No photo. Young seedlings dying are almost always the cause of overwatering, damping off, and/or too little light.

Rodney

You might also want to read through all the other problems with my seedlings posts running just a bit further down the page. The causes/solutions are all pretty much the same.

Dave

masterdan1, I can't see a photo but I wonder if you are referring to the seed leaves aka cotyledon. The first little green leaves that push up out of the soil? If so, they help nourish the plant until the young seedling puts out it's first true leaves. Eventually these seed leaves turn yellow and fall off. The reasons can also be what has already been suggested.

How old are your tomato plants? Once the plants reach a certain age, even some of the very bottom true leaves can wither and dry up. Please see if you can insert the photo of your plant as it will be a big help to actually see the plant leaves in question.

In Houzz, you just click on the spot in your message where you want the photo to insert, then click on 'Photo' in the message menu at the bottom of your message, then browse to where you have the photo stored on your computer drive, click on the photo to select it, then click OK. Houzz will upload and insert the photo into your message.

If you want to get better help, learn to post picture(s).

A Mary mentioned above, it is very easy to post picture(s) along with your comment/message. All you have to do is to store your pictures in your PC/lap Top in a file first. That can be done with a camera or with most cell phones.

LOWER LEAVES:

If by that you mean COT leaves , no worry. Once the seedling has several true leave, those COTs are not essential. But if yellowing continues upward, then there is a problem, Often over watering and lack of light and nutrients can cause that.

Sey

Mary's hunch was right; those are the cotyledons, i.e., the seed leaves. They're dying on schedule--this is normal.

And I'm pretty sure that Sey, Dave, and others who have grown tomatoes for a while will tell you that peat pots are problematic because they make properly regulating moisture levels almost impossible and encourage overwatering, so you should probably get them out of the peat pots.

I'm not sure what "veg starter compost" is. Do you mean a mix specifically for starting seeds?

It just says on the compost bag, ideal for starting veg seeds

So it's just compost, but it advertises itself as a good mix for starting seeds?

Yes exactly, it's organic

Should I be worried if some of the true leaves are a little yellow?

I would take that claim, that it's good for starting seeds, with a large crystal of rock salt. Seed starting mixes are 1) sterile, 2) fine in texture, and 2) formulated to drain very easily. Compost is generally none of those things. It's not that you'll fail if you don't use seed starting mix--I've used less ideal media and not had problems starting seeds. It's that results will probably be consistently better if you use a dedicated seed starting mix, and, more importantly, a compost shouldn't really be advertising itself as good for starting seeds.

That said, your plants are old enough that they don't need to be in a sterile mix or anything now. But compost is still problematic because it's very good at holding water, and excess water is one of the top problems people have with young plants. You might want to repot in plastic pots or SOLO cups and use your compost mixed with some things to help it drain faster (perlite, sand, vermiculite, in some combination). When you repot, plant the seedlings deep, i.e., set them rather deep in the pot and bury them up to their first true leaves, right below where the largest leaves form a "Y" on that plant in the foreground.

It's hard to tell for sure on photos, but your seedlings seem to be doing well. The above suggestions are really to prevent problems from arising as they grow. They look a tad pale--could be a sign of too much water or not enough light, or could just be the photo.

I must have been typing when you asked about the yellow true leaves. So I guess the plants ARE actually pale. If you do a search on this subforum, you'll see that over and over again, yellowing leaves on seedlings are due to peat pots and/or overwatering. Lack of light is a common issue, too. Do you have grow lights, or are you growing in a windowsill?

Indoors on the windowsill at the moment but I do have a greenhouse, but I thought it might be a little cold out still. What would be the coldest temp that they could stand?

Well, coldest temp they can stand without keeling over and coldest temp without suffering long-term damage to production is two different questions. The first is easy--as long as they don't freeze, they will live. The second is less easy--I have yet to find a definitive answer that has solid research behind it, and you won't find definite agreement or consistent reporting of experiences here between the various gardeners.

They will grow very slowly, however, and sometimes not at all, until temps get up into the 50s. However, if you don't have a lot of plants, you can bring them into the warmth of your house at night. It's kind of neat in the early stages to see them visibly growing overnight after having had a day out in good light and a night in a toasty house. It can get pretty easy to forget and leave them out when it's too cold, though.

People can give better advice if you give us your location. You can have that info display after your username. Go into >Your Houzz > Edit Profile >Advanced Settings>scroll a little more than half way down and you'll see a field called "Climate Zone for Garden Forums". You can put your geographic location, zone, etc. there.

Those plants are too small for the cotyledons to be shriveling. Looks like overwatering and/or issues with growing in pure compost to me. They should be transplanted into potting mix IMO.

my tomato plant grows to like 8-10 inches and still has the cotyledons, so i agree that while they're usually the first to go as the plant matures, the seedling is rather young for them to be almost completely dead already.

I agree.

Agree also. The plant(s) is not big enough for normal abortion of cot leaves.

I see some burn and crisping.

I would repot them into soil less mix and fertilize with a balanced synthetic fertilizer.

Compost , depending on its age and the material that it is made from, can be very poor in nutrients and it can also steal some the nutrients that is added to it.

Sey

Looks like burn to me too.

Looks like you are keeping the soil far too wet. The fiber pots are only going to make things worse as they'll pull moisture from the soil causing it to dry out and need more water. Definitely back off on the watering. If you can transplant into plastic pots or cups, do so. And if you do, lighten up the soil with something like perlite.

Rodney

P.S. Are you in the UK?

Huh. Some of mine start getting ratty at about 2 sets of true leaves, with some lasting almost until I plant out. But I don't have an ideal setup and am pretty rough on seedlings, as I have to move them a lot, etc. I didn't realize it was that unusual for them to get worn looking at that point.