Tree fertilization can be a major revenue stream for tree service providers, but in order to be held in high respect in the local community, as well as arboricultural communities, it needs to be provided in a respectable and judicious manner. Many factors are involved in determining fertilizer need.

It depends

When my daughter was a college senior, she and her cohorts were of like mind in their perspective: some people were out to take advantage of others, some were victims, and still others just were unlucky and the circumstances of life caused them to suffer unjustly. Many of them went off to law school with this mindset and began their studies with a black and white outlook.

As time went on, they were exposed to various sets of curriculum and clinical experiences. Slowly and surely, their perspectives changed. As they neared graduation, the black and white outlook softened to charcoal and light gray, and then eventually to a mix of hues. Now in their early years of practicing law, the answer to just about any question of negligence, guilt, innocence or damages is “it depends.” The same is true for the question of whether or not trees need fertilizer – it depends.

Location of tree

Tree fertilization is just not as simple as your client might think at first glance. It’s quite common for clients to call and request a fertilizer application because their neighbor just had one or because “the tree doesn’t look quite right” or because they saw an advertisement in the newspaper. These are all good reasons for them to raise the issue, but not a slam dunk to have it done.

Perhaps the most important consideration in determining the need for fertilizer is the location of the tree on a given property. Regardless whether a tree is in the middle of a yard of turf, not surrounded by turf at all, surrounded by perennial flowers and ground covers, in a well-watered landscape, in a nonirrigated landscape, in an urban setting surrounded by concrete, near a shopping mall parking lot surrounded by asphalt or in the middle of a forest, specific maintenance procedures should be followed for each setting.

On each site, localized conditions are responsible for overall tree health, including nutrition. The conditions are both existing and ongoing. Where trees and turf are co-located, the turf maintenance has a significant impact on the trees, as the roots are comingled. Removal of grass clippings tends to increase the need for tree fertilization, while clipping return decreases it. Due to the arrangement of tree and turf roots, grass roots get the first shot at soil-applied nutrients. When turf is fertilized, a certain volume of elements is absorbed by the turf and some by the tree.

Another part of the comingled factor is density of turf. Root competition from thin turf is much easier on trees than root competition from thick turf. Thin turf contains fewer roots, as well as more open spaces in the soil profile for applied elements to travel downward. Additionally, when turf roots are heavily fertilized on a frequent basis, a greater volume tends to be available for uptake by trees.

Yellowing leaves may be an indicator of the need for added nutrients. Photo: John Fech

Natural forest trees

Where do trees tend to be the healthiest? Most foresters agree that trees grow best in naturally occurring forests, where the forest floor is covered with a combination of tree leaves, seeds, flowers, bark, decaying logs and the like. Mycorrhizal fungi, which aid in the absorption of nutrients, are also likely to be present. As they decompose, these fallen tree parts recycle nutrients back to the soil to replenish nutrients removed by vigorously growing trees. In these scenarios, tree roots grow extensively, provide support for other trees and easily uptake necessary nutrients. Unfortunately, most clients do not have a natural forest for a backyard; as such they need well-trained tree service providers to determine if fertilizer is needed, how much to apply, and when to apply it.

Symptoms and indicators

In addition to a customer drawing your attention to a particular tree that may be in need of fertilization, there are several other indicators. Generally, most trees are supposed to be green in color, at least during the bulk of the growing season. Deviations from that norm, either a general yellowing of the leaves or needles or a condition where parts of the leaf stay green and others turn yellow, are symptoms that indicate that a tree may be in need. A second indicator is abnormal or stunted growth. If the new growth doesn’t match the expected orientation or arrangement of others of the same species, this may be an indicator that a need exists. A third is the reduced amount of shoot growth produced in the current and/or previous years. This symptom can be observed by finding the bud scars on the branches and noting the amount of growth produced in recent years and making comparisons between them.

Soil testing

Testing is necessary to gain insight on the health of the soil. Begin by selecting a soil testing lab with experience in making recommendations for tree health, not just for crops. When you suspect a tree may need some added nutrients, take several soil cores from under the drip line of the tree, several between the drip line and the trunk, and several in the zone extending outward from the drip line.

Next, remove all turf and thatch from the top of the core as well as any gravel or other extraneous material. Mix the cores together in a plastic bucket, using only soil in the 4 to 24-inch level for testing. Remove tree root pieces that may have found their way into the sample.


When the topic of fertilization is brought up, most of us think about nitrogen (N), phosphorous (P) and potassium (K). In many situations, the NPK is sufficient, but the soil test shows that micronutrients are lacking or the pH is too low or too high for the species. If symptoms of a possible lack of adequate soil nutrients are present, and products containing only NPK are applied, it is doubtful that tree health will improve. On the other hand, nitrogen is needed by both turf and trees in greater quantities than any other element. With all of these foundational factors in mind, it’s all the more important that a tree care provider fully consider the soil test report, the site conditions and the current maintenance regime when determining whether or not to fertilize trees.

Fertilizer won’t help solve the problem of girdled roots. Photo: John Fech

Not a cure-all

Though many situations exist where added nutrients will improve tree health in a landscape, fertilizer is not a cure-all for all tree ailments. Fertilizer will only bring relief when there’s a lack of nutrients. Poor performance due to girdled roots, sunscald, deep planting, compacted soils, rotting roots, soil being placed over the roots, etc., are significant, but fertilizer is not the appropriate treatment. In many cases, applying fertilizer products in an attempt to treat problems such as these will decrease tree health rather than increase it, due to the stimulus effect of the fertilizer to use stored carbohydrates to produce growth.

As such, fertilizer is not a good option to try when it’s difficult to determine what’s ailing a particular tree. Comments such as, “Well, we don’t know what’s wrong, but let’s try fertilizing it and maybe that will help,” are poor examples of good tree care, if not fraudulent.

One size does not fit all

Hopefully at this point in this article the message has been delivered that every tree is a little different in terms of its need for added nutrients. Certainly, a “one-size-fits-all” program, where all customers receive the same fertilizer treatments on a predetermined schedule, is inappropriate.

So, what to do? To the largest extent feasible, treat each tree as an individual. At the very least, use the following approach.

1.Routinely inspect each tree on the property. Make notes about the various aspects of tree health, including observed pests, previous pest symptoms, factors that contribute to tree stress, as well as aboveground nutrient deficiency symptoms.

2.Soil test as described above.

3.Apply light amounts of elements that are noted to be lacking in the soil test report; incorporate compost if organic matter is lacking; aerate soil to increase oxygen percentage and make other appropriate treatments, which might include changes in the landscape to separate trees from turf, reduced fertilizer or irrigation to nearby turf.

4. Retest the soil.

5. Fine-tune a second round of treatments based on the soil test report and visual observations of tree leaf color and amount of shoot growth produced in a growing season.

Consideration of all the pertinent factors that relate to fertilization in the overall context of tree health is important.