Epsom Salts

Planted out my tomatoes last week and already they are showing yellowing on the older leaves.  I remember from last year that Epsom Salts did the trick on this but can't remember what dilution to use.  I bought the pack in the Chemist and not in the Garden Centre so there are no instructions for feeding tomato plants on the container

I added a tablespoon to a large can of water, raspberries got some last week. They ain't dead yet.