Have you add any slow release fertilizer to it at all? I think that would be the first place to start. Also has the area got waterlogged? being too wet at the roots is just as bad as being too dry.

Here in the desert, I would have diagnosed it as a salt build up in the soil. In the UK that is probably rare, except near the sea, or where rock salt (sodium chloride) has been used as a de-icer, instead of road salt (calcium chloride).