Fertilizing Landscape Trees and Shrubs

Fertilizing Landscape Trees and Shrubs

Author(s): Scott Dewald

Most Nebraska soils are fertile enough to support tree and shrub growth without applying fertilizer. However, when woody plants exhibit poor growth or reduced vigor, yet have had adequate moisture and are not experiencing pest problems or other environmental limitations, the proper application of fertilizer may be necessary.

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Overview

Landscape trees and shrubs are occasionally subject to adverse soil and environmental conditions. Redistribution and compaction of the original soil profile during home construction, poor drainage, removal of twigs and leaves that normally decompose to provide nutrients, heavy grass sod competition, and extreme pH levels are some factors that contribute to nutrient deficiencies. Supplemental nutrients may sometimes be needed to help trees maintain vigor.

Most trees and shrubs will not require fertilization. Vigorous growth is a good indication that a tree or shrub is receiving a sufficient nutrient supply. Less vigorous plant growth and poor color usually indicate an insect or disease problem, environmental stress, poor root development or damage, moisture deficiency, or other factors not related to fertility. Being familiar with normal growth for a specific tree or shrub species and the associated site conditions will help determine the need for fertilization.

Determining Whether to Fertilize

Trees obtain nutrients for plant growth from the soil through their roots. The major nutrients include nitrogen, phosphorus and potassium. Of these, nitrogen is the most likely to be deficient. When needed and applied properly, nitrogen will give the most immediate growth response.
There may be a combination of symptoms in a nutrient-deficient plant. Some of the more easily identified symptoms are general lack of vigor or growth. Soil tests can provide additional evidence of nutrient deficiencies where tree growth is slower than expected.

It is recommended that two soil samples be taken, one from the 0 to 6-inch depth, and the other from the 6 to 18-inch depth. If the average nitrogen concentration of the two samples is less than 10 parts per million (ppm), then the plant may respond to nitrogen fertilization. Phosphorus and potassium should be applied to soils that test very low for both depths. Unfortunately, since the relationship between soil nutrients and landscape plant growth has not been well documented, specific guidelines for the amounts of many nutrients needed by trees and shrubs are not available.

Reading the Fertilizer Label

Most fertilizers for trees and shrubs are a composition of three major nutrients: nitrogen (N), phosphorus (P2O5) and potassium (K2O)*. The amount of each nutrient present in the fertilizer is listed on the fertilizer container by three percentages representing nitrogen, phosphorus, and potassium, respectively (Figure 2). For example, a fertilizer with the analysis of 10-6-4 has  10 percent nitrogen, 6 percent phosphorus and 4 percent potassium. This means that 10 pounds of 10-6-4 fertilizer will contain 1 pound of nitrogen, the same amount of available nitrogen as 5 pounds of 20-6-4 fertilizer (10 percent of 10 lbs.=1 lb.; 20 percent of 5 lbs.=1 lb.). Some soil-active herbicides that control weeds can damage trees if not properly used.

To prevent plant injury, avoid using fertilizer-herbicide or “weed-and-feed” combinations. Herbicides should be applied  separately at the rates suggested on the label

Fertilizer Application Techniques

Fertilizers can be applied to landscape plants through foliar sprays, trunk injections or applications on or beneath the soil surface. The method selected depends on the type of fertilizer being used, the specific purpose for fertilizing, soil conditions, tree location, the presence of quality turf, ease of application, equipment used and cost.

Foliar sprays, trunk injections, and trunk implants are limited in the number of nutrients they can supply to woody plants. They are recommended for applying micronutrients like iron or manganese when availability from the root zone is reduced due to soil pH, moisture relationships or other conditions. Trees under moisture stress should not be treated with trunk injections or foliar sprays.

Woody plants take up nutrients through the root systems and, in most cases, respond best to soil applications of fertilizers. Recommended soil application methods include surface broadcast, drilled hole technique, and soil injection.

Nitrogen

Research indicates that surface-applied nitrogen is as effective as nitrogen applied below the surface. Nitrate nitrogen will move through the soil to the roots as water moves downward. Due to the potential damage to grass under trees, surface applications for woody plants where good grass cover is present should not exceed 1.5 pounds of actual nitrogen per 1,000 square feet of ground area per application. This will reduce the risk of “burning” the grass. Homeowners who fertilize nearby turfgrass do not need to apply additional fertilizer for their trees.

Fertilizer can be applied with a spreader calibrated to apply the recommended amount. Start 2 to 3 feet from the trunk and move outward in concentric circles to 2 to 3 feet beyond the drip line (end of branches). Care should be taken to avoid overlapping. Immediately irrigate to move fertilizer into the soil profile. This method is quick and inexpensive but may cause excess grass growth or burning of quality turf areas.

Phosphorus and Potassium

Most soils in North America contain enough phosphorus and potassium to satisfy the needs of trees and shrubs. Deficiencies of these two nutrients typically occur only in areas where much of the surface soil has been removed. If a  soil analysis indicates the phosphorus level is less than  6 ppm (Bray #1) or 4 ppm (sodium bicarbonate),  or potassium is below the range of 75 ppm (extractable K), then applications of one or both of these to bring the nutrients to these respective levels would be beneficial for tree and shrub growth. Levels of these nutrients above their respective ranges should be avoided because of the negative effects excessive amounts can have on the uptake and utilization of other nutrients.

When phosphorus or potassium applications are recommended, vertical holes or injections into the soil pro-file work best. These nutrients are less mobile in the soil and therefore need to be applied near the roots. The drilled hole technique requires the most time, but can also be the most efficient, especially with nutrients that are less mobile. Use a soil probe or auger to dig 1- to 2-inch diameter holes  12 to 18 inches deep and 2 feet apart. These holes should be no closer than 2 to 3 feet from the trunk and should extend at least 2 to 3 feet beyond the drip line of the tree or shrub. Approximately 250 holes are needed per 1,000 square feet, or 4 square feet/hole.

Distribute the recommended amount of complete fertilizer equally among the drilled holes. To reduce turfgrass injury, keep the fertilizer level in each hole at least 4 inches below the soil surface. Depending on the size of the hole, the top can be filled back with soil or, if possible, by pressing the hole shut with the heel of one’s shoe. If one uses a soil probe, the grass plug can be replaced. Irrigation following fertilizer application will help to prevent injury to turf, but do not flood the area because dissolved fertilizer may burn turf.  An advantage of the drill-hole method is that in compacted soils, the holes may help increase water and air distribution into the soil profile.

Liquid or soluble injection is an alternative to the drill- hole method. However, since equipment costs are high, it is best done commercially.

Root feeders may be effective for applying phosphorus and potassium if they are inserted 12 inches or less in the soil and if the recommended fertilizer rates are used. They are convenient, but the cost of nutrients in water-soluble form is usually higher than dry granular fertilizers. Due to the higher cost, there is a tendency to cut back on the amount of fertilizer applied.

Regardless of the method of fertilization, irrigation following the application will help prevent injury to turf. Again, do not flood the area since dissolved fertilizer may burn turf.

Fertilizer Application Rates

Calculating fertilizer rates based on surface area is the recommended method for Nebraska. The amount of fertilizer recommended is based on the number of square feet in the growing area beneath the crown spread of an individual woody plant. This method accounts for situations where the root system is restricted by paved areas, foundation walls or other obstructions in the soil. Application rates greater than recommended amounts can make trees more susceptible to insect and disease problems and drought injury.

The worksheet in this NebGuide shows how to calculate the amount of nitrogen fertilizer needed. First, the square feet under the plant is determined, then the actual pounds of fertilizer are calculated based on the product’s concentration. If nutrients are needed, the recommended rates are 1.5 pounds of actual nitrogen or 1 pound of actual phosphorus or potassium per 1,000 square feet of area under the tree’s drip line or a shrub’s bed area.

The formula for finding the square feet area is: square feet of circle, (area) = π × radius2 = 3.14 x radius × radius. The radius is the distance from the trunk to the end of the branches (Table I). The formula for pounds of “actual”  N fertilizer needed = 1.5 lbs of “actual” N × (area/1,000  square feet). Then, fertilizer mixture needed = (lbs. of  “actual” N/% of N in the fertilizer) × 100.

In an example with a radius equal to 20 feet, the formula would be the following: Area = π (20)2 = 3.14 × 20 ×  20 = 1,256 square feet. Actual nitrogen needed would equal  1.5 ×  1,256/1,000 = 1.9 pounds of N. If the fertilizer had a 34  percent N composition, the amount of fertilizer needed would then be 1.9/.34 = 5.6 pounds of fertilizer.

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Tree Care