Low N in coir/peat - advice wanted

I've been using Pro-Mix HPCC

Nutrients during bloom: Maxigro/Maxibloom/Cal-Mag/Silica in grams/ccs: 2.5/2.5/4/2.5

This produces elemental ppm of 120/58/170 around 950 total using blue-lab meter.

I adjust PH of solution to 5.5 first 3 weeks and 5.8 beyond that.

Symptoms: Throughout the grow fan leaves at all levels but especially near the top will slowly turn lime green and yellow. No wilting, no brown tips or other discolorations, no notable difference between vein and between vein color, or any other symptoms.

This seems to be an indicator of low N. I wouldn't think 120 ppm N to be an issue, but I can try bumping it up to around 150. PH possibly slightly low. I can try bumping PH of solution up to 6-6.2. The Pro-Mix HPCC is peat based with chunk coir. I'm not sure on the exact percentages but this might indicate 5.8 solution is a bit low.

I'm thinking about doing both, bump N to 150 and PH to 6 and see what happens. Any thoughts?

I'll check runoff next time I water... but N can be absorbed at lower PH than other nutrients so I'm thinking the PH must be quite low to produce a N deficiency but it that were the case it seems like I would have other deficiencies as well.

Hence my confusion.

How about a pic and information on lights. Types and distances over the plants.

Thanks.

No pics, but it's straight forward, plant looks normal except for yellowing leaves starting around week 3. Yellowing can occur at any level on the plant mainly on the large fan leaves. Slowly progresses. Plants finish okay and weight is okay but still, I know some leaves can start to turn yellow towards the end of the grow but this is happening way to early.

Light is LED 3000K 90CRI 44w per square foot, grid of 23 emitters at 30w each spaced over a 32-72" area. I've been keeping the lamp around 12" from canopy. PPFD readings at that distance vary from 800 on the edge to 1200 in the middle. Average PPFD is around 1000 (1480 PPF over 1.5 square meters). Temps in there around 75F.

I've done similar with hydro and had no problems.

Since switching to peat/coir it's been an issue. Some strains worse than others.

The reason I entertain PH being low is because this mix is peat heavy. The chunk coir probably makes up 30% by volume and there is some perlite in there too. I add the cal/mag at 4ccs per gallon because I was getting cal deficiency (spotty chlorosis and brown spots) when I first started using this stuff.

Cal/mag did clear up that issue. The yellowing that is occurring now is not spotty or interveinal, it's pretty much the entire leaf. If the leaf gets yellow enough it will drop off before any browning happens. This seems to be classic N deficiency symptoms.

You don't want to post pics, and you seem to know the issue, so best of luck! However, small tip...nitrogen deficiencies will not begin at the top..if they are yellow/lime at the top because of a nitrogen def..then the rest of the plant will have lost the leaves!

It seems these 'hybrid' mediums (coco and peat mixes) cause a lot of issues for the grower. I think the tendency is to treat it like a coco grow, when in fact it should be handled more like a peat grow and in some cases the opposite occurs. I would contact Promix and ask them what pH they recommend in this particular mix and go from there. Also, if you think you're short on N, bump it up. If it is indeed a N shortage, your plants will green up instantly. If the added N doesn't do the trick you've eliminated it as a variable.

Agreed on the hybrid mediums. This stuff does need some cal/mag added, but unlike straight coir I think I've been running the PH too low.

Solid flush for all plants
2 control plants NPK ppm 120/58/170 PH 5.8 as normal
1 PH boost to 6.3
1 PH boost to 6.6
1 PH drop to 5.5
1 N boost NPK ppm 150/63/180 PH 5.8
1 TDS drop NPK 100/50/150 with PH 5.5
1 TDS drop NPK 100/50/150 with PH 6.5

I will hopefully get some good indicators, depending on what happens.

Ah, an experiment...I like it. Keep us posted.

24 hrs. later and four plants now have some upward clawing and browning on leaf margins. These are the N boosted plant, the normal feed with PH boost to 6.3 the reduced feed with PH 5.5 and 6.5. This doesn't provide the obvious pattern I was hoping for. The PH boost to 6.6 shows no clawing. Both controls remain unchanged. The normal feed with 5.5 remains unchanged.

-No real pattern to it, but with the additional symptoms it's looking like a magnesium deficiency (thanks @hotrodharley ).

-He also mentioned heat stress. I've raised the lights a bit and turned the temp down a couple degrees.

-Going into flower with peat it makes sense 5.8 ph was on the low side.

-May have been some salt buildup over time. I over water by about half gallon so maybe I should start with plain water to field capacity first then add nutrient solution on top of that... would flush out a little more than a gallon and I will save a few cents on fertilizer.

-Also, because it's not all coir, and because I use tap water I've only been using the cal/mag at half the suggested rate. I'm unsure on whether I need to go to full rate if I make these other changes.

DR. VonDankenstine said:

This is for all you DIRTBAGGERS out there(coco/peat/soil-less medium/perlite/hummus/vermiculite/work castings/charcoal/gravel/rock/bark/soil/ or any-other medium you may use in the place of soil or dirt) This recommendation does not include hydro applications---(rock-wool/hydro ton/fytocell/DWC/hydro/aero mist/ or any other hydroponic system PH which should be maintained at 5.5 to 5.8PH.

In the germination/seedling/ and vegetative phase of growth MJ will do very well at a PH range of 5.5 to 6.5. You also hear of coco users maintaining their PH at about 6 but the fact is that even coco will do well for the first 3 phases of growth at the same PH. Although potting mixes have less problems with nutrient PH lockout than outdoor soil, for maximum flower development/weight/size/ and vigor your PH should be above 6 and at or below 6.5 in the flowering stage. The reason for this is that the most important nutrient P(phosphorus) in flowering is easily absorbed and assimilated by the plants in the 6 to 6.5 range(its actually higher than 6.5 but we start to run into micro nutrient availability above 6.55). Your plants will do fine from 5.5 to 6.5 but for the best bud development in flowering it needs to be above 6 and below 6.5. Hope the info helps.

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I think I'm going to PH to 6.4 next round.

Nitrogen is mobile, if the plant needs it it will translocate it from the lower leaves first. Yelliowing on the upper leaves is something else.

You can't feed a soil or peat/coir grow as you would hydro. If you had good success in hydro, I'd go back to it if I were you. Why the change? You write how much you give but not how often, which has a role in how much builds up in this mix you're using. Some people say you treat coir like it's hydro but why would they say this when it's very different? The high dilution of hydro makes a difference. You have to use a higher ppm.

polishpollack said:

If you had good success in hydro, I'd go back to it if I were you.

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I had good success with hydro. Now I'm trying to have good success with a soilless medium. There's a learning curve but that's cool. One thing I like about soil/soilless is the ability to pull each plant out and manicure. I read in various places that soilless was similar to hydro, but that doesn't seem to be the case according to Dankenstein except for rockwool. Even straight coir seems to do better at 6 or higher. It's just a case of me soaking up bad information.

In this peat coir mix I've been feeding once a week, watering in-between with PHed tap water. The "normal" feed I mentioned comes out to around 1000 ppm. I was running the same basic feed I used in hydro 100/57/153 and started adding the cal/mag at half rate then started adding Si at half rate which puts the numbers at 120/69/170

DLI has been around 43. The dehumidifier has been a relatively new addition, keeping the room at 50%, slightly higher in the grow space.

I can ditch the Si, maybe even the cal/mag if there are other environmental stressors that were contributing to the issue. PH seems to be the primary consideration, but until things are under control it seems that dropping the DLI under 40 and going easy on the PPMs is a good idea. Once things look straight I'll work on bumping the DLI and ppms back up until I run into a problem.

So I have a plan, but am open to any advice that would help get this media working well.

Observe & Report said:

Nitrogen is mobile, if the plant needs it it will translocate it from the lower leaves first. Yelliowing on the upper leaves is something else.

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Makes sense. One issue I've had with identifying the problem is that until yesterday, all of this grow and the two previous, never showed any upward clawing or browning. Veins are slightly more light green/yellow than leaf tissue but without looking close it appeared homogeneous. N deficiency seemed to be the closest thing to this. In the deficiency/toxicity pictures most problems (including N deficiency) show obvious intervenial cholrosis. Due to the new symptoms and info it does appear to be a Mg deficiency caused by low PH.

If you are only feeding once per week in inert media they are starving.
Most coco growers are feeding every watering.
Sounds like potentially a minor nutrient deficiency with the mutated top growth

OneHitDone said:

If you are only feeding once per week in inert media they are starving.
Most coco growers are feeding every watering.
Sounds like potentially a minor nutrient deficiency with the mutated top growth

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In the past I've tried feeding in between with lower ppm and ended up having rapid onset of issues, burnt tips and leaf margins. I assumed it was too much fertilizer, backed down to once a week and problem fixed. I'm using about 1.5 gallons of media per square foot. The previous strain wasn't showing these current symptoms as badly. I will say that the once a week feed with 100/57/153 is fantastic in veg, nice green leaves and explosive growth, so it's difficult to believe I'm not feeding enough.

My experience in hydro also lends some credence to the PH problem. When I was able to monitor solution in real time I noticed in veg PH tends to rise to between 6-6.1 overnight and daily adjustment was necessary to keep it at 5.5. In transition PH would stabilize and getting deep into flower PH would swing down, needing to add some PH up to maintain 5.8.

Something similar seems to be happening here. In veg the nitrates are getting pulled in causing a upward PH swing which makes the 5.5-5.8 PH workable while the plants are still growing. Considering the tendency for some PH drop in flower and the amounts of peat in this mix I'm pretty sold on the idea that 5.8 is too low to carry through flowering.

But I will keep it in mind and if PH correction doesn't do the trick I'll look at other things like whether I've been under feeding.

It's unlikely to be mag def if you're giving calmag. Pics would help.

It's in there, it's just not being taken up is my guess. PH too low in soil and the plants have difficulty absorbing P, Mg and Ca. That's what seems to be the case at this point.

so The promix peat for me drops the soil ph down to 4.8-5.3 in a few days.

What happens is I water let’s say 6.2 I’m given a few day window of time before it drops back to those numbers ..it’s our job to figure the sweet spot of raising it enough so the drop is steady and slow enough to only get to about soil ph 5.5 from my knowledge so far before you need to water again I have problems at 6.2...


Medium takes a few good days to dry out so for me it would still be wet when it reached its low 4.8-5.3 with the blue lab soil ph meter...

Plant health looks better so far with 6.5 I haven’t plugged the soil lately to see what she’s doing ph wise.....
That low acidity causes all kinds of lockouts that create problems by guessing what’s going on...
I’ve been tryna get good info on how to run this stuff properly and it seems like Some trade secret iv dug hard and never found much good info

So 5.8 is horrible from my experience I run cobs also so I feel like my experience can relate to yours

5.8 is dropping that soil ph easily into the 4s in a few days time

Ever test ph of run off ? It’s scary I was using straight peat promix before

Now I run the peat coco one in hopes the coco lessens some of those pesky peat problems

I’m stupid and don’t know how to turn umoles into ppdf
But I found plants don’t mind 800-1200 umoles sometimes the 12 is a lot for strains and they start acting funky with diff things happening discoloring ..yellow tips ..tacoing

I got a orakle by in house doing kinda as you say right now that’s part of why I started the 6.5 regiment to see how plants responded and I like so far can comment back if something changes