The good and bad of fertilizing trees

In the not-too-distant past, the "chicken soup" theory of tree fertilization prevailed. It was viewed as a surefire treatment for every tree problem. Even if it didn't work, it couldn't hurt, right?

This notion has gradually given way to more contemporary ideas that reflect a better knowledge of tree growth and health.

The basics

First, let's review some basic fertilizer information. Fertilizer is not plant food -- it is a compound that supplements natural materials needed for plant growth. Whether this is beneficial depends on the circumstances.

Missouri's typically clay soils are usually rich in the minerals needed for growth. Unless soil tests indicate a deficit of certain nutrients such as phosphorus, potassium, or calcium, the most important ingredient is nitrogen, which is also the most limited in supply.

In nature, nitrogen is doled out slowly to plants in the soil after dead plants decompose. Chemical fertilizers tend to make much more nitrogen available at one time. This forces plants to use their carbon resources for growth, and limits production of natural compounds for defense.

This means that fertilization involves tradeoffs. We know now, for example, that fertilization may make a tree more susceptible to aphids and fire blight. In addition, foliage may be more nutritious to bugs and cause higher populations to develop.

Several studies have found that fertilizer decreases tolerance to drought, which may in turn increase susceptibility to trunk borers. Other studies have found that fertilization also decreases storage of carbohydrates, which may lower the ability of trees to recover from stresses.

Fertilizer tips

Trees are not necessarily healthier when they are fertilized.

To be more in synch with natural systems, fertilizer should be applied in small doses or in slow release forms, such as composted organic matter, urea formaldehyde, or coated fertilizers.

If trees are not being maintained on a regular basis, they may be better off without fertilizer. Newly planted trees should not be fertilized if they cannot be watered regularly.

Very sick or weak trees should probably be fertilized little, if at all.

For maximum tree growth, improve soil to permit good drainage and aeration. Add a wide, shallow ring of woodchips or mulch.

Current guidelines

Research has shown that broadcast application of fertilizer on the soil around a tree (from the trunk to the ends of the branches, or drip line) is an effective method. Trees may benefit from holes bored into the ground, but this is likely due to the resulting addition of oxygen to the roots.

Some researchers have concluded that the best results are achieved when fertilizer is applied in spring just before growth starts. Others prefer fall. If slow release fertilizers are used, the timing becomes less critical.

Here are suggested rates for applying nitrogen to the ground surface around trees, expressed in pounds of pure nitrogen per thousand square feet. These will become standards for tree care by the American National Standards Institute (ANSI) in the near future.

Establishment period (1-3 years after planting): 1 lb. or less per 1,000 square feet.

Established trees (planted at least 3 years): 2-4 lbs. per 1,000 square feet at 2-4 year intervals.

Mature trees (planted more than 20 years): 1 lb. per 1,000 square feet at 2-4 year intervals.

Joe Garvey is a district forester with the Missouri Department of Conservation.