Dark spots on leaves

Strain: Critical Mass, Regular
Indoor grow
Vegetative stage, 8 weeks old
Plagron power roots 1x a week; 1ml in 1l water
Plagron terra grow 1x a week; 2.5ml in 1l water
Adjusted water pH with white vinegar to 6.5-7.0
Soil: Plagron light mix
18h light schedule, approximately 30cm from lights
2x 20W Cosmorrow Full spectrum lights

I noticed for a few weeks these darker spots on my leaves. I thought it would go away, but that doesn’t seem to be the case. Can you identify the issue here?

Hi, I’m a beginner and only on my 2nd grow, first indoors so still learning. I don’t know your nutrient brand but you might be going a bit heavy on a young plant in early veg. Why not try alternating nutrients with plain pH adjusted water?
Nebula from GWE wrote a great article on this subject:

She inclines to overwatering as the likeliest cause as well as pH issues at the roots. Do you check the pH of your run-off? If not, maybe worth doing with a good pH pen and of course, really waiting till full dry-out before rewatering. Good luck!

Hello my friend!

I would check the feeding schedule and the pH levels. If you’re combining nutrients, you might be overfeeding the plant. Also, the pH should be between 5.5 and 6 — yours is too high.

I believe the issue might be a combination of all these factors. In that case, watering with plain water could help her recover. Keep in mind that the damaged leaves won’t heal, but you’ll be able to tell if she’s improving by looking at the new growth.

Hi Eli, I was always of the impression that target pH for soil should be between 6 & 7. I thought 5.5-6.3 for coco. I think he is in a soil mix.

I’m using Plagron Terra Grow, which has a recommended maximum dose of 5 ml per liter of water. I initially started with a half dose — 2.5 ml/L. However, after noticing brown spots and yellowing leaves (signs of phosphorus and nitrogen deficiencies), an experienced grower recommended I increase the dose to 4.0 ml/L.

I checked the soil pH using a pH meter and found it to be around 5.0–5.5, which is too acidic for soil. At that level, the plant struggles to properly absorb nutrients. My goal is to bring the pH closer to the ideal range of 6.0–7.0.

I don’t think overwatering is the issue — if anything, I was underwatering, allowing the soil to get quite dry between waterings. But I noticed the leaves deteriorated more during the driest days, so lately I’ve been keeping the soil slightly more moist.

My tap water has a pH of about 8.0, and even after watering the plant several times with it (sometimes with nutrients, sometimes without), the soil pH still wasn’t rising enough. I added dolomite lime granules, but they seem too slow-acting for immediate correction. I checked once the pH of the water runoff which was 6.5-7.0, the same pH as the water i gave it, but the soil pH did not adjust accordingly.

I read on the RQS website that carefully adding baking soda to the water can help raise soil pH more quickly. I mixed 1 tablespoon of baking soda in 5 liters of water, then gave 2 liters of that solution (with nutrients included) to the plant. This resulted in the soil pH rising to around 6.0–6.5, depending on where I measured in the soil. I’m hoping this doesn’t modify the soil microbiome too much but at the moment, i need an immediate fix for my soil issue because my plant wont survive.

Here is a photo of the top new leaves, but you can see the problem is the surrounding lower leaves.

Well done! The new growth looks much healthier. From what I’ve read (as a newbie also) the old leaves won’t change colour. As it looks like you are a few weeks into veg, you can maybe remove them soon in a first defoliation. Maybe other more experienced growers will have thoughts on this.
Re soil pH, I used to check this regularly with a cheap Amazon stick probe but had doubts about its accuracy so I bought an Apera P20 and now only check my water pH in and out and am happy with this solution. I havent used the metal stick probe for the soil since then. Your new growth looks to me like you are on the right track.