Fertility Management for Landscape Trees

We have recently seen an increase in homeowners and tree care services interested in testing soil and leaf tissue for better nutrient management for ornamental landscape trees. While these services can certainly improve performance and appearance, fertility management for trees and other deep-rooted plants can be challenging. Please keep in mind that these comments are not meant to replace the advice of a reputable arborist. When establishing new plantings, it is best to select species that are well suited to the climate and soil types in your area.

The proper method for collecting soil samples for trees is not well defined. Some may argue that the sample should be collected to the rooting depth of the tree. Since most trees can root 10 feet deep or more, this is not very practical. A more appropriate method would be to collect the samples to a depth that can be impacted by a fertilizer application. This would be a 6-12-inch-deep sample in most situations. Deeper samples may be helpful to better understand the pH further down in the root zone, but incorporating fertilizer that deep may not be possible. Keep in mind that a tree’s root system has access to much more soil than a typical soil sample represents. So, ideal fertility levels are relatively low. Trees in a well-maintained landscape or lawn setting often do not need additional fertilizer than what is already being supplied around them.

Leaf tissue testing can be an excellent tool to help determine what nutrients may be lacking in a tree. Since soil testing has limitations, the leaf tissue can help fill in the holes. Deciduous landscape trees in the Midwest often struggle with micronutrient availability due to the alkaline subsoils that sit on top of limestone bedrock. Iron and manganese deficiencies are very common. The symptoms of micronutrient deficiency are generally chlorosis of the newer leaves. A challenge in diagnosing a nutrient deficiency visually is that damage from pests and pathogens can look very similar. One way to potentially differentiate a nutrient deficiency from disease is to look for a pattern. A nutrient deficiency is more likely to follow a distinct pattern from bottom to top or top to bottom and on the individual leaves themselves from the outer margins working in or the middle working out. Disease damage tends to be more random.

Unfortunately, one situation where soil and leaf testing does not help much is when an old, well-established tree begins to show decline. Once this has begun, fertilizer is not likely to correct the issue because it is often the result of changes in the surrounding environment.