Oak trees can benefit from annual applications of complete fertilizers after they reach two years old. Summer is a good time to feed your oaks, provided ample irrigation is applied both before and after the fertilizer. Either a slow-release or quick-release fertilizer can be used as long as the maximum application rate on the quick-release formula is never exceeded; this could burn the roots.
Select a complete and balanced fertilizer formula that has roughly equal parts nitrogen, phosphorous and potassium with a guaranteed analysis of 10-10-10 or 20-20-20. If possible, opt for a fertilizer with micro or trace-nutrients like iron and zinc to add to the soil fertility.
Water your oak tree deeply until the soil is drenched, a few hours or a few minutes before applying the fertilizer product.
Cast the fertilizer around the root mass of the tree starting two feet out from the trunk and extending at least a foot past the drip line of the canopy in a wide doughnut shape. Use the recommended dose indicated on the product label for the size of tree and square footage of soil.
Scratch the fertilizer into the top two inches of soil and water until fully wet and beginning to dissolve into the soil. Keep the soil moist with deep watering at least once every 10 to 14 days during the summer to get the fertilizer nutrients down to the roots.