Apply or renew mulch over the root system of the citrus tree. One thing that citrus trees really hate is having to fight with grass growing right up against their trunks. Grass is so greedy it will gobble up any nutrients it can get at.
Clear grass away from the base by hand, or carefully spray with Zero. When all the weeds and grass have gone, give the roots a really good drink and then put a 10 centimetre or more thickness of organic mulch (old manure, grass clippings, compost) over the entire root area. Make sure that the mulch is not contacting the trunk (because this will encourage root and collar rots).