Trees that may benefit from fertilization include younger trees once they have been established, trees with low vigor or off color foliage or trees with dieback of twigs at the ends of branches. It is not advisable to fertilize trees that are newly planted, have limited growing space or have recent damage to root zones according to the ANSI A-300 Standards for Tree Fertilization. Analyzing the nutrients available in the soil and foliage will provide a more complete assessment of nutrients that are needed. The pH of the soil also affects tree uptake of nutrient deficiencies.

Trees should be fertilized so that they are available when root growth is active. The type of fertilizer applied will affect the availability. Quick release fertilizers are readily available while slow release fertilizers are composed of at least 50% water insoluble fertilizer so they become active over an extended period of time. Slow release fertilizers are preferred over quick release fertilizers for trees and shrubs.

Timing Applications

Trees and shrubs should be fertilized in late fall months (October and November) after hardening of the plant. Early Spring (mid March to April) is also an appropriate time to fertilize after the soil is free of frost.

Fertilization Rates

Slow release general fertilizer should be applied at rates of two to four pounds per 1,000 square feet per application. It is important in our area to avoid over fertilizing trees especially mature or over immature oaks. Using excessive fertilizers contribute to over growth of new shoots and foliage that are more likely to become infested with insects like aphids or disease pathogens like powdery mildew.

Application Methods

There are several methods available for applying fertilizers. Surface application is accomplished by using a spreader or evenly distributing the pellets by hand. Surface applications are not recommended in turf areas or where runoff is possible.

Sub-Surface dry application is accomplished by placing the fertilizer into evenly spaced holes. The holes should be two to four inches in diameter and spaced 12 to 36 inches apart. The holes should be four to twelve inches in depth. Sub-Surface liquid application utilizes the same spacing as dry application but the fertilizer is mixed with water and injected into the soil. This method is preferred by the Certified Arborists at TreePro Professional Tree Care because a slow release liquid fertilizer consisting of 50% coated urea is available for a longer period. It is also more efficient to mix liquid fertilizer with other micro-nutrients or mycorrhizae.

Mycorrhizae Applications

Mycorrhizae is a beneficial fungus that helps to improve root growth, improve nutrient and water uptake and reduce drought stress. Mycorrhizae fungi increase the surface absorbing area of roots 100 to 1000 times depending on soil conditions. Mycorrhizae also release enzymes that help to dissolve hard to capture nutrients such as organic nitrogen, phosphorus, iron and other “tightly bound” soil nutrients according to Mike Amaranthus, Ph.D at Oregon State University. Mycorrhizal fungi form an intricate web that captures and assimilates nutrients, conserving the nutrient capital in soils.

At TreePro Professional Tree Care we inject these beneficial fungi into the soil of trees that are stressed, are low in vigor or have damage to root zones from compaction or construction. Mycorrhizae in combination with mulching have provided new life to declining or stressed trees.

Soil Management

The goal of soil management is to protect and improve the quality of soil. Soil develops over thousands of years and is comprised of air, water, mineral particles, organic matter and organisms. One half of soil is pore space which is filled with equal parts of air and water depending on texture and soil saturation. Most of the solid portion of soil is mineral particles. Organic matter may make up only five to ten percent of the volume of soil.

Organic matter is critical in holding soil particles together, storing nutrients, and feeding soil organisms. (2002 University of Minnesota Extension and Regents). Soil structure is critical to root growth and tree and shrub health. Clay soils are low in oxygen and organic material. Low oxygen levels in soil make root growth very difficult. Compacted soils where the pore space has been reduced also make root growth difficult. Compacted soils are a common problem for trees in construction zones where tree protection is not adequate. Areas of high foot traffic from pedestrians or farm animals also create compaction.

Improving these poor soil conditions can be accomplished by adding organic material such as arbor mulch or soil aeration. A surface application of mulch will breakdown and incorporate into the soil over time adding organic material and nutrients. There are several methods to achieve better soil aeration for compacted soils including drilling holes and adding pea gravel, installing aeration tubes, or high pressure air aeration.

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