I'd say they could use 8hrs of darkness and possibly too much water or fert? Hard to say. I let mine dry out a tad between watering and they are looking fine. Mixed my fish emulsion at half strength after first true leaves and repotting to 4" pots. My lights are on 11 hrs right now.

Do the plants look like they are turning away from the light? As in too much?

Nope, none are turning away from the lights. I let the pots get really dry before I water them again. Whats odd is it is almost exclusively the heirloom seedlings doing this, the hybrid seedlings are as green as can be with no issues. makes me think that it is the lights are maybe to close to the seedings or are on to long (18 hours a day) none of the seedlings are leggy at all, have awesome thick stems.....I have never grown heirloom tomatoes before, so this is throwing me for a loop, cause I am clueless as to what to do for them.....

When my seedlings are 5 - 6 inches tall and potted up in 4-inch pots, the top leaves are up in the florescent light fixtures due to my set-up. And they aren't turning yellow due to their close proximity to the bulbs. Well, sometimes they actually get burn spots from touching the heat, but not yellowing.

If the plants are yellowing from the bottom leaves up, you know, beginning at the cotyledons and first emerged true leaves and drawing the green out day by day upward toward the top leaves, and the yellowing is fairly even like progressing from the tips of the older leaves then inward toward the stem in an even yellowing progression ... NITROGEN DEFICIENCY.

Nitrogen is mobile in plant tissue and is absolutely necessary for photosynthesis and cell division (new growth). Therefore, if the plant is not getting sufficient nitrogen from the potting mix (via the roots) for new growth to progress, it will draw stored nitrogen from lower leaves and transport it to the growing tips.

What varieties are they, not that it maters. I'm wondering about that mix your using and ph? Either that or salt from the fert? Or lack of food? Or its too much light. Got me.

Try giving them less light. Sounds like everything is the same as I'm doing minus the fert, soil and light levels.

I'd try giving them less light and see what happens. How often are you fertilizing them? Is your soil mix a soiless mix?

All of my plants are heirloom and they are super healthy so far. Knock on wood.

You may also have a minor iron deficiency.

Try 1 tsp Ironite or Fertilome iron supplement (also contains other vital micronutrients) and 1 Tbs. Miracle Grow or similar plant food (rose or tomato formula) per gallon water and fertigate the entire plant - leaves and roots.

I was using Miracle Gro "Quick Start" 4-12-4, says it safe for all flowers & veggies it's a planting and transplant solution at 1/2 strength to water them. I have only used this 2 times. I do have Miracle Gro Tomato plant food that I HAVE NOT used on the seedlings, it is 18-18-21. So I should be using the fertilizer now as the label stated strengths? Even on such young 3 week old seedlings? I was afraid I would burn them thats why I have been using 1/2 strength.....I live on an island, and the garden center doesn't open until monday, I will get some Ironite or Fertilome, add it to some Miracle Grow Tomato plant food and try that.....for tonight and tomorrow I will also cut back on the T-5 lights to being on for 16 hours straight, cutting it down to 12 hours straight. And yes, they are in a soiless ProMix......thank you everybody for your helping me along with this. I have always had such good luck with growing tomatoes, I am really at odds with what is going on here. I would be heart-broken if I lost them for a stupid reason on my part___:>(

Scroll through the pages of this link and see if you can find a picture that matches your symptoms.
http://www.salinitymanagement.org/Salinity%20Management%20Guide/sp/sp_7a.html

I'm assuming you're using florescent tube lighting. And I don't think there's a thing wrong with 16 - 18 hrs. per 24 with florescent tube lighting. I'd actually put the lights closer to the plants. Like within an inch of the tops.

I did not mean full strength Miracle Gro, sorry. My watering can is 2 gallon. I should've said 1 tsp. Ironite plus 1 Tbs. Miracle Gro per TWO gallons. Sorry. Glad you caught that. Yes, use it half the recommended strength. Just to get a little nitrogen booster and confirm whether it's nitrogen deficiency. Just once. Don't overdo it.

You might also add a Tbs. Epsom salts to that same 2 gallons of water. If it's a bother to get it, forget the Ironite until you try the nitrogen boost and Epsom salts. But the Ironite is sure to boost the health if you happen to buy some later.

I wish you'd post pictures to confirm whether it's nitrogen deficiency for sure. It's just that's my first inclination if your plants are 6 inches tall or taller and yellowing from the bottom up.

I only grow Heirlooms and I had this problem last year and to a lesser degree this year. What I found in my research was that when the plants are grown in colder weather, they lose the ability to process Phosphorous. So I put all the pots on a germination mat. I have grown several sets of about 20 plants this year without that difficulty. I have a set up with 3 light fixtures that I can move up and down. The plants sit on a table. I use a space heater under the table just so its heat travels up around the edge of the table to the plants. It gets down to the high 30s in my garage where I raise them but generally gets no colder than mid 40s. Anyway, this maintains a warm area for the plants. I keep my lights on for 18 hours as well. I germinated these in Jiffy pellets which expand when you add water, then transplant the whole pellet into a 4 inch pot with a potting mix I made last year. I haven't begun to fertilize until this last week and the plants are now 4 to 6" tall. I have a blight problem here in Hemet so I have treated these seedlings with Serenade since I first transplanted them. I think that the Jiffy pellets have quite a bit of fertilizer in them and my best seedlings in these two years have come from their use. Good Luck!!

Ok all you helpful people, I quickly created a fototime account and uploaded 4 pictures of my yellowing seedling leaves. I raised the lights to get them out of the way. If you look closely at some of the leaves they even curl a little bit and dried up....the hybrids are very lush, and my heirlooms are yellowing_____:>) help, please!!!!! I hope this link works http://www.fototime.com/inv/39417CE44222633

Okay, glad you posted the pictures Click for Pics because that IS NOT Nitrogen deficiency.

The yellowing shown in your pictures is intervenal chlorosis, yellowing beginning at the leaf margins and progressing towards midribs - leaving a lacy yellow pattern between the remaining green ribs, and is the result of micronutrient deficiency. Maybe iron, maybe magnesium. Those would be my first two guesses as they are the first to show up because soilless growing media do not contain them in sufficient quantity for plant health.

Either way, use the Ironite or Ferti-Lome liquid iron suppliment. Both have complete micronutrients. Until you purchase one or the other, give your plants a dose of Epsom salts, one tablespoon per gallon. Yes, one full tablespoon per gallon as some of your plants are getting to an advanced stage of chlorosis. Epsom salts = magnesium sulfate. I think it will help. Cannot possibly hurt.

You may already have sufficient nitrogen, phosphorus and potasium (NPK) in the soilless potting mix because you already dosed it with Miracle Gro ... evidenced by some of the other seedlings being plenty green, okay? It's just sitting there available to the yellowing plants but they need a vitamin pill to assist their ability to take up the NPK. Think of it that way. All plants are not created equal.

It will take a day or three for the little ones to green back up. But they will. Well except for the leaves where they've gotting tan, brown and crusty. Those are gone. You can trim off the crispy parts. But the leaves that are yellow and green lacy pattern, those will green back up.

hoosiercherokee....thank you so much so explaining and helping determine what the problem is. I do have epsom salts in the house and will do that right now. I know people like me can be a pain in the butt asking so many questions in these forums....sorry_:>( Will let everybody know how the epsom salts did.....thanks for making my pic link active too_____:>) oh, and I do have a fan gently blowing on the seedlings for a couple hours everyday.

Yea, I think hoosier hit it right on the nose with that one. Next year, try a fertilizer like Foliage Pro 9-3-6 that also has all the essential micronutrients in it, but for this year, follow his advice.

- Steve

So do you think I will have to be "nursing" the heirloom tomato plants all summer if this is a tomato type deficiency. My hybrids, in the same type promix soil are doing great, it's just the heirlooms (I have about 10 different type heirloom toms growing)...or maybe I should re-pot them soon to a different soil? Living on an island I don't have access to a varitiy of fertilizer products, but if there was one that excels with tomato need and requirements I would certainly buy some over the internet....any suggestions on the best fertilizer for heirloom tomatoes?

MV, this is no problem at all. We've all had these issues with tomato seedlings. Some of it has been trial and error and some of it from others before us. That's what these forums are for.

Basically your plants look good. I've had plants that looked that bad or worse spring back out of it. Don't expect it overnight. And expect to lose some of those badly wilted leaves. Just trim off the really bad stuff and go on with it. New leaves will emerge quickly.

The fan is great for air circulation but remember that the little ones you have are growing in totally artificial light and have not had direct sunlight to harden off. So it's not like if they were in a greenhouse or plastic hoophouse where you could turn on a fan and really create wind over nearly hardened off plants. Your plants have leaf tissue that still is very tender and thin. Keep the breeze very gentle so as not to dry out the leaf tissue.

Or perhaps it be wise to use a true soil mix (vs. soiless)after potting up to lessen the chances of nutient defiecencies?

I always start in a soiless seed starting mix and within the first two weeks of life the plants go to a half potting soil/half compost mixture. No complaints yet! Ok, You got me, Is it ever going to warm up!

Any big box store or even some drug stores should sell Miracle-Gro soluble plant food, no need to order something special online. I just bought a 6.25 pound container in Lowes for $8.87, which should be enough to last me for all the 150 seedlings I have going right now and then some. It is supposed to contain micro-nutrients. I dilute it to about one third the recommended dose, but use it every time I water the seedlings that have true leaves. I use plain rainwater before they get their true leaves. BTW, it says right on the Pro-Mix bag you have to use supplemental feeding as it doesn't contain fertilizer.
Once outside though Miracle-Gro is kind of expensive to use so I buy bulk fertilizer in 50 pound bags and also use compost for the actual garden.

I am relieved to say, my tomato plants are recovering well. They all have had a small dose of epsom salts diluted in water while it took me 5 days to track down some irionite. Living on an island, irionite is carried in the couple of nurseries here, but not this time of year...but I was able to find some....diluted some ironite and gave the toms some of that and they have become quite happy________:>) should I continue to use the irionite in my regular diluted watering of fertilizer as a suppliment? Or only if they become stressed again....I am guessing this all happen to begin with because I was not giving my toms enough fertilizer in the first place, thinking the promix I was using had fertilizer in it a little bit.....they will continue to be in the promix for another 60 days.....for all that have helped me along with this, a BIG thank you....I was so worried I was going to lose all my tom plants....

If you use something like miracle grow you should not need to continue using ironite or epsom salts.

oh, based upon all you have said you could have some kind of pH issue going on and that effects nutrient availablity