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Tomatoes plants yellowing

What could be the cause of my friends tomato plants yellowing? There are several varieties, gardeners delight, princess of the night, bloody butcher - I gave her some of my excess seedlings. 

The plants seem stunted generally, the leaves all smaller than on my plants - in both cases, the plants are in grow bags placed on a capillary box, and have been watered and fed regularly. On my friends plants the leaves are pale green, but definitely more of a yellow colour - this is pretty universal for the whole of the plant.
My friends greenhouse gets a little less sun than mine, but doesn't have any shades. The doors have been open throughout the day, and there are automatic vents.
There is no sign of blossom end rot or leaf curling.
There isn't a thermometer in my friends greenhouse, and while our summer temperatures this year haven't reached the eye-watering temperatures experienced in some parts of Europe, it has been a sunny and hot summer. 


  • Pete.8Pete.8 Posts: 10,967
    Possibly too much water?
    If the compost and roots remain wet all or most of the time due to the capillary box, the plants will not be able to absorb sufficient nitrogen. This will cause pale or yellowing leaves. Roots need air around them every bit as much as they need water.
    There could be other reasons.
    A photo may help us give more suggestions.

    Billericay - Essex

    Knowledge is knowing that a tomato is a fruit.
    Wisdom is not putting it in a fruit salad.
  • Danish PastryDanish Pastry Posts: 34
    edited September 2022

    I don't find the soil is waterlogged with my capillary boxes (when I stick my finger into the soil it's slightly moist rather than wet), the plants just suck up water as and when they need it - mine are fine, much bigger leaves and plants than my friends and beautifully green and a reasonable crop. 
    I wonder if her greenhouse has been too warm as she has no shades, but I've also seen images of iron deficiency that look similar. Although with regular feeding iron deficiency shouldn't be an issue. 
  • Pete.8Pete.8 Posts: 10,967
    I agree that if they've been fed then deficiency shouldn't be an issue.
    Your friend could try seaweed extract - it contains lots of micronutrients that may not be available.
    Tomatoes will stop growing when temps are above 30c, but that shouldn't cause yellowing, which is caused by a lack of nitrogen where it's needed.
    Assuming it's not a lack of nitrogen in the compost then the usual reason is too much water preventing uptake, but if that's not the case in this example, then I don't know.

    Billericay - Essex

    Knowledge is knowing that a tomato is a fruit.
    Wisdom is not putting it in a fruit salad.
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