Why my kumquot tree leaves turning yellow?

Question

My young about 1 meter tall kumquot tree in the ground is now bearing ripen bright yellow fruits in the middle of Sydney winter but the leaves are turning yellow even I keep the soil moist and feeding it with fertilizers.

However the leaves are not falling and the tree just looking unhealthy.

Answer 1

Yellow leaves can be caused by many things including lack of nitrogen, insufficient light, water-logged soil (plant roots need oxygen to thrive), dry soil, or iron deficiency. ... If new leaves are yellow with green veins, the problem may be a lack of iron.
https://www.gardeningknowhow.com/edible/fruits/kumquat/growing-kumquat-trees.htm