Fruit drop can be linked to many different cause.

Citrus normally shed large numbers of fruitlets shortly after blossoming in the spring and at early fruit set (pea size). However, it is also common for a sudden drop of small fruit (20 mm diameter) to take place in the summer, when warmer weather places stress on the tree. The problem is particularly severe in young navel orange trees and may be related to lack of water at, or soon after, fruit set. Do you think any of this applies to you?

Also diseases and lack of nitrogen or trace elements may also be responsible.
Root rot can also be another stress. It can start with loss of little fruit then move onto leaves if not caught in time. First it start with the very fine roots then moves on. Also a mix can compact with time. Do you over water? Is the mix drying out in a rapid amount of time, or at least moist when it rains that much? How long has it been since you provided a full repot with a fresh porous mix?
Does this apply to you?

Also, fruit formation cycle when the fruit is less than about 1 inch in diameter can be an issue. It is common for a tree to drop some of its fruit during the growing season as part of the natural thinning process. If more fruit drop than normal is identified, this is often a sign that the tree is under stress, which could be caused by a late frost, a lack of water (or even too much water) or an inadequate supply of soil nutrient fertilizers. Do you fertilize on a regular basis with all the basic nutrients that citrus need? Do you flush your mix out regularly with fresh water and then fertilize again? Does this apply to you?

To help remedy excessive fruit drop or the undersized growth of the fruit ensure that the tree is properly fertilized every year early in the spring growing season and until fall, with a fertilizer specially formulated for citrus trees.

Many here can suggest great fertilizers that work well for them. For me Foliage Pro has been a G*d send.

Of course there could be other reasons, but this is a good head start for you to think about and work with

Mike

I had a feeling it compacted. That is a sure sign of decline on just about any tree and sometimes it can take you by surprise and happen over time.
I would also look for pests since almost always the two are very much related.

If you want to go the easy organic route in lowering the pH solution you provide your trees, try 1 capful of vinegar per gallon of water. That should do the trick.

As for fertilizer, there are many recomendations on this, but I say to find one that closely relates to a plants need. I use a liquid fertilizer with micronutrients in a 12:4:8 NPK ratio, which MG does have. It's in a yellow bottle. Foliage Pro is a great choice too which is what I use.

You can either purchase a very porous mix, amend one, or make one. I make most of my own.
I use 1 part peat or Turface, 1 part perlite, and 5 parts of bark.
You can also purchase a good heavier mix which is primarily bark such as Fafard Mixes.

Any of these options or others will dramatically improve your growing skills and the vitality of your tree.
A mix that allows a frequent oxygen exchange is a must and this happens with a very porous mix.
Whenever roots are deprived of oxygen (O2) they soon begin to die, so a mix that will compact rapidly should be avoided.

Mike

I would definitely recommend that you use Foliage-Pro as Mike suggests. Ever since I followed Mike's suggestion to apply Foliage-Pro with white vinegar to my citrus, they have looked great and produced fruit. If you can't find Foliage-Pro at a local garden center, you can order it online, which is what I did.

Good luck!

Jerry

I will sometimes use 1/4 strength at every watering per gallon depending on my situation.
Or sometimes I will use 1Tbs per gallon once weekly.

The vinegar is always the same. 1 bottle top capful of white vinegar per gallon at every watering when using tap water.

Mike:-)