Citrus are Mediterranean plants. They like well drained soil and in the ground only need a good deep drink once a week. The soil should be slightly acidic. Citrus food contains sulfur as well as micronutrients the plant needs so you should have had enough nutrients if the tree was fed regularly at least every three months or so. The only thing I know of that can cause global yellowing is because the nutrients are not being taken up even if they are in the soil
1. Sour soil- pH out of range. If you have added compost to hold water it can make the pH less than optimum. High pH makes micronutrients like iron, zinc, magneseum, boron, and selenium less available and that will show up as deficiencies in the leaves.
2. The soil has compacted and the rootball is not evenly moist. Some of the roots in the center die and less nutrients get taken up by the plant. Leaves wilt and fall off.
3. The soil has too much water holding capacity. Compost based soil mix. It is fine if you don't have a lot of issues with monsoon rain and you don't water on a schedule but only when needed. Otherwise the soil stays too wet, fungal issues crop up and the roots start to rot. If it happens quickly the plant wilts and dies with soggy soil (phythoptora). If the roots have not gotten as far as rot, but is constantly moist, the air space is lacking an the roots become stressed which will show up as generally poor health on the tree with leaves yellowing and falling. The same thing happens when the tree does not get enough water, the leaves shrink, yellow and fall off. Trees will come back better from lack of water than when they get too much.
The other reason for citrus trees to be stressed is from virus.
Huanglongbin or citrus greening disease. It is a relatively new disease but it was spreading.
https://www.californiacitrusthreat.org/h ... eening.php
Tristeza virus is common worldwide. Hawaii has some of the most virulent variants of the disease and most of the trees have it. That is why most citrus trees grown in Hawaii are grafted onto resistant rootstock. The resistant rootstock will still show signs of the disease eventually and the tree will die but it will take about 20 years vs 3-5 years on susceptible citrus stock. One of the symptoms of tristeza is that the tree will actually become very productive and give a lot of fruit for some years and then the bark will start to peel off. Cutting off the dead branches help but if the bark starts to peel off the main trunk, the tree's days are numbered.
https://idtools.org/id/citrus/diseases/f ... e=Tristeza
Happy gardening in Hawaii. Gardens are where people grow.