What might be causing PH drop in the rootzone?

Plants look healthy besides some minor cupping close to lights. They are almost 4 weeks into flowering.

I've been doing the 1:1.5 extraction analysis for the rootzone 2 times a week and I can't understand why the PH level drops to between 5.1 and 5.5. Measurements are taken before watering and I'm watering every 2 days (1 off 1 on 1 off 1 on...). The thing is that PH of water that goes in with nutrients is 5.8-5.9 and I have always heard that in coco with healthy plants ph swings up in time, not down.

In week 2 readings were 170-280ppm in the rootzone with 1300ppm going in. I've been slightly increasing the ppm of nutrients with every watering and now I'm at 1500ppm going in and rootzone at 450-470ppm. Canna says that target value for EC in the rootzone should be 1.1-1.3 which is 550-650ppm. I'm still not there yet so sounds like a slight deficiency. Trying to dial in with my new sealed co2 environment.

I have heard that too high ppm can lower PH, but I don't think that's the case.
Another thing I've heard it that bad bacterial growth in the rootzone can lower ph, but that too doesn't seem to be the case, because I have 2 different strains in the room and one is 2 times bigger than the other, which means that the bigger ones drink water faster and it's highly unlikely that they are the ones being more bacterial than the smaller ones. And here's that: I have now increased the ph of nutrients going in from 5.9 to 6.4, but still the same drop and the bigger ones show me lower ph than the smaller ones - my last readings were 5.5 vs 5.1 and one week ago it was 5.3 vs 5.5. Do you get it?

What could be the reason?

Welcome to ICMag!!

Coco is not what I have the most experience with, but I can help you get the help you're looking for. The more information you can supply us, the more help we can be. So please, what type of lights, wattage, type of hydro system, you're in coco (and posted in the correct forum for it, good job), what type of water you're using, the type of nutrients and way you're mixing them, temps/humidity and CO2 levels. To start with, lol

As water evaporates from the coco, the ppm goes up and the pH drops. Coco also absorbs certain elements and can release them later, under the right conditions. Too high of a nutrient concentration will burn roots and cause root rot (another cause of pH dropping). This is the main reason you want to keep your ppm as low as healthily possible. In my DWC reservoirs I consider 5.3pH to be the bottom end of the 'healthy' range for cannabis. Your pH seems just fine, as evidenced by your remarks on how healthy they look.

Also, there are two common ppm meters @ .5 and .7 conversion. The .5 conversion is a lower number than the .7 for the same EC.

Does this help?

If your coco gets too dry the pH will drop, and the EC will spike (increase). Keep your root zone wetter than you think it should be.

For example, I grow in 3gal smart pots under 1000W HPS bulbs. If we were to hand watering, I would soak the plants twice per day. Once 30-1hr after my lights turn on, then again 5 hrs before they turn off.

If your fertilizer contains (to much) NH4 and the carbonate hardness (dKH) of your water or to be exact the final nutrient solution (after adding ph-) is too low. That could result in NH4 letting the medium PH drift down over time.


To rule that out, you need the following pieces of information.


1. How much NH4 per Liter are you feeding your plants. Or how much NH4 does your fertilizer contain and how much do you put in per liter
2. What kind kind of acid at what percentage do you use to ph down? How much of that do you add per liter.
3. What is the Carbon Hardness of your water?

Much of what I do is my own work. I feed based on run-off, but my numbers are somewhat different. It's interesting to hear of other people interested in the root zone, and actively checking it.

Two things come to mind. First, I guessed you were about 3 weeks into flower. Are you doing something that took a while to show? It's forgotten news now, but I used to watch for the ph drop 3 days running, then switch to full flower food. It was only a useful guide in recirculating systems. The plant goes through a change, and drops something. I seriously can't remember. I toyed with it for a while, but saw no result from this timing indicator. Second thought on my mind addresses a more gradual change. Your root zone is getting a higher EC reading. Food is acidic. More food, more acidity.


Canna pro has been coming out the bag at higher than 1.2 for ages. Each time you have given a reading, it's both ppm and ec, and I'm unsure if your doing math or reading it. A lot of the numbers are so strange looking to me. 1300 in, but only 280 at the root? Where did it go? A number of feed elements can't be force fed to the plant. Things like K are there in abundance, so the plant can get them, but won't take it all. With just 280 of your 1300 left, I don't need a calculator to see the plants are out of food. Is this measured on a just water day, and why do you have a just water day. That's not natural. The plant won't know what sort of root system to grow.

I really don't get it... but the only thing we can measure is EC. PPM is measured by boiling a beaker dry and seeing what the debris weighs. It can be calculated roughly from EC by using a mathematical factor. Which is times 500 or times 700. Yes.. it's not a standard. PPM calculated from EC can be out by a huge margin. If you can't tell me which one you are using, the information is useless to me. EC1.0 is both 500ppm and 700ppm. My meter even gives both. And I dare say its the most used meter on the planet. So to round up, if you want to talk ppm, boil off a liter and see how many micrograms you have. Or just use EC so people can understand what you're doing.

Sorry for the lecture. I'm trying to get the message of ppm across to a wide audience. Stamping out something so ingrained is hard work though. I may need a large bat before I'm done

My two cents in my experience Coco is the most easiest medium to grow in I've never needed to check the runoff in Coco make up your solution to the desired ppm and pH top feed or bottom feed one or the other don't mix them up. Pick your plants up and get to know the weight wet and dry.
By the way are the two plants the same? don't sound like clones to me size/ height does not always matter. I've gotten better yields from smaller plants at times.

I didn't expect so many responses. Thank you for that.

I'm trying to understand what you meant by that ppm vs EC thing and I'm using a 0.5 (hanna) calibration which is the most popular version of ppm if people are talking about ppm's imo. Anyway, from my observation every 0.1EC = 50ppm and my bluelab combo meter reads both. To me ppm's seem give me more accurate readings. I don't know much more about it though.

[FONT=Arial, Helvetica, sans-serif]"Are you doing something that took a while to show?" - what did you mean by that?[/FONT]

GreenCrack said:

1. How much NH4 per Liter are you feeding your plants. Or how much NH4 does your fertilizer contain and how much do you put in per liter
2. What kind kind of acid at what percentage do you use to ph down? How much of that do you add per liter.
3. What is the Carbon Hardness of your water?

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Damn this is going too hard on my mind but I'm using Canna nutrients.

1) I don't know that, but basically Canna Coco A+B are the main nutes.
2) I recon adding about 0.01-0.04 of Ph minus which is "Hesi Ph-" to be exact which consist of 59% phosphoric acid.
3) Carbon hardness? Umm.. I'm using RO filtered water.

I clarify: 0.01 - 0.04% of Ph minus per liter (where is edit button?)

Douglas.Curtis said:

Welcome to ICMag!!

Coco is not what I have the most experience with, but I can help you get the help you're looking for. The more information you can supply us, the more help we can be. So please, what type of lights, wattage, type of hydro system, you're in coco (and posted in the correct forum for it, good job), what type of water you're using, the type of nutrients and way you're mixing them, temps/humidity and CO2 levels. To start with, lol

As water evaporates from the coco, the ppm goes up and the pH drops. Coco also absorbs certain elements and can release them later, under the right conditions. Too high of a nutrient concentration will burn roots and cause root rot (another cause of pH dropping). This is the main reason you want to keep your ppm as low as healthily possible. In my DWC reservoirs I consider 5.3pH to be the bottom end of the 'healthy' range for cannabis. Your pH seems just fine, as evidenced by your remarks on how healthy they look.

Also, there are two common ppm meters @ .5 and .7 conversion. The .5 conversion is a lower number than the .7 for the same EC.

Does this help?

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I think I should start a grow journal when I plant new ones in few weeks. I didn't know here are so many people willing to help. I come from Rollitup and people are totally bitches there.


But I'll give you few answers anyway.

I'm using powerful ePapillon lamps that hold Philips double ended bulbs in them which can be dimmed to superlumen mode at 1150W without losing lumen per watt. And of course I'm running my lamps at full power and they are moving on the light rail. Not much, like 40cm. My lux meter reads about 50 000 - 80 000 lumens at canopy and there's 1150W of light per every 1.64 square meter, so 700W per square meter.

18L of coco in every pot

12 plants per one lamp (1150W)


RO filtered water.

- Canna Coco A+B
- Rhizotonic
- Cannazym
- little bit of extra Canna mono Ca and Mg, but thinking about changing it to Calmag+, then I would not need to add them seperately.
- Canna PK 13/14
- Canna Boost

The way I'm mixing is in a big cistern with cement mixer, then pumping it through drippers.

Been following some custom feeding schedual so far that was created by one big commercial grow company, but I'm thinking about making my own schedual based on Cannas feeding chart, because I have my own genetics that are unique. I would just like to be able to read the plants accurately. This is why I've started doing the rootzone analysis lately.

I have upgraded my room to be a sealed Co2 setup with AC units and fog makers. Maybe I will need a dehumidifier in the end of flowering, but maybe I could just add some heat and let the AC units cool it down - both together should be quite effective at lowering humidity.

I'm thinking about doing one perfect coco grow before I move on to RDWC system. What I mean by that is that I've been doing mistakes every time and it's 7th grow already. My best results so far have been 1.8 grams per watt and I would like to get it over 2gpw before retiring from coco the quality was pretty nice btw, although I ran co2 till the end with high temps. I would like to have better quality and higher gpw.



The biggest thing I have learned lately is that I should not let the coco dry out so much. I'm watering more often now I have been watering one day off one day on, but now I will try make it every day or even 2 times a day. I still have big pots (18L).

Ahh, I should make a grow journal.. This would be more correct place for all that information.

You should be watering at least once daily or ideally multiple times a day in coco coir (like you mentioned you have switched to in your stretchy growth thread). Coco coir has a natural ph slightly above 6, each time you give feed water it resets the root zone ph to your feed's ph and it should rise back to its natural ph slightly above 6 as it eats the fertilizer. Overfeeding would cause ph drop and ppm buildup in the rootzone and then other quickly distinguishable symptoms to the plants, so its probably not your issue.

FarmerWilly said:

Plants look healthy besides some minor cupping close to lights.

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^thats a sign of heat or transpiration stress, whats your humidity like?
Im thinking its a combination of vapor pressure deficit/transpiration and underfeeding. If the plant is transpiring (sweating) to stay cool and drinking the water faster than its eating the fertilizer the root zone ph drops. You probably arent feeding enough times during the day to keep up with how fast the plants are drinking and eating.

Midwest toker said:

You don't need to add cal-carbonate to correct the pH of coir. Natural pH of coir is around 6.4 pH.

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Midwest Toker said:

I never check my coir for pH, every feed to waste set the coir nutrient and pH back to what your feed input is.

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jidoka said:

How many times a day are you feeding? The more often you feed the higher your ec can run without building salt. I use a 2.4 ec under t5s fed enough times that ec does not build up

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"[FONT=Arial, Helvetica, sans-serif]You probably arent feeding enough times during the day to keep up with how fast the plants are drinking and eating."

Yup, I think it's as easy as that. Major mistake of mine that I've made. Can't wait to see how the plants will do next time when I'm doing it right from the start
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Oh one more thing, I have seen some springtails. I have read they are not harmful, but in the other hand they are probably living on dead roots, right?

So I'm probably having root rot to some extent. Consequence of letting coco to dry out maybe.. in combination with high ppm.

I'm starting to get somewhere...

They only show up if they smell something rotting/decomposing to eat, same with fungus gnats but yeah springtails are mostly harmless it seems. You have any damage to the leafs that look insect related?

Why do you want to switch to RDWC? Really thats just asking for more problems. Drain to waste multifeed is the best system imho.

FarmerWilly said:

Oh one more thing, I have seen some springtails. I have read they are not harmful, but in the other hand they are probably living on dead roots, right?

So I'm probably having root rot to some extent. Consequence of letting coco to dry out maybe.. in combination with high ppm.

I'm starting to get somewhere...

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Root rot will definitely drop pH. And it drops it pretty quick too.

You also asked about my humidity. I bought a very accurate hygrometer that was recommended by my local calibration lab, but I'm still not sure where should I put it. At the moment I am keeping it between the lamps on top of canopy, but slightly, just very slightly between the buds, not above the canopy. This is where I guess is the most harsh environment for the plants (close to lights), so I'd like to see whats going on there - 32C and 60%

When diving into the canopy, then it's a whole different story. Deep into the canopy it's like 27C and 70%.

Its very confusing to hear people talking about temps and humidity, because they almost never say where is it measured.

Even if they say canopy, where's that?
I understand what leaf temp means (ir).
And then there's ambient room temp.

I would personally measure top of canopy and inside of canopy to get max and min temp/rh range

For first 5 weeks or so I tried to keep RH up with 2 foggers constantly running. Then it was much higher RH, around 80%.

Ibechillin said:

Why do you want to switch to RDWC? Really thats just asking for more problems. Drain to waste multifeed is the best system imho.

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It's either RDWC or Aeroponics. I just want to see what this plant is capable of. I will get over my problems. Everything can be done well.

The root rot/springtails probably developed before you switched over to doing multi feeds, that should help avoid those issues from happening next cycle. Canopy level just means the average height of all the plants in an area, where your hygrometer is at sounds fine slightly below the top of plants. We narrowed down the reason for the plants getting so tall during early flower as the large swing in day night temp difference, and that the top nugs are foxtailing and top leaves are cupping because they are too close to lights and getting too much IR/heat.

I asked about humidity because it has to do with transpiration rate, this is a vapor pressure deficit chart. The gold areas are where you want to be for maximum transpiration/growth rate. Deep in the canopy 27C and 70% is perfect so i wouldnt want to raise the lights anymore, just need to control stretch/final height so top colas remain happy as well.



The chart is recommending you run 80% humidity to match the 28C - 32C temps your room is running with high dose CO2 like you did the first 5 weeks flower. Id be concerned about budrot etc running 80% humidity all the way through flower personally. Ideally after week 5 I would reduce CO2 dosing, room temperature to 24C - 26C and humidity to 70% to accommodate optimal VPD range.

FarmerWilly said:

You also asked about my humidity. I bought a very accurate hygrometer that was recommended by my local calibration lab, but I'm still not sure where should I put it.

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Some photos would help a lot.

Sounds to me like you're not doing any canopy management. I work to keep all the flower sites in the optimal light zone. Too close to the light and they get cooked, or are very tiny and rock hard. Too far away, or too shaded in the canopy, and they are big fluffy and low quality flowers.

Train the tops if necessary, clip all the flowers growing a stem away from the main branch (they're looking for light) at week 2'ish.

You should have even air flow throughout your entire grow area. Your temps and humidity should be very consistent. Cooler at the intakes and a few degrees warmer as it exits the top of the canopy.

This is how you grow a lot of cannabis, all the same quality.