I would not worry about that little bit of yellowing. Yellowing and turning brown at the leaf edges is often due to salt build up. This is often due to not watering with enough water., You should water until water runs out of the pot. Otherwise, the water evaporates and leaves salts behind. This is made worse if fertilizer is included in the water.

You can rinse the salts away by watering with copious amounts of water. like 2 - 3 volumes of the pot.

Chlorosis

Of course it is a chlorosis as the leaf has a yellow tip, but many things cause chlorosis. I agree with Vlad that the problem is a mild soluble salt build up in the soil. Do as Vlad suggested, and flush the soil with water in the volume of 2X the volume of the soil.

Every year we get fantastical explanations for chlorosis in the fall. All anyone has to do is stick their head out the window and look for fall colors. If you see Maple trees turning red and yellow your citrus will have chlorosis soon enough, unless you supplement them with artificial light. At least a dozen maybe more reasons this is clearly not a salt build-up. Toronto Canada gets only 150 hours of sunlight in October. A small fraction of that if you are growing in the window. Yes Jeff you need a grow light and a grow tent if possible.

It is that time of year.

Sonnet 73: That time of year thou mayst in me behold

BY WILLIAM SHAKESPEARE

That time of year thou mayst in me behold

When yellow leaves, or none, or few, do hang

Upon those boughs which shake against the cold,

Bare ruin'd choirs, where late the sweet birds sang.

In me thou see'st the twilight of such day

As after sunset fadeth in the west,

Which by and by black night doth take away,

Death's second self, that seals up all in rest.

In me thou see'st the glowing of such fire

That on the ashes of his youth doth lie,

As the death-bed whereon it must expire,

Consum'd with that which it was nourish'd by.

This thou perceiv'st, which makes thy love more strong,

To love that well which thou must leave ere long.

LLO thanks for the Shakespeare poem. It was a ray of the season. However, I do not believe the yellow tip on the afore mentioned leaves is just fall coloration, Fall coloration would color more than the tip and more leaves than just one.

Classic signs of salt build-up are yellowing leaf tips and a white chalky buildup on the soil surface and the interior of the pot rim. You would also have wilting leaves as the salt prevents the uptake of water. The leaves are not wilted, he does not have any white chalky buildup on the surface or sides of the pot. This is likely new potting soil as he grew from seed. Peat moss potting soils are designed to last 1-2 years. Highly unlikely that after 2 months the potting soil needs to be replaced. The seedlings are only 2 months old. He is using a starter fertilizer and only once a month which makes the probability of a salt build up all the more unlikely. He mentioned a negative Celsius, I don't think Toronto has had any hard frost yet so he probably lives further North. Days are 55F and nights are 45F in Toronto. Crappy weather for growing citrus. Given the available light, the leaves will most likely turn to yellow sooner than later.

LLO, you are correct about a white chalky buildup on the surface and on the pot due to a salt buildup.. This occurs ONLY when the salt buildup becomes highly EXTREME. Luckily, in Jack's case the buildup is in the early stage. If his tree was in the extreme stage of soluble salt buildup, the tree would be in a much worse condition.

Thanks so much for all the responses! As "Lime Orange Zone 6a" said, the seedlings are in newer soil. I will be sure to water more deeply though, I haven't been doing that.

The one seedling that has yellow has advanced very slightly since I posted, but seems to have stopped. The strange thing is it's just that one seedling. I have 4 in that room, which has the best light in my home. But I also have... 10 downstairs (I planted some seeds from a few lemons I had, every single one germinated and some gave me twins!). I've started taking them off the window sill overnight, so maybe that helped? I'm in central Alberta, it's currently -8C (18F). They also got fed a couple of days ago.

I have been wondering if I should get them a supplemental light source for the winter. It's very bright and sunny in the winter where I am, but I know we don't get a lot of useable light for plants in the winter. Has anyone gotten a light for their citrus? Is there a recommended amount or type?

Thank you again!

A few things to consider: Not all seeds will show the same vigor in growth and form. Some will be stronger and some others will be weaker. Second, next to a window it is always colder compared to the rest of the room. Nothing much can be done about it except that you can use heat mats. If you do use heat mats then I suggest using a pebble tray with water to increase the humidity around the plant. Put the pebble tray over the at mat and plant over the pebbles making sure the pot does not sit in direct contact with water. lastly 311 fertilizer once a month during growing season is too little fertilizer. How much of it do you use per gallon/liter of water? In winter they will slow down and need less though.

Regarding lights you will get a wide range of suggestions. Since your seedlings are small I suggest to use a 2 tube good quality led light on a timer. Get something that has color temp of 5000K or higher. Hang the light about 6-8 inches above plant. Rotate the plants every few days so that all sides get light evenly from Sun.

I will look for a light. It's still just the one little seedling that has yellow tips. They're starting to go brown now. I've moved them all off the window sill and have given them a spot with a little bit of warmth and some air flow. Hopefully whatever is bothering that one seedling doesn't affect the other three. I'm wondering if I should cut off its affected leaves?

I wouldn't cut of any branches.

Just want to say, they look beautiful and pristine to me! Congrats on getting so many to germinate!