Fertilization program for trees and shrubs

Is Fertilizer Necessary?
It is a misconception that native trees and shrubs in the urban landscape do not need fertilizer. Although they were growing fine in the forest before man arrived, human occupation has significantly changed conditions for plants. The forests within the communities surrounding an area are no longer native forests, but are urban forests with all the stresses of urbanization. Where decomposition and periodic fire once recycled nutrients in the native forest, these processes have changed.

Control of fire, home and building construction, paving, soil compaction, and even garden cleanup, have all reduced the natural sources of nutrients.

All this contributes to plant stress. Nutritionally stressed plants are more susceptible to insects, diseases, droughts and other environmental stresses.

Symptoms of nutrient deficiencies show up in plants as poor vigor, abnormal color, and reduced leaf size. Nitrogen deficiencies show up as a yellowing of the mature leaves and poor growth. Leaves of phosphorus deficient plants have reddish or a dull purple cast. Deficiencies of potassium show up as slow growth and mottling of the leaves in extreme cases, but is difficult to detect. If a plant is growing poorly and exhibits unusual leaf symptoms, find out what is causing them rather than applying nutrients which may not be needed. The practice of adding nutrients just to make up for any potential nutrient deficiencies may cause salt build-up in the soil and water pollution. It also wastes money and time.v

Major Nutrients
Nitrogen (N) is the nutrient needed most by trees and shrubs. Soil phosphorus (P) and potassium (K) levels are low in the coarse soils of the Basin. Fertilizers with these two nutrients are not needed annually unless soil testing indicates a deficiency. The use of a complete fertilizer every year (one that contains phosphorus and potassium in addition to nitrogen) is usually not necessary. Since mycorrhizae help plants extract these nutrients from the soil. Mycorrhizae are fungi that help the plant more efficiently absorb water and nutrients, and in return the plant supports the fungi with nutrients and energy.

In forest soils, nitrogen added every two years is sufficient for mature trees and the application of phosphorus and potassium every five years is usually ample even with deficient soils. Rather than using a complete fertilizer, apply the P and K separately. This will enable a more precise application of N.

Fertilizer Application
Fertilizers are applied to trees in a number of ways. They can be spread (broadcast) over the soil surface, injected as a liquid into the soil under pressure, placed in holes in the soil, sprayed on the foliage (foliar), or injected into the trunk of the tree.

Soil surface application is the easiest and can be done with a lawn fertilizer spreader. When applied to bare ground, rake the fertilizer into the soil and cover it with mulch. Irrigate immediately thereafter so the fertilizer will penetrate into the soil. Use a light irrigation to avoid runoff.

Another way to fertilize trees is to drill holes at least three-quarters to one inch in diameter under the canopy of the tree and up to one-half the radius of the canopy beyond the drip line. Begin drilling holes three feet away from the trunk of the tree to avoid damaging the large flare roots. The holes should not be over eight inches deep and should be placed one to one and one half feet apart. The shallow holes enable an application of fertilizer to reach the roots of the tree. This is especially important when the trees are growing in grass which may use the fertilizer before it gets to the tree roots.

Pressure injection of fertilizer into the soil is very effective and less work intensive compared to drilling holes, but it does require special equipment. Because the fertilizer is in liquid form, the injections can be spaced two to three feet apart and not deeper than eight inches. Deep watering probes can be used if they have a chamber to hold the fertilizer. Most of them require a special tablet that slowly dissolves as the water flows over it. Regardless of the length of the probe, apply the fertilizer in the plant’s root zone, no deeper than eight inches below the soil surface. Deeper placement will waste fertilizer and contribute to pollution of ground water.

Foliar application should not be used for general fertilizer applications. It is not an effective way for trees and shrubs to absorb N, P and K. It is however, effective for micro-nutrient applications of iron or zinc but only if tests show there is a need.

Trunk injections often do more harm than good because of the wounds they leave in the trunk. This method is generally reserved for situations where all other application methods are ineffective. This requires unique equipment and the specialized skills of an to arborist.

Fertilizer Selection

Sources of nitrogen fertilizer are divided into water soluble nitrogen (W.S.N.), or fast release nitrogen, and water insoluble nitrogen (W.I.N.), which is slow release. The fast release types, inorganic salts of ammonium or nitrate, dissolve readily in water and include ammonium sulfate, ammonium nitrate, ammonium phosphate, and sodium nitrate. These forms of nitrogen are sometimes used in encapsulated slow release nitrogen formulations. Urea is an organic form of nitrogen that is also fast release. This may be coated with sulfur which is insoluble. As micro-organisms feed on the sulfur, the coating deteriorates and the nitrogen becomes slowly available to the roots of the plant. The fast release nitrogen forms are all immediately available to the plant and, unless encapsulated or coated, are easily over-applied. This results in waste, plant burn and water pollution, so use them according to the directions in this fact sheet.

Slowly available forms of nitrogen, ureaform (UF) and isobutylidene diurea (IBDU), are soluble in water. These forms of N provide lower concentrations of nutrient over a longer period of time. This decreases the likelihood of plant burn and increases the probability the nutrients will be used by the intended plant rather than passing on to pollute streams and lakes.

Slow release nitrogen is often listed on fertilizer labels as W.I.N., water insoluble nitrogen. Nitrogen fertilizers are classified as fast, medium or slow release depending on the amount of W.I.N. they contain. Fertilizers containing less than 15% W.I.N. are classified as fast release, between 15 and 30% are medium, and more than 30% W.I.N. are slow release. The label shown in Fig. 1 indicates 3.2 % slow release N so divide 3.2 by 20, the total N, which equals 16% W.I.N. This fertilizer would be classified as a medium release fertilizer.

Do not use "weed and feed" fertilizers near trees. The chemical in the product that kills weeds will also kill trees and shrubs. If a pesticide is needed, purchase and apply it separately from fertilizers. Use the pesticide only according to directions on the label. Also, do not use manure, it can cause polution.

It is best to determine fertilizer application rates in terms of the actual nutrient applied. Fertilizers contain a guaranteed analysis which shows the percentage (by weight) of nitrogen (N), phosphorus (P₂O₅) and potassium (K₂O), in each bag (see Fig. 1). Some fertilizers contain a fourth number which may be iron or sulfur. Neither is recommended for use in the Tahoe Basin, unless a soil test indicates differently. By law a complete fertilizer contains N, P₂O₅ and K₂O. An example of a common complete fertilizer is 20-2-4, which means the bag contains

20% nitrogen, 2% P₂O₅ and 4% K₂O by weight. You can use a simple calculation to determine how much fertilizer to apply to reach a recommended level of a particular nutrient. If it is recommended you apply 2 lb. of actual N, using the above fertilizer, simply divide the 2 lb. needed by 20 percent, or 0.20 (the fertilizer’s N content). The result is 10. To get the recommended 2 lb. of N, apply 10 lb. of the fertilizer. This is only an example.

The Amount of Fertilizer To Apply 

The amount of fertilizer to apply and when depends, in part, on the age of the plant and the time of year. Fertilize newly planted trees and shrubs when they are planted if a slow release fertilizer is used, or six weeks after planting if a fast release fertilizer is applied. Fertilize young plants at least once with no more than one pound of actual nitrogen per thousand square feet the first year. Late summer and fall, after the tree has stopped growing, is the best time to fertilize. This does not promote late soft growth as once thought. Fertilizer is absorbed by the roots in the fall, giving the tree a good start next spring. In fact, there is a noticeable difference in growth between fall fertilized plants versus those getting a spring application with nitrogen. Use a complete fertilizer every couple of years if soil tests show a need for it.

The amount of actual nitrogen to use is two to four pounds every one to two years per 1,000 square feet. If trees are growing normally, no fertilizer is needed, but a half pound of N per 1,000 square feet will keep them healthier. When a granular fertilizer is used, one way to calculate the amount of fertilizer needed is to first measure the trunk of the tree at four feet from the ground. If the trunk is less than six inches in diameter, apply 0.1 lb. of actual N per inch of trunk diameter. A tree with a five inch diameter trunk would need 0.5 lb. of N or 2.4 pounds (lb. of actual N ¸ % N in fertilizer -- 0.5 ¸ .21 = 2.38 or 2.4) of ammonium sulfate (21-0-0). Spread the fertilizer evenly under the plant's canopy and extending beyond by one-half the radius of the canopy. If holes were drilled, equally divide the fertilizer by the number of holes and place it in the holes.

When the trunk of the tree is over six inches in diameter, 0.2 lb. of actual N are recommended per inch of trunk diameter at four feet off the ground. For example, if a tree has a trunk diameter of eight inches, you would use 1.6 lb. of N or 1.6 ¸ .21 = 7.6 lb. of ammonium sulfate. Again distribute this evenly around the tree.

Once the fertilizer is applied irrigate it into the root zone to prevent volatilization of the nitrogen from occurring. Volatilzation is the loss of nitrogen as a vapor into the air. When lawns are growing over the root zone of the tree, apply the fertilizer on dry grass and water after application to wash the fertilizer off the leaves of the grass to prevent burning. Do not overwater, which increases the risk of surface run-off and the leaching of fertilizer below the root zone.

Tips For Protecting Water Quality 

· Do not use fertilizers near stream zones, shorelines or on water saturated soils. 

• More is NOT better with fertilizers - do not over fertilize.
• Use only the nutrients necessary -do not guess, get a soil test.
• Do not fertilize over snow.
• Fertilize in the late summer and fall when the trees have finished growing and before the soil freezes.
• Do not overwater - nutrients are lost by runoff and leaching.
• Landscape with Sierra Nevada natives. These plants are adapted to local conditions and need little, if any, irrigation or fertilizers once they are established.
• Sweep all fertilizers, soil and plant clippings off paved surfaces to prevent the nutrients they contain from washing into streams, storm drains and Lake Tahoe.

A good fertilization program for trees and shrubs promotes healthy plants that are resistant to insects and disease. However, improper use of fertilizers can damage plants and pollute streams and lakes. By understanding and following the information in this fact sheet, you can have a beautiful, healthy landscape and protect the area.

References