Shade trees such as elm, liquid amber, maple, birch and box elder provide protection from the sun and are beautiful in the landscape. Trees also provide beneficial oxygen as part of the photosynthesis process. Shade trees need a regular schedule of fertilization to remain strong and vigorous. Soil testing, application timing and type of fertilizer contribute to a successful fertilizer program.
Trees need 18 nutrients to support healthy growth, according to Rutgers University Extension. A regular schedule of fertilizing shade trees encourages vigor and supports resistance to disease. A strong tree is able to repel harmful insects and disease. Nutrients from fertilizer also help trees tolerate environmental stressors such as drought and nitrogen overload in the atmosphere. The purpose of tree fertilization is to supply specific nutrients found to be deficient.
Soil testing is essential before applying fertilizer. Soil tests analyze soil pH and macronutrient content and provide recommendations for fertilizer application.. Alternative labs tests analyze soil for its overall biological activity and nutrient deficiencies. Healthy soil contains billions of microorganisms in each teaspoonful. The National Sustainable Agricultural Information Service provides a list of alternative soil testing labs (see Resources).
Tree roots that absorb fertilizer are located in the upper 18 inches soil under the tree. Feeder roots extend beyond the tree canopy. Fertilizer should be applied in a circle around the tree several feet beyond the canopy drip line. Grass growing under the tree competes for soil nutrients. Do not apply turf grass herbicides under the tree canopy, the toxic ingredients may be absorbed by tree roots.
Fertilizer applications made in early spring and late fall are most beneficial to tree nutrition. Fertilizer applied in late fall provides nutrients for vigorous spring leaf development. Avoid fertilizer applications during summer growth period because it disrupts the hardening off cycle of winter. Fertilize deciduous trees every other year and conifers yearly.Application methods include drill holes, fertilizer needles and surface broadcast.
"Selection of the best formulation should be directly related to the results of a soil test," according to Rutgers University Extension. Nitrogen is the nutrient most used by trees and plants and must be continuously replaced. Both synthetic and organic fertilizers have adequate nitrogen to support tree health. Organic fertilizer is derived from natural sources such as fish waste, seaweed, bone and blood meal, bat guano, cottonseed and animal manure; which provide all the essential 18 nutrients.