Trees require plenty of nutrients in the soil in order to thrive. The primary nutrients trees need are nitrogen, phosphorous and potassium. There are a variety of other nutrients tree also need but these are usually available in large enough quantities in the soil without additional fertilizer. Over-fertilization is just as much a concern as too little fertilizer, so understanding when to add additional feeds is just as important to tree health as using the proper type.
Perform a soil test at planting and every three to five years to see if fertilization is necessary. Soil test kits and instructions are supplied by county extension offices upon request or for a small fee. Alternately, fertilize when new shoots grow less than 6 inches or yellowing leaves appear in spring and summer.
Use a nitrogen-rich fertilizer unless otherwise indicated by a soil test. Purchase a fertilizer that has no added herbicides, as these damage trees.
Fertilize young trees at planting with a quick-release fertilizer, applying .1 pound of nitrogen per 100 square feet. Apply .4 pounds of nitrogen per 100 square feet in quick-release form for trees less than three years old that you want to encourage growth. Use .1 pounds of nitrogen fertilizer for trees planted into lawns or those you wish to slow the growth. Apply .10 pounds of nitrogen per 100 square feet for established, mature trees.
Spread the fertilizer around the tree in a circle 12 feet away from the trunk. Water immediately after fertilizing so the fertilizer nutrients leach into the soil where the tree roots are.
Fertilize mature trees growing in the middle of lawns in spring and again in fall. Apply .1 pounds of nitrogen per 100 square feet in a circle around the tree. This prevents excessive fertilizing of the lawn in the area, which otherwise may kill the grass.