How can I make my Valencia Orange tree grow sweet oranges?

There are two possibilities here.

One is that yours may be a genetic variant that simply produces fruit lacking in normal sugar content. Not all trees are the same, just like people. Some are sweet, some are not.

The other is that maybe it isn't a Valencia.

I doubt if it's a fertilizer problem. I also doubt there is much you can do to improve this short of getting another tree

How To Grow Sweet Oranges

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ntroduction Healthy trees and a sweet, delicious harvest are the goal as you tend your oranges throughout the year. Instructions Difficulty: Moderately Easy Things You'll Need * Compost Makers * Fertilizers * Garden Hoses * Mulch * Orange Trees * Shovels Steps 1 Step One Purchase an orange variety adapted to your area. For example, Washington navels grow well in California, while Valencia oranges are adapted to Arizona, Texas and Florida, as well as California. 2 Step Two Plant the tree in a warm, sunny area where the soil drains well. 3 Step Three Mulch to conserve water. 4 Step Four Water the tree deeply once every 7 to 10 days in midsummer. Water less often if it rains or if the weather is cool. 5 Step Five Fertilize every four to six weeks from February to August. 6 Step Six Protect trees from frost if temperatures are forecast to drop below 28 degrees F. 7 Step Seven Harvest oranges when they taste sweet. Timing will vary by variety and growing area. Tips & Warnings * Most nurseries in citrus-growing regions offer a nice selection of container-grown orange trees. * You can grow citrus from seeds, but there's no guarantee that the seedling will be identical to its parent. A seedling can take from 2 to 15 years to bear fruit. * The more likely that winter temperatures will fall below 28 degrees F where you live, the less likely you'll harvest a quality crop.

Either the type of orange you have is naturally sour or there's something wrong with your soil.

For oranges to become sweet, they need at least 2 days of cold weather. I lived in Florida and that was how my orange trees sweetened. If it was ever too hot, they would be less sweet.

Another thing could be that the nutrients in your soil are gone and you need to go to the garden store to by something to replace it.