boosting a tired poplar?
 

What would be best to feed a variagated poplar, it's always done well, in
about 10yrs now and kept to a height of 8'. Has three to 250/300 daffs
around the base and is in the centre of a lawn. It's come into leaf but
somehow looks a bit tired. What can I feed it with thoughts to the daffs
round the base?

Thanks
Shan