Trees that grow in the urban environment can experience many environmental stresses, including compacted and poor soils and competition with turf for water and nutrients.
Soils, especially in newer housing developments, may be significantly deficient in the nutrients necessary for the optimum growth of trees. In construction and building processes, the native topsoil is often stripped away from the area during excavation for foundations and infrastructure, then the subsoil is distributed over the site, severely compacted by equipment, and then covered with a very thin layer of topsoil sufficient to seed and establish turf.
Trees planted in this kind of environment may never reach their potential for growth, and often struggle to survive. These areas are denied the natural cycling of nutrients back to the soil through the decay of leaves and natural debris, which we clean up and haul away every fall. The result is an environment that is less than optimal for tree growth.
While fertilization can’t address issues of soil compaction, it can help to promote the growth and vitality of your trees and alleviate nutrient deficiencies in the soil. Any fertilization program for your trees should begin with a soil test that will provide you with valuable information about pH and any of the major or minor elements that may be deficient in your soils.
The three nutrients that are most essential to the growth of trees are nitrogen, phosphorous, and potassium.
Each of these three elements are represented in that order by the three numbers that appear on any fertilizer bag. A fertilizer with an analysis of 10-10-10 has nitrogen, phosphorous and potassium each at 10 percent of the total volume of the bag. There are a number of micronutrients, such as iron, manganese, and copper that are used in very small amounts by trees, which when deficient can adversely affect tree growth. A comprehensive soil analysis will show the availability of both major and minor elements in your soil and will provide recommendations for addressing any deficiencies.
The typical fertilizer spread that may be applied to lawns is rarely enough to meet the requirements of trees, as the nutrients are quickly taken up by the turf and never reach the trees’ critical root zone. Additionally, products applied to lawns that contain both fertilizer and a broadleaf weed killer such as 2-4D or related chemicals can have an adverse effect on trees, so be certain to read and follow labels carefully when applying these products.
A proper fertilization program should be tailored to the findings of the soil analysis. Phosphorous-deficient soils are rare in Vermont, so it is unlikely that you will need to fertilize with a mix containing phosphorous. In fact, excess phosphorous is a major concern as it can be carried through runoff and leaching to nearby waterways.
Fertilizer is most effective when applied in spring, as trees begin their new growth flush and roots are most active in taking up water and nutrients. Granular fertilizer applied to the surface should cover the area out to the edge of the canopy of the tree and requires sufficient rain or irrigation to carry it into the soil making it available to tree roots.
Fertilizer can also be applied in liquid form and injected into the soil under low pressure by a qualified applicator with specialized equipment. Fertilizing by liquid injection allows the nutrients to be immediately available for uptake by the tree, and less of the available nutrients will be lost to competing turf.
Trees that get the nutrients they need to maintain growth and vigor are better equipped to withstand the numerous stresses that adversely affect our urban trees.
V.J. Comai is a certified arborist with the International Society of Arboriculture. He lives in Charlotte.