Should I Fertilize My Trees?

Urban trees grow in an environment often quite different from their rural cousins. They are generally growing in places where they have not evolved to adapt. Roots grow under pavement. Road salts damage leaves and root systems. Vandals tear off branches and uproot entire trees. Construction compacts soil and tears roots.

Because of the environment in which they grow, urban trees often need our help. Their genome cannot anticipate all that people and cities will throw at them. The entire field of arboriculture is based off the fact that trees are growing in a human-impacted, not-natural-to-the-tree environment. However, the type of help they receive should be based on the needs of the tree, given its species and actual site conditions. Fertilization is often prescribed by tree care companies as one such remedy to improve tree health. However, such a prescription is often unwarranted.

Fertilization and other soil remedies should never be prescribed without diagnosis. Doing so is a waste of money and can lead to environmental problems such as elevated nutrient levels in waterways. Fertilization is useful in some cases, however. But there is only one reason why your trees should ever be fertilized –

Trees should only be fertilized because of a documented soil nutrient-tree need mismatch, which can be improved through the addition of deficient nutrients.

Chlorosis in red maple is often indicative of a soil nutrient deficiency for the species. However, a soil test is needed to determine if fertilization will aid the tree or if pH is the real culprit

​​Unfortunately, many tree care companies prescribe fertilization without so much as doing a soil test. There is a simple, likely-obvious reason why they do this – it makes them a LOT of money. Never agree to have your trees fertilized if your tree care professional has not performed a soil test. If an arborist even suggests fertilizing trees or other soil amendments without a soil test, look elsewhere for help with your trees.

There are many other reasons why an arborist may suggest fertilizing trees. Some reasons that are insufficient for prescribing fertilizer include:

1.Trees are newly planted 
You should never fertilize trees simply because they are newly planted. Some studies show that fertilizing newly planted trees can impede root growth, and lead to unhealthy top growth.

2.Tree roots have been damaged 
A common reason for fertilizing trees is to help improve tree condition after root damage. However, the only way to help recover from root damage is to improve root growth. Fertilization does not aid in root growth, but can often impede such recovery.  

3.Soil pH issues
If the trees on your property need nutrients in greater amounts. In this case, soil pH remedies would be a better option. However, the efficacy of such treatments is questionable at best.
 
As is the case with many aspects of arboriculture, the best answer to the problem is to plant the right tree in the right place. A tree species adapted to the soil conditions present on your property will often eliminate the need to fertilize at any point in the life of the tree. It can also eliminate the need to use other nutrient deficiency correcting solutions as well. Again, if your arborist or tree care professional prescribes fertilization or other soil remedies without a soil test, look elsewhere for help with your trees.