Plants in dire need of diagnosis

[The Unclean]

Hi all, I'm a first-time grower who's having a little difficulty with my plants. Specifically, they're showing leaf yellowing/death starting from the tips, but it doesn't seem to be a nitrogen deficiency because the very bottom leaves are untouched. I'd appreciate it if you could help diagnose these girls!

Required information:
-Royal Flush (Sativa-dominant)
-10 plants, of which only three or so are showing serious symptoms
-Early vegetative stage, started from seed four weeks ago
-Indoor, organic soil (Happy Frog) in a one-gallon pot
-500-watt LED lighting
-CO2 supplied via tank with a time-release valve
-80-degree temperature, 45% humidity, pH 6.8
-No pests detected
-Watering every three days or so
-Fertilizer info below

I've attached photos of three plants - the first is one of the largely untouched plants, the second is moderately affected, and the third is by far the worst (and may well be on the brink of death). You'll notice a significant difference in size and number of leaves.

Mostly healthy:

Problematic:

Dying:

At first I suspected this was just a nitrogen deficiency, but I've been adding half-strength doses of 20-20-20 and some kelp extract (root and foliar) since about two weeks from sprouting. I gave the affected plants a nitrogen supplement four days ago, but there's been zero improvement - if anything, the yellowing and necrosis have spread. Any idea what's wrong with my garden?

 

[djfireball]

I'm no expert but possibly
Magnesium/calcium deficiency, your ph is around the same as what my plants are at. Maybe lower the temperature a bit. I was running around the 80's and the dispensary told me to lower it.

 

[YardDog35]

Do you have a ppm meter? 20-20-20 is very high and beyond what they need now, even at half strength.

 

[The Unclean]

You're thinking it's nutrient burn, then? I'll try flushing the affected plants with distilled water. How long do you think it'll take for them to recover, or can the dying one be saved at all?

What I don't get is why certain plants seem unaffected if I've been fertilizing them all with the same solution. (Unfortunately I don't have a ppm meter, so I can't give you exact figures)

 

[Cloudblood]

Definite nute burn. Jack's 20-20-20 is really more for outdoors monsters and if your using something similar you'd have to cut it at least to 1/4 solution at a full size plant and maybe to 1/6 or less in a smaller plant.. Unless you have a GIANT resevoir or a huge grow pot 20-20-20 will do as much damage to roots as you see on the leaves. There are lots of products that will have a unneeded amout of nutes in it so people think the number will mean huge plants.....

If those leaves are crispy and brittle you should cut them off so the plant can work on the parts of the plant still vibrant and not repair the broken parts.... we all run into burn here and there trying to get ahead, it should not kill anything just flush it and stay off nutes a spell....

The reason only certain plants are affected is pure luck that your mixes don't go in 'hot' on each plant....The super nutes are sticking together and resisting a real good dilution and it seems only one plant took the brunt of it. Call it luck they are not all toasty.

i would use a starter mix at that size of plant....a -12-4-4 sort of deal cut down a bit or there abouts....the plant needs mostly nitrogen at this point in your grow anyway.

 

[The Unclean]

Thank you for the quick and informative reply! I've excised the necrotic leaf areas and flushed my whole garden just to be on the safe side, and as of this morning the symptoms are no longer advancing. How long do you recommend I wait before applying the starter mix?

(Side note, this is exactly what I love about the cannabis community - I can't think of another group that would be so good-natured towards a rookie making a boneheaded mistake)

 

[Cloudblood]

You can re-intro the nutes at your leisure because the flush should of saved the hot spot in the grow medium. Just go slow at first, I have been known to get crazy and make plants brittle at times in a rush out the door in the mornings....

 

[YardDog35]

You're thinking it's nutrient burn, then? I'll try flushing the affected plants with distilled water. How long do you think it'll take for them to recover, or can the dying one be saved at all?

What I don't get is why certain plants seem unaffected if I've been fertilizing them all with the same solution. (Unfortunately I don't have a ppm meter, so I can't give you exact figures)

The Unclean gave good advice.

Yes I believe it was nute burn, good you flushed. I would switch nutrients to something more suited to your plants needs. N is great for veg, P is great for root development, and K is great for flowering. You don't need much K right now. My advice would be to go with a one part like General Hydro's FloraNova, They make a grow, and a bloom, and they keep it simple. Once you get into flower you could start adding additives like a bloom booster and sugars. Just a thought.

With that being said, you're doing a great job! Even the plant that is the worst, really doesn't look too bad. I've burnt some plants, big time! You're plants look healthy, and not over watered, keep it up.

 

[dmagick2010]

I'm in need of a little help diagnosing a problem with my plant it's a White Siberian ( cross between White Widow and AK47). I'm Thinking its a heat or light burn but not sure. I had the plant outside in a greenhouse until my mother-in law started to poke around and started asking questions about it. then 2 weeks ago i brought the plant inside and put it in a corner where it could still get sunlight. I have a 150w HPS light due in tomorrow hoping this will solve the problem but here are some pix from week 6 till now 5 of the pix are of the problem