[Jay The Digger]

My tomatoes are around 3 feet tall and have several flower trusses on them, but the bottom leaves have gone yellow.  They are not drooping, but look like they are going short of something.  They are planted in growbags in a poly tunnel and are not over watered.

I am feeding them every week, so am not sure if they are not being fed enough, or not getting something they should.  After looking at the label of the food, it only has 0.02% magnesium !!!   Would this cheap food be the problem?   I am going to try epsom salts to see if this rectifies it.

Any other suggestions?

[Ivor Backache]

My indoor tomatoes are showing a third flower, but none of them have set. I have not fed them at all. When the first truss has set then I will use a high potash feed. 
You haven't said what the feed is but you refer to the label so it must be commercial. magnesium is a trace element so 0.02% is right.
The problem is in the bottom leaves and when the tomatoes develop you will cut them off to allow the sunlight to ripen the crop. Its the top leaves of the plant that pull up the nutrients, rather than the bottom leaves pushing it up the plant.
I think your problem could be overfeeding.

[harrymac]

It could be nitrogen defiency - leaves will turn pale green then yellow, starting from the lower leaves.

Magnesium is essential for chlorophyl formation. A deficiency will cause the leaves to go yellow, but starting between the veins in the leaves, with the veins remaining green for a time. As with nitrogen the leaves will yellow from the bottom up.

A picture would be useful.

[Jay The Digger]

Sounds like nitrogen then.  The whole leaf turned yellow rather than leaving the veins green.  The rest of the plant is healthy.

Do tomatoes lose leaves in this way as a matter of course? 

I've been told to pinch out the shoots between leaves but these produce flowers if left, so am leaving them.  Am I asking too much from one plant (no matter how well fed & watered?)  (Sorry I should have said, these are Bambino cherry tomato plants.  My other crop of St Pierre are doing fine.  Planted and fed in the same way)

It's all getting quite baffling :->

[harrymac]

Most tomatoes are what are known as indeterminate (or cordon) types, and are trained as a singe stem, removing all side shoots. It's true that if you leave the side shoots, eventually further trusses will appear. However the plant spends so much energy in producing extra greenery in the meanwhile, the fruits will suffer.

Maybe that's what your problem is - the plants are growing too fast for the nutrients availabe. Nitrogen deficiency can be the first sign of a general nutrient defiency.

[peterjf]

tomatoes tend to drop a few leaves from the bottom of the plant,

take the yellowing leaves off , then feed with a little mag ,

or even better feed with a little comfrey juice if you can