I have read that salt may be the reason for leaves on mature potted tomatoes to turn yellow.

Here is a link that might be useful: Tomato problems

What would happen if you left the growlights on 24/7?

I've done 24/7 when I got a late start growing translants. They grew somewhat faster and were fine. I popped good sized healthy transplants into the garden 4 weeks later. They grew and produced fruit normally. I think if I were trying to grow them under lights to fruiting the 24/7 would not be so good, but I haven't tried that.

In spite of all the "oops" moments I've had growing tomatoes, they seem to do remarkably well most of the time. Neglect may actually be a good thing as it eliminates most overwatering and the problems it brings.

Hey all, thanks for the help. I also found that yellow leaves with a pronounced purple on the stems and underside of the leaves is a sign of phosphorus deficiency. The problem is that my basement is pretty cool and I read that the plants will not take up much phosphorus even if it it available if it is too cool. I don't think I can justify adding to my $250 plus power bill in order to save 9 tomato seedlings so I will just cross my fingers and leave them alone. On the bright side, the sugar ann seeds I saved from last year are growing into very robust little plants.

found that yellow leaves with a pronounced purple on the stems and underside of the leaves is a sign of phosphorus deficiency...

That's true for established plants (as is the salt problem mentioned above). Seedings on the other hand normally develop the purple stem/leaf underside color (numerous discussions here on it) and it poses no threat or problem and disappears once established in the garden.

Yellow leaves in seedlings is too much water. ;)

Lights on for 24/7 has its advocates. 2 schools of thought: 1) that a period of darkness is needed to allow the plant to convert the sugars developed during photosynthesis to growth, and 2) the conversion takes place regardless of light or dark. It's up to you but I think most of us adhere to Mother Nature's plan of light and dark. ;) Again, there is a recent thread here citing scientific studies on the question with more details.

Dave

How much light in Lumens does a Tomato plant require? And How long? Lot of threads i read say 16hrs On 8 hrs off? I noticed at first my tomato seedlings were turning yellow but it the symptoms Vanished after. The only two things i changed was added more light power, and watered less.

Sumilea

In my experience yellow=overwatering. Also, neglect with tomato plants - works... if harmed they bounced back well... I can picture you hovering over your tomato plants throwing coffee and water at them... And they're still surviving (although a little yellow and with leaf roll)! Man, if anyone knew my regimine I'm sure they'd never allow me on this site again! But whatever I do right/wrong it works for me!

Digdirt

young seedlings can show signs of phosphorus defn too. Especially in cooler conditions. Yellow and purpling would be diagnostic for this.

Id try warming the seedlings from underneath and apply a mild/weak solution of water soluble bloom fertilizer when watering until you transplant or they perk up. You can place a heat mat or on top of a running appliance to get some heat.

When you start messing with daylengths of light you can have an effect on flowers. Type of light has some effect too. Nothing dramatic just something to consider.

16 hours on 8 hours off was found to be best in the research.

Darkness triggers metabolic reactions, though the effects are not as profound in tomato as some plants, darkness does have an impact. Give them a rest.

At one stage cool dark can even help promote more flowers.

Oh I agree with you mule. I'm not saying you can't have the symptoms of phos def. in seedlings, just that it poses no problem for them, is quite normal in many varieties (per research pics posted here rectently in other threads) and requires no action.

Indeed, often attempting to warm the environment to compensate for it only results in too rapid growth of tomato seedlings. Best to leave them cool I find, and thus have proper size for transplants rather than leggy giants with weak root systems - once transplanted the symptoms of the deficiency disappear rapidly with no lasting effects.

As to the lights issue - I also agree with 16 on and 8 off. In my experience, the quality and quantity of growth using 24/7 doesn't begin to compare to that achieved with 16/8 or even 12/12.

But I'm also pointing out that there are other schools of thought on the issue - it comes up for heated debate often on the Growing Under Lights forum and many rant and swear by it there. They have research on their side too - I just don't buy into it. ;)

Ultimately, it's the grower's choice.

Dave

So some of my tomato seedlings have yellowing leaves and they fall off soon after. It's affecting certain trays and all varieties. It's not over watering, cause most of the time, they are bone dry before I bottom water. Not sure if it's stress from being too hot, the avg temp is in the high 70s. Or overcrowding in the cells or what. Also the ones affected the most were planted on 12/20 The plants have normal top growth, I've fertilized them with a diluted complete fertilizer. I'm stumped, never had an issue, it's just certain trays? I know I need to pot them up, but is that the cause?

They are under 600w MH, 16 on 8 off

I know they're still small, but it may be nitrogen deficiency.