Over-Fertilizing Your Trees Can Harm Them

Well-meaning homeowners who want to stimulate growth or promote health in their landscape trees often feed them with fertilizers. Unfortunately, too much of a good thing can have the opposite effect and can actually harm your trees. In normal landscape soils, many trees require no feeding at all, and if you do feed them, it's critical that you use the right fertilizers in the right ratios. 

The Right Fertilizer With the Right NPK Ratio

Trees are usually grown for the appeal of their green foliage, so the best fertilizer is one with a relatively high ratio of nitrogen, which promotes green growth. Unless your soil is deficient in potassium or phosphorus (a soil test can tell you this), fertilizers for trees should have a high nitrogen number in the N-P-K designation. 

A good choice is a fertilizer with an N-P-K (nitrogen-potassium-phosphorus) ratio of 10-6-4, preferably in a slow-release formulation. Slow-release formulations are usually non-liquid products that use granules that are released gradually into the soil. 

Although balanced fertilizers, such as 10-10-10 products, can be helpful for many flower and vegetable gardens when used with discretion, such fertilizers can have a bad effect when applied to the soil beneath trees. Excessive amounts of these nutrients can create too much mineral salt in the soil, which will harm the beneficial soil microorganisms necessary to healthy trees. 

Stay at less than .20 pounds of nitrogen per 100 square feet of root zone application area, depending on tree species and size. Any time you exceed this recommendation, you will create a situation for on-site contamination or the potential for runoff pollution into lakes and streams. Extreme contamination of soil can harm the site for a very long time.

The Effects of Excessive Fertilization on Trees

You can actually kill a tree if you apply too much fertilizer. Applying high levels of quick- release nitrogen can burn the roots when applied to the soil and can burn the foliage when applied as a foliar spray or drench. And if the fertilizer contains too much potassium and phosphorus, it creates excessive soil salts that trees may be unable to tolerate. 

The most common ways to over-fertilize a tree include:

• Over-use of fertilizers that contain an equal ratio of all three essential nutrients (nitrogen, potassium, and phosphorus)
• Applying more fertilizer than the standard recommended application rate suggests
• Using fast-release rather than time-release fertilizers

Any or all of these mistakes will increase the chance of root damage to your tree. Too much fertilizer introduces toxic "salt" levels that not only harm the tree but also make the site unsuitable for future planting. 

Symptoms and Treatment for an Over-Fertilized Tree

Symptoms of a tree that has been over-fertilized include:

• A crust of fertilizer visible on soil surface beneath the tree drip zone (the area of the ground beneath the spread of the branches)
• Yellowing, wilting, and browning on the tree's foliage, starting at tree leaf tips and margins
• A tree that starts to drop leaves before dormancy begins. 

The tree may survive and the site can be much improved if you do a fairly simple, three-part treatment as quickly as possible:

1. Remove the dying or wilting leaves, if you have any, to reduce fertilizer remnants in the tree itself.
2. Water the fertilized area of the soil thoroughly to a "flushing" point. Copious supplies of water will be necessary to flush excess fertilizer from the soil. 
3. Cover the critical root zone with a natural plant-based mulch—preferably composted leaves and grass. 
4. Perform a second water flush over the composted mulch.