Bottom leaves yellowing on 4 week old plants

OK, so these AK-48 plants where sprouted almost 4 weeks ago. There are five plants, which I started under four 4' 40w fluorescent tubes. When it was evident that the plants were stretching badly (growing 4" in a day and falling over), I added two additional 150w HPS bulbs and a small fan. This seemed to solve that problem.

I transplanted them from their starting cups to 5 gallon buckets about a week ago. The soil is all Miracle-Gro moisture control potting soil. Except for the stretching, they have seemed extremely healthy until now.

As you can see in the pictures, the edges and tips of some of the leaves have begun to yellow. The initial two sprout leaves have completely shriveled, though I assume this is normal. On all of the plants the single bottom leaf has yellowed at the tip (second and third picture). Additionally, on a couple of plants the first real (bottom-most) fan leaves have yellowed slightly at the edge (first and fourth pic).

I read that this type of yellowing was sometimes a symptom of Nitrogen deficiency, especially because it was starting at the bottom and going up. So, two days ago I fed the plants with fertilizer for the first time with Miracle-Gro tomato fertilizer (18-18-21) at about 3/4 strength. Two days later the plants look the same: no better, no worse, and still growing steadily. The rest of the leaves are a healthy looking deep green.

Some additional information: I have been giving them a decent amount of water every 3-4 days. The water is filtered city water, which is then left out for several days. Also, the room gets a bit warm when we have hot days, but nothing over 90 degrees or so on the worst days. A fan keeps fresh air circulating from outside.

So, am I worrying over nothing or is there something I should be doing?

Pictures

You are worrying over nothing. Stop feeding them that's probably nute burn. Tips of plant starting to burn with super green leaves does not indicate N deficiency.

Just because you have nutes doesnt mean you need to use them. The plant is very green and MG has nutes already in the soil. MJ Plants are very hardy it takes some real talent to kill them, and usually the killing of them happens from over doing things and causing problems rather than neglect. Just water the thing and give it light and you are 90% there. LESS IS MORE. If you overfeed your plant, you will cause deficiencies due to toxicity.

Leaving the water out is unnecessary. I had a DWC plant that I put a 1/2 cup of full strength bleach into and left for 5 days (was out of H2O2 and had some algae that was pissing me off), it killed a few leaves that was about it, plant lived no problem. Entire grow room smelled like a pool. trying to evaporate the infinitesimal amount of chlorine out of your filtered city water is a waste of your time (unless doing so makes you feel better ).

One minor point if you happen to up pot, mix some extra perlite in MG has some but a little more is a decent idea, your soil looks like it might be slightly compacted (no big deal, if you dont overwater it will be fine).

Hey and thanks for the response. The only reason I didn't consider nute burn was because when this showed up I had yet to fertilize the plant. You think what's in the Miracle-Gro soil would be enough to burn it?

I know it's not much damage, but I was trying to be proactive about it. It is my first grow though, so you're likely right about me worrying over nothing.

I won't be transplanting again (those are 5 gallon buckets), but I'll keep the perlite thing in mind for next time. Btw, this is the soil I'm using. I'm not sure precisely what differentiates it from normal Miracle-Gro soil, so I figured I'd point it out. The soil seems pretty fluffy to me, at least the first few inches, but I'm not sure what is considered "compacted" either. I'm already being very careful about overwatering though, since the container is kinda big for the plant size right now.

Regardless, I guess if it doesn't get worse (and it doesn't seem to be) I should probably stop worrying.

Yea its so minor it might just be the MG. I've never used that particular one, I've used the regular potting soil and the garden soil before.(as well as fox farm/etc, IMO MG is just fine). I'm not sure what "Aquacoir" but sounds like it might have some coconut husks in it to retain moisture (this is a pretty common medium in growing ie Coco/etc). I was just judging by the pics you provided looked compacted basically you want some air pockets in the soil. If its fluffy like you say then no problem, MG tends to have a lot of bark/wood in it, its probably just the pic.

I've grown AK48 before, If you have a few AK48's you want the pheno that is less branchy more tree like although even the "tree" one tends to branch out pretty well on its own. They yield really well and both types seemed to stay short.

Here's a pic of some AK48 for ya

Cool, thanks again. I'll just chill for now and keep a close eye on it, maybe grab a PH tester since I should probably have one anyway.

Edit: Looks like you were probably right about the soil causing the burn.

Edit2: That's a beautiful bud! Right now all five of my plants are growing nearly identically. Mine seems somewhat tall right now, but I think that's because of the early stretching. The newer growth is much more compact. I might take some pictures of the whole setup tomorrow, just for fun and to get some general feedback.

Thanks man, that was a in between my normal run grow (I'm a licensed caregiver) that I just stuffed in there. I lost my HDD and my really good pics, but here are a couple more better looking pics of some dinafem Sweet deep Grapefruit that I also stuffed in there with the AK.

A short update:

The bottom-most leaves seem to have yellowed slightly more and I read that Nitrogen availability starts to go down at a PH of 8, so I'm wondering if the PH could have been causing what I thought was a N deficiency. I used bromothymol blue to test the PH of my tap water and it was pretty high, somewhere between 7 and 8. I don't have anything to test soil with right now, but I think it's safe to assume that it's probably high too since I've been putting this water in it for weeks.

Because of this, I added sodium bisulfate to the water before watering today and brought the PH down to around a 6. I then watered fairly generously. Even if this isn't the cause of the damage I'm seeing, I think it's safe to say that the PH needs to be brought down anyway. I'll probably bring the PH of my water down to 6.5-7 from now on.

I'm curious though; is the sodium bisulfate OK for cannabis or is there something else I could/should use? The only thing that worries me is that this is technically a salt. I have hydrochloric acid on hand and have heard of some people using vinegar and lemon juice. I also have some aquarium PH down, but it doesn't say what's in it. What would be best/safest to use? Would the acid end up breaking down into chlorine?

I added before and after PH pictures to my imgur album. I also took some more pictures of my setup.

i experienced this same problem last summer with a few plants i grew next to a creek in very sandy ground. the lower leaves started to yellow and die off. the plants didn't die but the yeild suffered greatly. i contributed this later to lack of Nitrogen... I wouldn't wait too long to see if what you have done fixes your problem. if it ends up being a lack of nitrogen you can save some of the damaged parts of the plant... but anything that gets too damaged will just die off and stay dead. I think some of the yellowing parts of the plant that aren't too bad will make a recovery if you fix the problem in time.

I took more pictures of the plants today, a few new leaves are brown on the tips, but for the most part whatever it was seems to have stopped spreading. Plus, the plants have grown about two inches in the last3 days.

They're about 14" now; in a few days I'll probably top them, make clones from the tops, and put them into flower.