did seem to be pretty accurate.

Larry3215 said:

I doubt its the acid thats changing the PH. I have been using plain old hydrocloric battery acid in my system for over three years with no side effects, and it does not change the EC at all.

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When you say Hydrochloric battery acid do you mean hydrochloric acid or sulphuric acid? I don't remember ever seeing a battery with hydrochloric in it.

Thanks for the chart. Lightbulb moment when I looked at it. The EC is probably rising because the water level is falling. facepalm.
Nothing to see here folks.

TheGreatSouthern said:

When you say Hydrochloric battery acid do you mean hydrochloric acid or sulphuric acid? I don't remember ever seeing a battery with hydrochloric in it.

Thanks for the chart. Lightbulb moment when I looked at it. The EC is probably rising because the water level is falling. facepalm.
Nothing to see here folks.

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Sorry man. That was a major screw up on my part. My only excuse is I was stoned, on opioids, and in a lot of pain.

I meant EC when I said PH. I also meant sulfuric when I said hydrocloric. I get my acid from the local auto parts store. A $7 container lasts me a long time - going on two years now.

Ive never used hydrochloric, so cant comment on it.

TheGreatSouthern said:

I lost about a week's worth of growth, and being autos I won't be getting that back unfortunately but it is what it is, they're in a SOG anyway so hopefully the losses can be clawed back to some degree by letting them branch out a little more than I would have.

Click to expand...

Forgot about this - stoned remember

On my first grow of autos, I ran three plants. The breader said they generally went around 10 weeks from seed. Other who grew them reported similar times.

During that grow, I screwed up three different times by forgetting to turn the sprayers back on after doing some maintenance. Each time, they were off, and the roots had no water being sprayed on them, for at least 6 hours one time and over 12 hours the other two times. Each time did significant damage to the roots - AND - really slowed down the growth. The roots actually seriously air pruned/died back each time to to one degree or another, and turned brown and I lost all the fuzzy hairs I am so fond of. The leaves developed serious looking spots, brown, crisping, etc. It looked bad.

BUT - each time they recovered. I do LST under a scrog screen. Those three autos filled up a 7 sq ft space pretty well, but the main point is, they ended up taking about 13 weeks to finish instead of the normal 10 or so. Each time I tried to kill them, they took a week off, but still finished off just fine despite looking really ugly. The harvest was excellent too, and very potent. I ended up getting just under 3000 grams wet from the three plants. I dont smoke, so I cant comment on flavor, etc. I make RSO and do capsules.

Anyway, my point is they can still finish up ok if you just give them more time. At least the specific ones I grew sure did.
I have several grow logs on Overgrow with tons of pics. Same username.

Im curious if you are seeing any PH difference between your rez water and the warmer water returning from the tables?

I dont know of anyone but me who has bothered to check for any PH changes due to temperature change, so Im really curious to know of anyone else can confirm it. You wont see it if there is no temp change, and the difference should vary with the difference in temp between the rez and tables, so you will need to check the water just as it leaves the tables if possible or grab a sample as its returning to the rez.

14 gallon tubs filled with hydroton,did i read that right?

Larry3215 said:

Im curious if you are seeing any PH difference between your rez water and the warmer water returning from the tables?

I dont know of anyone but me who has bothered to check for any PH changes due to temperature change, so Im really curious to know of anyone else can confirm it. You wont see it if there is no temp change, and the difference should vary with the difference in temp between the rez and tables, so you will need to check the water just as it leaves the tables if possible or grab a sample as its returning to the rez.

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Yes there is a difference, about 0.1. I just checked this morning. I couldn't say if that's due to temperature though.
The best way to do that would be to take a sample of the 20c nutes from the res and let it warm up to 28c, which will be the air temp today and check it again?

myke said:

14 gallon tubs filled with hydroton,did i read that right?

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That's right. 12 plants per tub in a SOG, 6 tubs per table. Each table is about 1.1M square and sits under a custom built LED comprised of 15x Samsung QB22A strips running about 11 amps in total, which works out to about 550 watts. So roughly equivalent to an HLG550 only a bit more efficient as mine has more chips driven a bit cooler. generally I just do single cola but I'll have to do these ones leaving the top cola and the next pair of branches to make up for the losses early in the piece, on account of them being autos.
I was doing the same set up in coco and getting about 1.45g/w dried and cured with an 8 week turn around with the Mendo Mass / White Widow hybrid. I know there are a few folks getting upwards of 1.8g/w these days, but I can't help but wonder if they are doing it on an 8 week rotation
To be clear it's a 10 week strain, and I do the first two weeks in 0.5L perlite pots during the last two weeks of the previous run so they are already 3 inches high when they get transplanted into the tubs and start their 8 weeks under the Samsungs.

So IME the hydroton makes the ph go up.I use to top feed 24/7 and the roots just follwed the water.I changed to just 2 times a day like coco i hand watered DTW. They grew much better and the DTW made the ph problems go away.

Oh thanks, yeah I was wondering if the hydroton might be doing it. Or more like it crossed my mind when it first started happening and I never investigated it because I figured it's probably pretty inert and not having much effect. Did you notice the effect on PH from the hydroton diminishing over time or does it just do it forever?

I only used that set up for one SOG run.I also was running a rdwc so I just switched everything to that.Ph is still a constant concern but I don’t get too excited if it’s not exact.