No one can answer whether you should re-pot the tree except you. All you need to do is slip the tree from the container and look at the root system.

As far as the amount of fertilizer, when Citrus Joe and I were at the University of California CCPP containerized screen house collection, the University fertilized their container trees at 300 PPM with EVERY watering. I "fertilize" my trees with every watering, due the the fact I used a slow release fertilizer. However, I also apply a simple N solution every now and then in addition, nitrogen is very quickly leached from containers.

Millet

Matt, another point you should be aware of is that the amount of slow release fertilizer (or any other granular fertilizer for that matter) to use on a container plant is dependent on the size of the container-- not the size of the tree. The product directions should provide that info.

Well,
we in Europe favour the once a week shedule on fertilizer questions. Means, in summer every week a fertilizer should be given to the tree.
Many people switch from mineral fertilizers to organic component, which a far from be good in how to give the plant the right dose.
So, organic sources must be incooperated in the compost which one uses for his plant, and 'refertilisation' has to be done on annual repotting shemes.
Additional nutrients can be supplies only by adding some mannure to the compost surface and leaching the nutrients into the pot, or by mixing mannure to the irrigation water, and irrigate with such a mix. Many of these organic sources have to be carefully diluted, to prevent root burn by high acid and salt content.

Mineral sources are compareable, because can cause severe root damage, if a to high dosage is used.
I use a salt compond mixture, named HaKaPhos.



Is shown on the label, about 3,5 oz up to 10 oz per 26 Gallons of water is the given rate for a tank mixture.
So for homeowners, this means only 0,1 oz for a Quarter-Gallon of water. Because of a better measurement the volume can be used, except the weight: 0,1 oz is a volume of 0,1 fl oz.

Experiences with the fertilizer are quite good. And the large bag lasts long....

That looks like a very good fertilizer with trace minerals. Is the +2 magnesium?

Yes, the +2 is Magnesium....

I like it, the fertilizer comes in large bags, as you can see. It is a salt, very good solube in water and leads to very good response in tree growth.
But: If the roots get cold, of the summer is a mess like this summer had been, you have to fertilizer carefully...