Low pH of Organic Soil - Calcium Deficiency

  • ILGM Girl Scout Cookies
  • Method: Organic Soil like super soil, specifically “Great Lakes Water Only”
  • Vessels: 7 Gallon Airpots
  • pH in is usually around 6.8 or 7.1. Even though everyone says you don’t have to in organic soil with lime I pH my water. I also have a bluelab soil ph meter. Current soil pH is 6.2.
  • PPM: I don’t know
  • I use worm tea mostly, plain water and sometimes supplement with organic calmag and occasional recharge
  • Indoor
  • 300W QB
  • Temp 75F
  • Humidity 50%
  • 4" exhaust
  • No humdifier or AC
  • Co2: No
  • Age: Prob 60 days veg?

I’m starting to get light green tops. I’ve gotten this before. I’m quite confident that it is a calcium deficiency. I noticed yesterday before watering that it was a little low (I can’t remember what it was) so I left the water with a higher pH. I believer 7.1. Today I checked the soil pH and it is 6.2 which I believe is outside the range for calcium availability. Today I noticed the difference in color of the tops.

The difference in color of the tops isn’t very extreme right now but I JUST pruned the lower leaves so I’m holding one up here for a reference. They are a little overwatered right now.

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I think your plant looks good. Lighter color on New Growth is normal. I have never owned a soil pH probe. But if your probe is correct at 6.2 ph is not bad. I normally get a run off and check ph . Good luck.


This is my concern…

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The new growth looks normal to me.
As far as soil Ph I’ve found that the Ph readings will be different based on how moist or dry the soil is. Drier higher Ph…wetter lower ph.
So based on how deep the probe goes I find say right after watering will show a lower ph but before watering will be higher.
The soil based on depth into soil will be different. As the soil dries the top will contain less moisture while at the bottom will have more moisture.
When I check a day or so after watering at 3” it’s maybe 6.8 at 6” 6.5 and 9” down is 6.0 and from what I’ve experienced is normal. This range allows the different nutrients to be used as needed even though the lower depth is 6 and doesn’t uptake calcium well etc but up above it’s in range for calcium.

Hope this helps


It does help. Thank you. I’m still a little nervous because I’ve seen this with previous grows. I wasn’t monitoring the pH but the same symptoms started showing and eventually turned to leaf chlorosis. Eventionally has seemed to correct on its own. Haven’t yet figured out what has fixed it in the past. Possibly through beneficial microorganisms via worm tea. I’ll just ignore and ride it out.

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EDIT: Just posting so I can hang my head in shame about my last comment in which I invented the word “eventionally.”

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Plant looks good. If you want to bring things up a bit; top dress some more dolomite lime like 14 cup and see if that helps when watered in.

I think you are just fine with your current PH and nothing to worry about. Yet.

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Yeah, maybe I’ll give that a shot. 1/4 cup per plant right?

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That’s a good place to start.

Yep I’m sure he meant 1/4 cup…
you may want to check the soil ph each day after watering and make notes of how it will change based on moisture and depth. Once you have a history of the soil ph profile then maybe think about top dressing some dolomite lime…either pelletized or powdered. Don’t use Hydrolyzed lime!
If you decide to add some dolomite lime I suggest using a little at a time and check ph like above. You can’t remove it but you can add more if needed.

Can you explain your process using the blue lab soil meter as I haven’t used one of those. How deep can you test the soil? Do you check right after watering etc etc.
I use a soil ph probe like this one and its 12” long so I do check ph at 3”, 6”, and 9” down a day or so after watering.

I’m new to it but the probe is made of glass and has a plastic protective sleeve. You have to keep a solution in the sleeve so the probe stays moist. When it’s time to use it rinse it with tap, use the plastic thing to make a 4ish inch hole in the soil then put the probe itself in. I honestly don’t know if it matters whether wet or drier. I need to check the manual.

Ookay so update: Since originally posting this thread 2 days ago I have watered once with straight dechlorinated tap water. I forgot to pH it because I’m a moron but after watering I remembered to check what was left in the bucket. It was 7.1pH. I checked the soil pH with my probe and it was 6.2 still. Next day soil ph still at 6.2.

The problem now is that the light green tops are persisting but are now also twisting up. I’m also seeing a little bit of taco. The heat is not the issue, the light is at 50% intensity and 20" from canopy. Temps are 75f ambient but according to my IR thermometer the canopy itself is at 70F. There is no fan directly blowing at them.

Can overwatering cause this?

They look normal to me.
New growth will be a lighter green and will look like that as they are “spreading” their wings. No taco going on just how the new growth looks until they develop more and spread open.

It’s a little hard to see in the pic but if you look at the large leaf it has a couple blades that are curved to one side kind of. The tiny growth is coming out twisted too. Seems like it is worse than a few days ago. I’ll leave it alone and post an update in a couple more days.

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Hps lights
Room is 76* 40%
Soil medium.
Plants got over watered and haven’t come back. Any advise is appreciated. Soil ph is 7 water ph is 6.1-6.3

Back sooner than expected. Checked the pH last night. One plant was at 5.9. I checked the moisture of the soil and decided to give a little high pH water to bring the soil ph up. I watered to with recharge, bonemeal, organic calmag and dechlorinated water at 7.2 pH. Each 7 Gallon pot for a half gallon of water. That was last night. This morning I checked the pH and it is exactly where it was before. Using my pH probe meter. It’s calibrated, kept in solution and is only a couple months old. I have Lime coming Monday. Provisions difficult to get during the apocalypse. Any other ideas in the meantime? Guessing I’d have to wait anyway because anything will need to be watered in. What if I did some kind of calcium foliar spray. That way the plants get what they need while I am trying to get the soil worked out. Thoughts? (No pics because they look the same as yesterday). Oh also, the previous post is not from me. It’s a thread hijack.

This is because the inertia of the soil overpowers the liquid. Unless it has high solids content it will do nothing for PH management. Also a warning: don’t ever exceed maximum PH for your medium to avoid any burning of roots. We’ve seen folks use PH 10 water to adjust soil PH which doesn’t work.

Yeah, I kind of wondered about that. Wishful thinking. Obviously I’ve been researching a lot. Just came across something. Someone is saying to water in using Nectar for the Gods Olympus Up then bringing back down with whatever their pH Down is called. I happen to own both but have rarely needed Olympus Up because my water pH tends to be around 7.2.

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Your soil PH isn’t that bad; I would top dress lime and re check your numbers 10 days or so after top dressing.

Okay thanks.

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