Phosphorous deficiency or something else?

Hi all, I’ve been looking at leaf deficiency charts and lots of other things that this issue could be but can’t figure it out for sure. I thought I’d ask the pros here.

My plants are 35 days old from seed and a few are starting to show some brownish dots and slight leaf discolouration. It looks like it could be phosphorous deficiency but can’t say for sure. They’re still in veg and seems to be on upper leaves only but starting to show slightly on the lower leaves now too. Trying to nip this in the bud before it’s too late.

Growing in pro mix HP with light doses of GH nutes. PH is in the vicinity of 6.5 when I water.

Let me know if you have any ideas? If it’s phosphorous I will start adding more to feedings

Ontario_Outlaw said:

Hi all, I’ve been looking at leaf deficiency charts and lots of other things that this issue could be but can’t figure it out for sure. I thought I’d ask the pros here.

My plants are 35 days old from seed and a few are starting to show some brownish dots and slight leaf discolouration. It looks like it could be phosphorous deficiency but can’t say for sure. They’re still in veg and seems to be on upper leaves only but starting to show slightly on the lower leaves now too. Trying to nip this in the bud before it’s too late.

Growing in pro mix HP with light doses of GH nutes. PH is in the vicinity of 6.5 when I water.

Let me know if you have any ideas? If it’s phosphorous I will start adding more to feedings

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looks like lack of nitrogen but I could be wrong.

Ontario_Outlaw said:

Hi all, I’ve been looking at leaf deficiency charts and lots of other things that this issue could be but can’t figure it out for sure. I thought I’d ask the pros here.

My plants are 35 days old from seed and a few are starting to show some brownish dots and slight leaf discolouration. It looks like it could be phosphorous deficiency but can’t say for sure. They’re still in veg and seems to be on upper leaves only but starting to show slightly on the lower leaves now too. Trying to nip this in the bud before it’s too late.

Growing in pro mix HP with light doses of GH nutes. PH is in the vicinity of 6.5 when I water.

Let me know if you have any ideas? If it’s phosphorous I will start adding more to feedings

Click to expand...

First off, your ph is a bit too high for pro mix..should keep it closer to 6, ideally 5.8-6.3, it's sphagnum peat moss, not soil! Secondly, this is nothing to do with nitrogen, you say you feed 'light', need more clarification what that means, but for now, the plant seems not to be getting enough magnesium and possibly calcium. I say try lowering the ph of your waterings to see if that helps, maybe they are in the soil but unavailable due to the higher ph, or you simply have to add more. Do you use tap water for waterings or RO? Phosphorus deficiency is not very common in mid veg as that's when the plant uses the least P, and the leaves don't get specks of dots for a P deficiency, they get blotches.

Thanks very much for the responses guys.

Blitz, I think you may be onto something with the PH. In my mind I thought of Pro-Mix as a soil and was keeping it on the high end of 6. So more than likely a PH issue as it does kind of look more like a calcium/magnesium issue after looking at some more photos of what those deificiencies look like. Could possibly be a combo of deficiencies with the PH being too high.

I use tap water that’s around 120ppm from the tap. I usually let it sit out with an airstone for 24 hours to reduce chlorine. By light nutes, I am using 1/2 what is recommended by GH and was using the grow weed easy website recommendations for GH week 3. I haven’t been adding any cal-mag since I’m using tap water but figured there may be enough from the tap water. I think I’ll up the nutes a bit and correct the PH and see how it goes.

I really appreciate the responses. This site has been amazing for learning so much

pro mix is more like coco than soil. The PH needs to be lower or you will start having the deficiences. Tap water is probably ok but a pinch of epsom salts wouldnt hurt once you dial in the PH.

Also it is a good idea to vary the PH. Sometimes 5.8 up to 6.3. It helps ensure the nutes are available.

Ontario_Outlaw said:

Thanks very much for the responses guys.

Blitz, I think you may be onto something with the PH. In my mind I thought of Pro-Mix as a soil and was keeping it on the high end of 6. So more than likely a PH issue as it does kind of look more like a calcium/magnesium issue after looking at some more photos of what those deificiencies look like. Could possibly be a combo of deficiencies with the PH being too high.

I use tap water that’s around 120ppm from the tap. I usually let it sit out with an airstone for 24 hours to reduce chlorine. By light nutes, I am using 1/2 what is recommended by GH and was using the grow weed easy website recommendations for GH week 3. I haven’t been adding any cal-mag since I’m using tap water but figured there may be enough from the tap water. I think I’ll up the nutes a bit and correct the PH and see how it goes.

I really appreciate the responses. This site has been amazing for learning so much

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I use pro mix as well, both the hp version and the organic herb and vegetable one, which for me, is the best! I use tap as well, never had to add cal mag and also use only the gh flora micro and bloom, and usually have to add epsom salt when flowering. Also use silica and flora nectar, and if needed, usually in mid flower, kool bloom a couple times. I used to ph at 6.5, and never below 6.2 as i thought too, it was soil. I would always run into issues and only after further reading, i saw that 6.5 would be the max ph you want! lol...prefers it lower, not like coco or hydro, but ya, swinging in that 5.8-6.3 range.
Also, your 120 ppm tap is quite clean, mine is 140-150 in warmer months and 120 in colder times, i know mine has a good ratio of calcium to magnesium, so i never really needed the extra cal-mag with my feedings, just mag, but yours may be different and it's possible even after correcting ph, you may need to add cal-mag. If only using the hp pro-mix, im not sure it contains lime or gypsum, which my organic one has, so you may need to add cal-mag ultimately.

Well this explains a lot then. I only have one grow under my belt and while I’d say it was pretty successful for the first try, I noticed the same leaf discolouration during flowering and thought it was just part of the process at that stage. I think I kept the PH a bit lower last time so the problems weren’t as pronounced as they are now. I’m definitely going to keep the PH between 5.8 and 6.2 going forward. Will see how the next few weeks pan out and add some epsom salt or calmag depending on how they do once the ph is corrected. I should have probably looked up recommended PH levels for pro mix but you live and learn. Thanks again!