I planted 4 seedlings in one pot and 2 in another, all from the same punnet. The pot with 4 plants has many yellow leaves but the pot with 2 plants has none and looks healthy. They both get the same treatment - water, thrive etc, and they are side by side. They are both in full sun (Port Lincoln). The 2 plants are not as tall as the 4 plants? I'm confused.
Yellow Leaves on Some Tomato Plants
Certified GKH Gardening Expert
There are many reasons that this can happen. With seedlings, it is likely either a water issue, a nutrient issue or a temperature issue. Watering is normally the big one. Make sure that the soil is not too wet or too dry in the pot with the seedlings with yellow leaves.
Since the pot has 4, they may have used up the nutrients in the soil too. Giving them a bit of fertilizer may help.
There may also be a draft coming down on the yellow seedlings, especially if you have them in the window. Check and make sure that they are not getting any cold air on them from some source like a drafty window or a door that opens and closes.