Orange trees are a low maintenance plant that requires a steady amount of food to produce excellent fruit. Whether using chemical or organic fertilizers, orange trees must be carefully monitored and their feedings consistent with the growing seasons. Unless using organic fertilizers, never feed a tree during its first growing season, as it can severely damage the root system and stunt the tree's development.
Incorporate mycorrhizal fungi into the soil annually beginning two planting seasons before planting the orange tree. Mycorrhizal fungi attaches to a plant's root system and enables them to gain more nutrients from the soil. This fungi tends to be absent from most urban soils.
Fertilize annually with an equal amount of nitrogen, phosphorus and potassium. These nutrients can be found in a standard balanced, or 10-10-10 fertilizer. Slow releasing standard fertilizer is more beneficial for citrus as it ensures that the nitrogen does not wash below the root line. Standard fertilizer should be applied and watered in annually.
For those desiring an organic alternative, incorporate fish emulsion fertilizer into the soil bi-annually beginning one planting season before planting the orange tree. For best results, fertilize initially in the spring as the tree is budding, and again once its fruit has been harvested.
Supplement annually with additional nutrients including magnesium, boron, copper and zinc. These trace elements are found in turf fertilizers. Similar to the balanced fertilizer, these nutrients should be applied and watered in annually. Test the soil and if necessary, supplement annually with iron supplement tablets. If the tree's soil is above a pH of 7, then iron must be added to the soil.
Fish emulsion fertilizer already contains most necessary trace nutrients. If the soil's pH is above 7, dissolve 6 tbs. black strap molasses into 3 cups of hot water and water in well. In addition, mix 5 cups of hot water with 1 lb. of coffee grounds and let sit overnight. Pour onto the tree's root system annually at the start of spring for added soil acidity. For best results, consider applying at night and watering in well.