Late fall is the best time to apply fertilizer to trees and shrubs. Most will only require nitrogen. A general rule of thumb is to apply 2-3 pounds of nitrogen for every 1,000 square feet of root area. Thatís not 2-3 pounds of fertilizer, itís 2-3 pounds of nitrogen. For example, the analysis for ammonium nitrate is 33-0-0. The first number always represents the percentage of nitrogen in the fertilizer. So ammonium nitrate is 33% nitrogen. To apply 2-3 pounds of nitrogen, one would need to apply 6-9 pounds of ammonium nitrate. This can either be applied at one time or preferably be split into two or three applications by applying part of the fertilizer in October, some in November, and the remainder in December. How do you calculate the size of the plants root area? Consider that most woody plants will have a root area that extends 2-3 times the diameter of the treeís canopy or limbs. Measure from the trunk to the outer edge of the treeís canopy, multiply that number by 2 to get the radius of the trees root area. Then plug this number into the formula for calculating the are of a circle, A = pi times the square of the radius. This will give you the area of the root system and allow you to calculate how much fertilizer to apply.