The reason Urea is used to supply nitrogen via foliar spays, is because the time for 50% leaf absorption of the chemical is only 1/2 - 2 hours.  Many chemicals take days or weeks.  The best time to apply foliar fertilizers is early morning, when the plant's stomates are open.
If temperatures are projected to reach approximately 85-F or higher during the day, applications should be made just after daybreak or after the temperature drops in the evening and the relative humidity increases.  Winter applications should be made at midday, when temperatures are suitable warm.  Avoid spraying to runoff.  Excessive liquid per tree causes pooling of the fertilizer at the tip of the leaf, which can result in tip burn.  Nutrient uptake is best when the application solution is at a pH from 5.5 to 6.6 .  Last note: For most cation nutrients (magnesium, potassium, zinc manganese and iron), application as a sulfate salt is equally effective as chelated products and less expensive.

https://citrusvariety.ucr.edu/citrus/acidless.html

All branches turn down sooner or later.

Many of my trees are grafted upon Flying Dragon (FD).  The oldest one is a 25 year old Cara Cara.  Presently the FD root stock is approximately 1.5 inches wider than the scion all the way around, The tree is now 11-feet tall and 12-feet around, healthy and has been from the original planting.   I have always been a big FD root stock fan.  FD has a long history of producing excellent tasting full size fruit.

Some members might not know the reasons to apply the first spray , called the winter spray, of low biuret urea.  This first spray is designed to increase flower number and yield of commercially valuable sized fruit WITHOUT reducing fruit size  Later applications are better than those that are too early.  This is because once this opportunity to increase flower number has passed, late applications of low biuret urea will still increase the retention of abscising reproductive organs. Citrus Production Manual

I think it was lost along with other posts by a hack on this site.

Vlad, that is why the tree is sprayed 2 months BEFORE expected bloom date (before the new flush of leaves)

The leaf deficiency symptom shown is an iron deficiency (leaves with green veins on otherwise yellow leaf)  An iron deficiency in citrus can be caused by either a lack of iron in the plant, or by high PH rather then a shortage of nutrient.

1.5 to 2 months before expected bloom date is the time to apply the first application of low biuret urea. 

Poncirusguy, as your tree is growing in a protected area, (an artificial 12 month summer), you can fertilize any time.

Yes it is OK to fertilize while fruit is still on the tree. Fertilizing can be done with fruit or no fruit on the tree.  However, in your location, I would hold off on fertilizing until Feb. 1st.  It is important to understand that warm weather temperatures, not irrigation of fertilization, play the  MAJOR role in stimulating untimely winter flushes of large mature trees. However, when unseasonably warm temperatures occur, young tees may flush more easily than larger, mature trees, especially trees planted the previous year. If such young trees flush  during winter months, fertilization may simulate additional tender growth, making trees even more susceptible.  Consider the degree of flush, tree size, time of year and geographic location, when making this decision.(Florida dooryard citrus guide).

Citrange and Pager, thanks for both of your great articles.  Much appreciated.

The deficiency is a nitrogen deficiency..  The symptoms for a nitrogen deficiency in citrus are:  Totally yellow leaves with no variation of color, or yellow/orange veins with some green to the sides.  Note: 1 inch of irrigation either from rain or hose can leach nitrogen 1 to 2 feet down in sandy soil.  A combination of low nitrogen and cool weather could also be involved.