Mandarin in a pot losing leaves and looking sick

 I have had a mandarin tree in a pot for about 3 years and it has been great
but now has yellow leaves and looks very sick.

Any ideas?

 
Yellow leaves usually indicates a lack of nitrogen, the other possibility is
that it is root (pot) bound probably time for a root trim and repotting or a
good feed.

 Just repotted it.  You were absolutely right!

Thanks for your help.

Cheers

Sandy