By Cornishsally
Cornwall,
United Kingdom 
Next question !- magnesium deficiency in tomatoes. Potato blight is all around ( chopped mine down yesterday) but according to this month's RHS magazine the patches on my tom leaves are definitely magnesium deficiency. Does anyone know what causes this please? I know it's said that conventional tomato feeds make the problem worse but in fact I hadn't even started feeding when it appeared. The tomatoes are in 2 Levington large growbags, 3 to a bag and each in bottomless pots with the contents of a third bag split between the pots. Plenty of fruit coming.
5 Jul, 2012
Answers
Ah, I did wonder if I'd been overwatering a bit. Thanks very much Bamboo.
5 Jul, 2012
Well you're damned if you do and damned if you don't - water supply is critical when the fruits are swelling, it needs to be regular and sufficient.
5 Jul, 2012
You will always suffer far less from this kind of problem if you try to avoid grow bags and other restricted planting. Many of these 'deficiency' problems, like blossom end rot, are the side effect of irregular watering which prevents the roots from absorbing the necessary minerals. You really can't beat planting them in the soil and giving them a full root run, assuming you enrich the planting soil with lots of compost.
I know that's not easy unless you have a greenhouse, but it is possible to erect a very cheap polythene structure just for a few tomato plants that will protect them from blight and give you a plentiful crop.
5 Jul, 2012
I add slow release fertiliser to my grobags in the hope that it adds the trace elements.....what do you think Bamboo its osmocote sold by miracle grow
5 Jul, 2012
There are different formulations of Osmocote, but the basic one does contain a balance of nitrogen, phosphates and potassium - magnesium isn't mentioned, Pamg.
5 Jul, 2012
I agree Bertiefox but the toms got blight last year in the greenhouse soil! Unfortunately the greenhouse is on a shared patch of ground that we all use for vegetable , and in my case fruit, growing and my neighbours grow spuds right up to the greenhouse. You know how fast potato blight takes over and last year none of us knew to cut it down immediately, so I think the spores blew into the greenhouse. I am just keeping my fingers crossed that my tomatoes will be ok this year!
6 Jul, 2012
Thanks Bamboo, I,ll read the label!
6 Jul, 2012
Acidic soil/compost, lots of water (magnesium leaches out readily when the soil is wet) on their own can cause magnesium deficiency - the high potash content of tomato food can make the situation worse, because it prevents the plants from taking up any available magnesium.
Use Epsom Salts as a foliar spray for fastest results (200g in 10 litres of water, plus a few drops of mild washing up liquid), or apply to the compost at 40g per m2.
5 Jul, 2012