The more sun, the sweeter your fruit will be
First, fruit trees need a minimum of at least half a day of sunlight each day during the growing season. The more sun, the sweeter your fruit will be. If your trees are in a bad location, consider transplanting when they are dormant in winter.
That said, many times the fruits on trees are not as sweet as they could be, because the soil they are planted in is lacking phosphorus and potash. It is important to apply a 0-10-10 fertilizer to the soil around your trees in November and again in January to provide these vital nutrients.
During the growing season, feed once after the blooms are finished, again in June and one more time in September with a complete fruit tree food. Make sure not to overwater, either. Allow the soil to dry out a little between waterings.
Established fruit trees respond best to deep and infrequent watering