Which deficiency? RDWC with 0.9 EC tap water

Hi to all OG,

I have a RDWC setup in a tent and my tap water is 0.9 EC. Being it is DWC it doesnt leave much for everything else the plant needs. I have been through a fair bit to sort out my water issue and am finally there. My next grow should be fine but I am finishing off the current grow still which has evidence of issues from earlier in the grow. What i found working with my 0.9 EC water was that i could give the plant close to what it needed during veg as well as flower if i watch and adjust the system closely. However, it seems to be during the transition from veg into flower I am not able to give the plant all it needs. I guess as the plant starts requiring flower nutrients as well as calcium, magnesium and nitrogen as it is still stretching and growing in size. During the 4 weeks from switch to 2 weeks into flower the larger older leaves at the top of the plant, maybe 4th of the large leaves down f rom the top of each branch (each branch has bud sites and nodes/leaves much further down than that) start to yellow off. I still got great results out of the grows but I am still curious to find out what it would have been even though I should not have this trouble anymore. In the photo below you can see the larger leaves at the bottom/foreground of the photo. They are yellowing between the veins but the veins are still dark. The purpling on the petioles is genetics in this case even though the plant is deficient in something. I have grown another strain in here in the same system and exactly the same issue. My guess is either potassium or calcium deficiency leaning towards potassium but would love to hear some others opinions:


I have a GG#4 that does what your plant is doing, only way worse and it’s only in veg.

You mentioned your EC from the tap was high, mine is also though not as high as yours - 0.4 EC from the tap for me. What’s the pH? What are you feeding the plants at?

I’ve found that these issues in DWC are almost always pH related if you’re feeding it a high enough EC.

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My issue is that I slowly up the feed and the highest i can get to without the pH starting to swing like crazy is 1.1 EC. If i go up to 1.2 then the pH starts to drop faster than i like. If i go up to 1.3 then the pH plummets down and the plant starts to get slight burn on the leaf tips. Any higher and it starts to fry. I have tried letting the pH swing between 5.6 and 5.8 with occasional swings up to 6.0 and i have tried 5.8 to 6.0 with occasional swings up to 6.2. Both times the same behaviour.

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Hmm, do you know your water pH before you start? Are you adding additional acid to drop the pH or is that just the nutes doing that?

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Just the nutes mostly. The tap pH is 7.2 so I do not need to use a lot of pH down after the nutes. If I battle the pH down swings a bit then i do a res change to go back to without the pH up and whatever it is the plant is dumping in to drop the pH. I assume it starts to get lockout and drops the pH to try and pick up what it cant even though the pH is not the issue. Perhaps letting it drop will allow it to pick up what its lacking but if i let it go it would be all the way down to 5.0 in no time and I am pretty sure that’s not good. :slight_smile:

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Hmm ok, last question - how big is your reservoir?

It sounds like you’re seeing something I’m seeing in my larger 18 gallon reservoir. I have a plant there too that is dumping nutes into solution no matter what pH I set it to. It does the same as yours, slowly leeches nutes into the water that drive the pH down. There’s another plant in the same reservoir that is having the same issue, but not nearly as extreme.

The bad news is that I have no idea what’s causing it. I’ve tried low EC, high EC, low pH, high pH and every combination thereof. Right now I’m trying to increase the oxygenation of my water to combat high temperatures, in case that’s the cause.

The good news is that I’ve been fighting this for at least a month and it hasn’t killed the plant yet?

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Some stoned thoughts:

Do you know the alkalinity of your water? Mine is high, so it buffers pH changes pretty well. I have to use concentrated acid, but if I don’t overdo it the pH is very stable.

Yours sounds not so high?

Are you using hard water nutes? Have you tried using regular nutes? If your alkalinity is low and your dissolved CaCO3 is low, then regular nutes might buffer the pH changes better.


Main issue with DWC is the roots sit in the solution so you generally need half the amount of nutes you’d need if you were feeding any other way. I’ve also seen a big nutrient requirement drop from the plants at around week 5 in dwc. They go from eating something like 700+ppm to wanting less than 400ppm almost overnight.

For reference, on a 4ft tall plant in DWC I never went passed 1.8 EC(~900ppm) at 0.5 ppm scale and my tap water is 0.2 EC which breaks down to about 70/30 calcium carbonate and magnesium. I still fight PH issues in flower regardless of nutes used.

I think with your tapwater so high out the gate, you may NEED to switch to RO/Distilled water, and/or switch to flood and drain or something else instead of DWC in order to have good results.


I’m having some improvements in my DWC by running higher than conventional pH and slightly higher EC’s.

pH stability is the #1 thing to get under control IMO. Once you can keep your pH where you want it, you can determine if you can reach a good EC/pH balance for your water, or as @HolyAngel said, decide to go with a different growing style.

Ebb and Flow is nice because you can reuse most of your old pumps and tubing, just need to build a good flood table. I’ve built a mini one out of totes and it’s nice.


If you do have low alkalinity water, a bit of baking soda might be all you need to fix your woes.

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@pufferfishfiend, yeah my water is alkaline and stable too. For your plants dropping the EC, they often do that if they start getting nutrient lock out. They drop exudates into the solution to drop the pH because it thinks that is why it cant pick the nutes up. Unfortunately, if you get the plant into that state for too long then dropping the EC does not stop the plant dropping the pH as it kind of gets stuck. Joe Crow is doing similar with high pH and High EC as you. I have been looking into that too.

@HolyAngel and @pufferfishfiend, all is well going forwards as i have my water situation sorted now but this current grow was just showing a little deficiency as a result of previous issues caused by my crap water. Even with this issue I have been getting great results grow after grow, however, now I also have good water for my future grows so look out! :slight_smile: primo buds on the horizon.

@HolyAngel, you use high EC for DWC. That could be the cause of your pH issues. I have found by using super low EC I still get the same end result but use much less nutes and I dont have to battle pH anymore. I could keep cranking up the EC and still get good end result but fighting pH swing all the time and wasting nutes. By low I mean 1.0 to 1.1 EC for most strains. And now that I have good water a decent amount of that will be nutes. Yay!

Also, once a plant has been swinging its pH down for a long time, then you cant just set the EC low and expect it to stop dropping the pH. The plant is in a state by that point. You need to treat it low out the gates and then its happy days all the way to harvest.


When you say “treat it low”, do you mean pH?

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No, treat it with low EC from the start. Once I get it going in the buckets as a small clone I only increment the EC by 0.1 per week. I give it a week to respond to the change. If i put it up by 0.1 at any point and if the pH starts to swing down during that week I back it back down 0.1. every now and then I will put it back up by 0.1 and usually it responds within one day if its going to push the pH down. and i will put it back down if it does. I usually end up around 1.0 to 1.1. The plants produce good weight and rosin pressing gives 26 to 28% return on a single press at low temp. Super tasty and gives you a belt. So it is not lacking, thats for sure. It seems they do not need much.


I’m still a believer in low EC in general. The plant I’m running at 1.3 EC is 6 months old at this point, so I still consider that relatively low EC :smiley:

This plant just has me pulling my hair out trying to figure out what’s going on.


They can be frustrating at times due to how much we are invested in them. Financially, timely, spacely, emotionally :smiley: Unfortunately with the underfeeding method i use you do not get to see your lovely rich green plants. They often look a little pale but are always growing strong. As you can see in the original photo. That was just after a mass defoliation.