Tree Fertilization
Reasons and Methods

Trees, like all other living plants, have many nutritional requirements for sustained growth and reproduction. There are various methods of determining the available nutrients in the soil, as well as methods of determining the amount of nutrients absorbed by the tree. When certain nutrients are deficient in the soil, fertilizers may be able to correct the deficiency. Fertilizers come in many forms and are applied in many ways. The amount and kind of vegetation in the area, as well as the reasons for fertilization are critical factors in determining the application method to use.

MACRONUTRIENTS

Nitrogen - Promotes green leaves and stems

Phosphorus - Used in the production of roots, flowers, and fruit

Potassium - Aids in flowering and fruiting, sturdiness, and disease and stress resistances

Calcium - Aids in cell manufacturing

Magnesium - A prime element in the development of seed and chlorophyll

Sulfur - A primary element of proteins and contributes to the green color

Nutrients

Trees and other woody ornamentals require large quantities of six macronutrients and lesser quantities of ten micronutrients. The most important of the macronutrients is nitrogen, which is the most limiting nutrient in the soil. Phosphorus and potassium are also needed in large quantities, but are generally available in the soil. The other three macronutrients are also usually present, but need to be added when a deficiency is noticed.

The micronutrients (iron, manganese, zinc, molybdenum, boron, chlorine, copper, carbon, oxygen, and hydrogen) are needed in lesser amounts and are generally found in sufficient amounts in normal pH-balanced soils. However, a deficiency in any of these ten nutrients can affect the health of a tree.

Before beginning any fertilization program, the amount of available nutrients must be determined. Adding too much of a nutrient or adding the wrong nutrients to the soil will not correct a problem and may even be damaging to the tree. A soil sample is the most accurate method of determining the available nutrients, the soil's acidity, and the soil's ability to store nutrients in a usable form. Small core samples are taken from the entire area and then dried, mixed, and analyzed. Most state land grant colleges and cooperative extension services provide these services at reasonable rates.

While the soil analysis determines the nutrients that are available, a foliar analysis determines the nutrients that are actually taken up by the tree. Leaves are collected from all over the tree, dried, and analyzed. Even though certain nutrients are available in the soil, the foliar analysis points out nutritional deficiencies caused by soil acidity (pH).


Fertilizers

Fertilizers are available in many different forms and combinations. Many of the terms used are confusing and misleading. A complete fertilizer is one that contains "significant" amounts of the three primary nutrients, nitrogen, phosphorus and potassium. A fertilizer analysis, the relative percentages of these three nutrients, is listed on the label and is referred to as the NPK number. For example, 100 pounds of 13-13-13 fertilizer has 13 pounds of nitrogen, 13 pounds of phosphorus, 13 pounds of potassium, and 61 pounds of inert (filler) materials. Incomplete fertilizers only contain one ro two of the primary nutrients. These are not lesser fertilizers, but are used to correct specific deficiencies.

Fertilizers are also available in organic and inorganic forms. An organic fertilizer is made from natural sources, such as plants, animals, and unprocessed minerals. They contain carbon compounds and may also contain certain amounts of inorganic materials. They are generally slower in releasing their nutrients to the soil. Inorganic fertilizers are man-made and release their nutrients relatively quickly and may cause plant "burn."

Water-insoluble nitrogen, (WIN) is a slow release nitrogen and is less likely to cause plant "burn" or be leached from the soil. Ammoniacal, Urea, and Nitrate nitrogen are inexpensive, synthetic, water-soluble sources of nitrogen. Chelates or chelated micronutrients are water soluble compounds that are readily available to the plants and may be applied by foliar or soil sprays.


Environment

Many environmental factors affect the need for various nutrients. Higher concentrations of plants increase the competition for nutrients. The plant species also influences the need for fertilizers. Grasses use large amounts of nitrogen and potash and leaves smaller amounts available for the trees. Vegetables and bulbs use large amounts of phosphorus. Soil type influences the amount of fertilizer needed. In sandy soils, nutrients are leached, move quickly into and through the root zone. Clay soils have more ability to hold these nutrients until plants can absorb them. Younger, more vigorous trees use more nutrients than do older, slower growing trees.

Fertilizers can be applied for several different reasons and desired effects. One reason may be to increase the growth of plants. Increased growth means an increased need for pruning and other maintenance. To maintain a healthy tree, minimum amounts of nutrients can be applied. Different relative amounts of nutrients within a fertilizer or "incomplete fertilizers" may be used for specific situations. Increasing potassium may increase root growth and stress resistance, while increasing nitrogen may increase stem elongation.

The existence of mycorrhizae affects the effectiveness of roots. It improves water and nutrient uptake, increases resistance to drought and root diseases, and survival of newly planted and established trees. Several methods are available to increase the amount of mycorrhizae and decrease the need for fertilizers.


Application Methods

Numerous fertilizer application methods are available. The best method for a given situation is based on the soil and foliar analysis. competing vegetation, soil type, desired effects and other considerations, such as use of property and location of ground water. No one method is best suited for all situations.

The most frequently used, easiest and least expensive method is broadcasting a granular fertilizer on top of the soil. A spreader is calibrated to deliver the desired amount of fertilizer over the root zone. After the application, the area must be watered to dissolve the fertilizer and wash it off the grass and into the soil. This method has the advantage of being inexpensive, using simple nutrients taken up by grass and other plants and thus not available to the trees. Smaller and more frequent applications need to be applied when grass is present.

Another method of applying fertilizer is the spraying of a liquid, water soluble fertilizer. This method requires a tank and spray mechanism, which can be as simple as a hand sprayer to as complicated as a large tank sprayer mounted on a trailer. It has the disadvantage of requiring more expensive equipment to apply. An advantage is being quick to apply after the desired formulation is mixed.

In order to eliminate nutrients being taken up by grasses and other shallow rooted plants, fertilizers may be applied directly into the root zone. One method of this type is soil injection. Fertilizers are mixed in a tank similar to the above method, then a soil probe is pushed into the soil to a depth of six to twelve inches. These application points are spaced on a grid pattern that is two to three feet apart. The advantages are that the fertilizer is injected directly into the root zone where it is needed, applying water at the same time and adding air spaces by breaking up compacted soils. This method has a disadvantage of requiring expensive equipment and potential of rapid leaching.

Another method of applying fertilizer directly into the root zone is the drill hole method. This method uses a protable drill with a bit two to five inches in diameter. Holes are drilled in a similar pattern and depth as the liquid injection method. Granular fertilizer and a soil amendment, such as peat moss, perlite gravel or sand are placed in the holes. A broadcast application of fertilizers helps to eliminate the patchy effect visible when grass is present. This method also has the advantage of aerating the soil, but requires the location of utility lines before starting.

A third method of applying fertilizer directly into the root zone is through the use of fertilizer spikes. These spikes are two to three inches long and made of a compacted fiber impregnated with fertilizer. The fertilizer is released slowly as the spike disintegrates in the soil. This method requires very few tools, but is expensive when treating large areas and slightly compacts the soil where the spike is inserted.

Foliar applications are generally used to correct deficiencies of minor nutrients. It is not adequate to provide all necessary nutrients. These sprays are most effective when performed just prior to the onset of active growth. One or two treatments per year are needed. This method also takes expensive equipment to deliver the fertilizers to the foliage, is only effective on certain plants with certain deficiencies and the spray may drift to non-desired locations.

Tree implant and injections are two techniques that apply the fertilizer directly into tree tissue. These methods introduce the fertilizers directly into the xylem of the tree and depend on the transpiration system of the tree to move the nutrients systemically throughout the tree. They are most effective on trees over four inches in diameter, which have minor nutrient deficiencies. The disadvantages are that the technique is critical to correct application, repeated applications cause coalesces and overdoses may cause severe damage to the cambium and xylem. The advantages are that they are very effective in treating specific deficiencies and deliver nutrients directly to the tree with minimal waste.

The last and sometimes overlooked method of fertilizing is the mulching of trees. The decomposition of the plant materials returns the nutrients to the soil in a manner similar to that which naturally occurs. This method does not deliver nutrients at a known rate, but has the added benefit of reducing competition from other plants, moderating soil temperature, reducing water loss and reducing the use of equipment around the base of the tree. This method is able to be used in combination with other methods when only a few nutrients are deficient.


Conclusions

Fertilizers are necessary when soil and foliar analyses determined that certain nutrients are deficient. The specific recommendations made by these analyses should be followed so that the deficiency is corrected without wasting fertilizers or poisoning the plants. A fertilization program should be only one part of a complete plant health care program. This overall program should include the needs of the landowner and the needs of the plants on his or her land.

Further information and specific recommendations can be obtained from the county extension agent or a qualified consulting arborist in your area. For a list of consulting arborists, contact the American Society of Consulting Arborists at 15245 Shady Grove Road, Suite 130, Rockville, MD 20850 or (301)947-0483 . For a list of certified arborists, contact the International Society of Arboriculture at P.O. Box GG, Savoy, IL 61874 or (217)328-2032. 

Latest Update 09/08/03