Yellowing leaves on seedlings

I've got a couple of seedlings (lowryder 2), at around 3 weeks or so. They have both had exactly the same treatment and are planted in Miracle Grow under CFL 18 hours a day. It is a 70watt CFL (4300 lumens) and the growing area is surrounded by a reflective mat with some yeast also bubbling away in there. I water them sparingly and have not added any nutrients. However on one of them the leaves have started to look quite yellow! The problem is not nearly as noticeable on the other plant, which is also growing faster. What could cause this? Pictures attached.

It's my first time growing so apologies if this is a silly question- I did search around the forums first but didn't find anything that looked like my problem.

Thanks in advance!

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Click on this link. http://boards.cannabis.com/plant-pro...ing-forms.html

Fill it out the best you can and post it here. We need some more information. :hippy:

First time grower

Your Equipment:
.1) Type and wattage of lights. (MH, HPS, CFL's, tube fluorescents, LED's)
1xCFL, 65W, 4300 Lumens
.2) Distance from tops?
5cm
.3) Reflector type? (cool tube set-up, bat wing, enclosed reflector, bare bulb...)
reflective blanket -see picture Attachment 275453
.4) Is there a consistent fresh air supply?
Yes
.5) Do you have an exhaust fan and a circulation fan?
No- area is open to the room
.6) What are the bulb wattages, kelvin ratings, and schedule?
65W, 6400K, 18hours on/ 6 off

Your medium:
.7) Specific brand and type of soil, (coco, peat based soilless...) and anything you've added to it. (vermiculite, perlite, worm castings...)
MiracleGrow All-Purpose compost
.8) Size of container.
Small 5cm across, 7-8cm tall
.9) Did you use peat pucks (or similar) to root clones or germinate seedlings?
No

Your nutrients and water:
10) Source of water? (tap, bottled or filtered) What's it's ph before adjusting?
Tap- dunno ph
11) Method of checking water ph. (ph pen, test strips, aquarium test kit...)
N/A
12) Method of adjusting water ph. (phosphoric acid, white vinegar, hydrated lime, PH Up...)
N/A
13) Specific brand and N-P-K ratio for each bottle. List dosages (quantity per gallon) and current feeding schedule.
No feeding
14) How often are you watering between feedings, and how much per watering?
When compost looks dry
15) Any additives or tea's? (Superthrive, CalMag, molasses, Mother's Earth...)
No
16) Are your ph levels stable, or do they fluctuate?
Dunno
17) What is your ingoing water's ph? ...your runoff ph?
Dunno
18) Do you foliar feed? If so, with what, how often, and at what time do you spray?
No

Your growroom:
19) Indoors or outdoors?
Indoor
20) What size of closet, room or hut?
Small room roughly 2m square
21) What are the temps and humidity levels while lights are on? ...With lights off?
Probably 18-21 in the room, maybe a bit more within the blanket when lights are on
22) Have you seen signs of insects in the growroom?
Occasionally, flies etc

Your strain:
23) What strain are you growing? (Indica dominate or Sativa dom?)
Lowryder 2
24) From seeds or clones?
Seeds
25) Is this an autoflower strain?
Yes

Ok a few suggestions:

First you need to get a ph tester. PH is key, and can cause many things to happen.
I would recommend getting a ph pen. It doesn't have to be one of the expensive ones. I use a Milwauke ph pen and it cost me $15 US.
Also do some searching and reading around here about ph levels and flushing. :thumbsup:

Second is I would recommend transplanting into the next larger pot size. The new soil needs to be amended with some perlite.
Marijuana likes an airy well draining soil. Mix the soil and perlite at a rate of 2 - 3 parts soil to 1 part perlite.
Again do some searching and reading about how to properly transplant. :thumbsup:

The last bit is that most people say that autoflowers perform best on a 20on/4off cycle.
And it would be better if you had one of those bulbs for each plant. (Once they have a few more nodes and are showing healthy growth.)

If you could get all this in line, I'm sure you will see better results. :jointsmile:

Here's a diagram to start your understanding of ph. :hippy:

Thanks very much for that advice- I'm going to test the soil pH and get some perlite tomorrow.
I just checked online and apparently the tap water in my area has a pH of around 8, so that's what they've been watered with.
I'll keep this thread updated in case anyone has a similar problem in the future.

Thanks again!

No problem, let us know how they turn out. :hippy:

dont use compost for seedlings and as tiki said ph is key. spend a few bucks more and get fox farms ocean forest and see the difference. good luck:thumbsup:

You want to read the lables. Check the NPK ratios of different products. I'm unable to bring up the NPK ratio of MG's compost but I'm going to guess that since your young leaves are fading green and turning yellow, that there isn't much fert to this. Funny, because MG potting soil is loaded with fert. If you want a good soil to grow in, try ordering this stuff. Sunny Girl Gardens You can buy it online or at a store if you live in western Washington, Oregon. It will be shipped to you address and you just add water, nothing else. Comes in two parts, one in bottom of bucket other part on top of the bottom part. That's all you need.

Thanks everyone for the replies. I had a look at the packet of MG compost but it didn't specify the balance of the fertilisers. It does say that it contains enough food to last the plant for three months though so there should be something in it! Sadly I live several thousand miles away from Oregon, otherwise I'd give that Sunny Girl Gardens a shot.
Anyway, so I've now repotted in 15cm pots with a mixture of compost and perlite (I threw the original soil away) and from now on I'm going to use bottled water with a pH of 6.5 to water them (Clearview Spring, 25p/2L from Tesco if anyone is interested), so hopefully this will solve the pH problem. Our tap water here is very hard in addition to being quite alkaline, so that probably didn't help things much. Here's a couple shots of them in their new home.
I'll post again when I've done the pH test on the old soil and after they've had time to get over the shock of transplanting.
Thanks again!

If you were to add more soil, that part of the stem under the soil will grow roots. :thumbsup:
This will help fix the slight stretching that you have, and make sure that you let the soil really dry out in between waterings.

Happy growing. :hippy:

Thanks, I'll get onto that- will post more pictures if the situation changes

You should put a small fan blowing some wind on them if you haven't already. That will knock them around and strengthen the stems; otherwise they will fall over when bigger.

Thanks for all your suggestions- after a while when it looked like they wouldn't pull through, they are now thriving!

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A few drops of vinegar in your water will bring your PH down, for two plants buying distilled water isn't a big deal but you could also boil/microwave the water as well then adjust your PH levels.