The oranges on my tree are sour. How can I help them turn sweet?

See if these help. Having the same problem with the neighbors tree that I take care of.

http://homeguides.sfgate.com/oranges-grow-sour-64213.html

https://www.hunker.com/12400226/how-to-help-an-orange-tree-produce-sweet-oranges

https://www.gardeningknowhow.com/edible/fruits/oranges/oranges-taste-sour.htm

https://www.gardeningknowhow.com/edible/fruits/oranges/oranges-taste-sour.htm

Most orange trees, other than Valencia & particular juice oranges, are "grafted trees" onto a sour orange root stock. This would include the loved Navel oranges. Your tree may be more of an ornamental orange rather than an eating orange - check your tag that was on your tree when you bought it if you still have it. You can do a few things to help make your oranges a little sweeter but how successful that may or may not be, I don't know. Oranges & other citrus produce we used to care for were tree ripened.

Those big Navel orange groves you see like in Arizona for example, are grafted trees that were grown from very small trees on sour orange root stock. If you are lucky enough to find Navel orange trees at Nurseries, they will be grafted trees. Ever notice & wonder "how" those Navel oranges are grown &/or started without seeds?? That's how. A Navel orange cannot be grown from a seed.

Oranges aren't naturally sweet until ripened on the tree. Store oranges are usually picked while still green or are only starting to ripen & are sprayed while in warehouse under processing to make them artificially ripen. Then they are sent to stores where you purchase them.

A few companies offer "tree ripened" oranges & those will be naturally sweeter. An orange doesn't become sweet on the tree until it gets to a certain growth stage when it stops growing in size & begins to naturally form sugar. How do I know this?

My hubby & I used to work on an 80 acre citrus farm where he was the farm manager. He & I were both trained to "graft" tree starts for Navels, Blood Grapefruits, & Mineolas on sour orange root stock trees. Once the graft takes hold & begins to bud out branches & leaves, which shows graft has taken hold, the sour orange trees above the graft are cut from the trunk of the tree & cut is sealed, thus leaving the graft to continue growing & becoming a productive tree.

I can't even begin to tell you how many trees we grafted during that tenure.

It's possible that the type of orange tree you have is a sour orange tree.

It also may be the soil. Oranges need just the right amount of nitrogen throughout the growing season to produce sweet fruit. Fertilizers should not be added until the tree begins to grow. Also, too much fertilizer can produce leggy growth and a reduction of fruit.

I am from Florida and I know that the oranges need one good frost to sweeten them up. Makes them sooo good!