cordyline leaves have turned yellow

Could anyone help please. I have had 3 cordylines for about 3 years now,all side by side in containers in a very exposed seaside location. Each year the leaves on 2 of them have started to turn yellow and I've managed to deal with that by adding epsom salts (for the magnesium) to the soil. The same thing has happened this year but this time to the one that has never had any problem as well,thing is that the epsom salts don't seem to be working this time. I've read that on some plants yellowing leaves are a sign of lack of iron but all of the articles I've looked at about this are talking about ericacheous(sorry about the spelling!)or lime hating plants like hydrangeas and rhodedendrons.Does anyone know if it would do any harm to my cordylines to try some iron -I don't want them to shrivel up and die!Any input would be greatly appreciated, thanks.

 

They could be getting a bit big for their containers barney, starvation is a likely problem after 3 yrs in the same containers.
Think I'd try a good dose of nitrogen

 

I'll try that , thanks Pete