I looked up Vigoro 6-4-6 to see if that formulation also contained the required trace minerals need by the tree. However, only the front of he bag is shown, the total formulation must be on the back of the bag. Anyway, a fertilizer having 6% nitrogen, as Vigoro 6-4-6 does, should be applied 5 times a year for a citrus tree 2 years of age. Always be sure not to place any fertilizer against the tree's trunk. lastly. the basin that you cut out of the grass is quite small for a citrus tree's root system. If you place fertilizer on the grass to feed the tree's roots as they expand outward, the grass will use up most of the nutrition. Therefore, the tree will not receive the nutrition it requires. Citrus are heavy feeders, requiring more nutrition than most plants.