Fertilizing Shade Trees: the basics
[excerpts from Rutgers Cooperative Extension Fact Sheet 031, "How to Fertilize Shade Trees"]
March 2001

Fertilization is just one of many cultural practices that can maintain or encourage plant development and vigor, improving resistance to certain insect or disease attackers, and increasing tolerance of environmental stress. Eighteen soil elements have been identified as essential for woody plant growth. The importance of a complete soil test prior to any fertilizer application cannot be overemphasized. Without the proper sampling and analysis completed prior to treatment, there would be no definitive decision making process by which treatments would be deemed necessary. Nutrient management within IMP or Plant Health Care (PHC) management models takes into account current levels and plant growth demands, and must play a major role in fertilizer recommendations and subsequent applications.

Shade trees planted in home landscapes and lining streets have been introduced into artificial and highly competitive environments. Rooting space is severely limited by the completed and heavily engineered soils required for urban and suburban infrastructure. Grading and soil restructuring has removed vital nutrient reserves held in the humus or upper levels of the soil profile. Utility rights-of-way, line-of-sight, and other factors hamper proper crown develo9pment. When grown under these stressful environmental conditions, shade trees can succumb to damage from opportunistic pests and diseases. maintaining or increasing tree vigor through proper fertilization can play a vital role in the future success of our shade tree resource.

Research has shown that the majority of tree roots responsible for nutrient uptake and distribution are located within the top 18 inches of soil. Though these roots were previously thought to extend only to the drip line of the tree, they actually extend well beyond the edge of the canopy, depending on the tree species and growing conditions. This area should be the target area for fertilizer applications.

Early spring and late fall are considered the best times to apply fertilizer for shade trees. Most trees have their greatest ned for nutrients in the spring, so an application after leaf fall will provide the best results during the following year. the health and vigor of shade trees can also be improved with spring fertilizer applications. Avoid any applications that may stimulate growth too late in the growing season, as the tree will not harden off in time for the winter season. Deciduous trees can be fertilized once every year or two. Conifers may be fertilized in the early spring, the frequency of which dependent upon the vigor and desire growth. Several fertilizer formulations are available in today's market. Selection of the best formulation should be directly related to the results of a soil test. Fifty percent (50%) organic fertilizer with a 3-2-1 ratio of nitrogen, phosphorus, and potassium (N-P-K) is commonly used where K and P are not severely deficient.

For the individual property owner, a correctly completed soil test will show the soil type, nutrient status and current pH and desirable pH ranges for the tee question. Fertilization rates will be based on these results, which for most sade trees will range from 2 to 4 pounds of N per thousand square feet.

Steady supplies of balanced soil nutrients support healthier root structure and shoot development, consistent growth, and a higher tolerance of abnormal environmental conditions such as drought, disease infection, and pest infestations. There are several different methods available to apply fertilizer to shade trees. The method selected will depend on soil and growing conditions, time, fertilizer formulation, experience, and labor and equipment requirements.

According to the recent released Tree Fertilization Standards, ANSI A300-Part 2, "The reason for fertilization is to supply nutrients determined to be deficient to achieve a clearly defined objective. That objective should be accomplished in a manner most beneficial to the plant." Proper tree care, selection a nd maintenance based on this and other modern guidelines in the field of arboriculture will ensure the health and longevity of shade trees throughout our communities.