Also, to allow the tree to get enough of the water and fertilizer you give it, you need to take away all the grass growing around the tree in a much bigger circle than you have right now. A good citrus fertilizer should be applied 3 times per year, in spring, summer and fall. It would be sprinkled on the ground all around the tree to a width beyond what is known as the "drip line" which is where rain would fall if it dripped off the outermost leaves.
The roots of citrus are very near the surface of the soil, so the grass is right there amongst them. The roots extend out further than the reach of the branches, so you'd need to clear a circle at least a meter from the trunk of the tree all around, and enlarge it every year as the tree grows.
Be very careful when removing the grass too, so that you don't damage the feeder roots of the tree. In your hot climate it would be a good idea to add some sort of mulch, such as hay, straw or wood chips in a layer a couple of inches thick over the bare soil around the tree. This will help keep the soil cooler while the tree grows a thicker canopy, and will also help you keep the weeds and grass from growing back. But do not put the mulch right up against the base of the tree, this could cause a disease called "foot rot" or trunk rot. Keep it maybe 20cm away.