Maintenance programs should be developed for trees and shrubs in both residential and commercial landscapes. A good maintenance program includes monitoring and controlling insect and disease problems, suppressing weed competition, and making timely applications of water, mulch, and fertilizer.
Tree and shrub fertilization is especially important in urban and suburban areas where soils have been altered due to construction. These urban soils tend to be heavily compacted, poorly aerated, poorly drained, and low in organic matter. Even where soils have not been affected, fertilization may be needed as part of a maintenance program to increase plant vigor or to improve root or top growth.
Fertilizer Objectives:
How and when to fertilize landscape trees and shrubs depends on:
- Maintenance objectives (stimulate new vs. maintain existing growth)
- Tree and shrub ages (generally more for younger and less for older plants)
- Plant stress levels
Table 1 provides information on different formations of chemical fertilizers and compares them to speed of reaction, leaching, soil reaction and the number of pounds of fertilizer required to obtain roughly 1 pound of nitrogen for every 1,000 square feet of property.
| Table 1 - Chemical Fertilizers, Analysis, Speed Of Reaction And Effect On Soil pH. | ||||
Fertilizer |
Analysis |
Speed of |
Soil Reaction |
Pounds of each fertilizer |
Ammonium nitrate |
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Ammonium sulfate |
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Urea |
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Ureaformaldehyde |
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Di-ammonium phosphate |
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Calcium nitrate |
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Potassium nitrate |
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10-10-10 |
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Osmocote |
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Additional information on fertilizer analysis and formulation.
Determining the Need to Fertilize
Visual inspection of trees and shrubs is often the best overall factor to use in making fertilization decisions.
Look for:
- Poor or chlorotic leaf color (pale green to yellow)
- Reduced leaf size and retention
- Premature fall coloration and leaf drop
- Reduced twig and branch growth and retention
- Overall reduced plant growth and vigor
In addition to observing signs of possible nutrient deficiencies on plants, soil and foliar analysis can be used to help determine or confirm whether supplemental fertilization is needed.
Fertilizer Selection:
A variety of fertilizer types exist:
- Complete (N-P-K) vs. incomplete (one or more select nutrients)
- Organic vs. inorganic
- Fast release vs. slow release
- Dry (granulated, pellet, spikes, pulverized, encapsulated) vs. liquid
To help determine the type of fertilizer to apply, consider the following: type of plant, time of year, desired rate of plant reaction, application methods and equipment cost, proximity to water sources, effect of soil type and pH, type of deficiency, and results of a soil test or other sampling method.
Most landscape plants benefit from a slow release nitrogen fertilizer that may be organic or inorganic. Keep in mind that nitrogen is readily leached (washed through soil) but phosphorus and potassium are not, meaning they require less frequent application.