Why are my lemon tree's leaves turning yellow and falling off rapidly?

I have three citrus trees - Lemon, Lime and Orange. Currently, the leaves of the lemon tree are turning yellow and falling off at an alarming rate. The other two trees are not showing such symptoms. I have tried using tomato feed, as I am struggling to find citrus feed locally. Any ideas on how to solve this issue?

There can be various reasons why the leaves of citrus trees turn yellow and fall off. It can be due to overwatering or underwatering, excessive fertilization, pest attack, especially scale insects, or chilling at night. Citrus trees need to be kept in frost-free areas with good bright light, the soil surface should be slightly dry from October to April and nicely moist during other times of the year. Feeding should be done only from April to September using citrus feed or acid-lovers feed such as Miracid. Note that lime causes iron deficiency and yellowing of young leaves.