Trees, like other living things, need the right combination of nutrients to grow and flourish. If your tree is planted in soil that lacks essential nutrients, adding fertilizer can help provide the best environment for growth.
Benefits of Proper Fertilization
- Increases tree growth.
- Reduces susceptibility to certain diseases and pests.
- Can help reverse declining health.
Urban landscape trees often grow in soils that lack the proper nutrients for satisfactory growth and development. In these situations it may be necessary to fertilize to improve plant vigor. However, do some testing to make sure your tree really is lacking in nutrients, because fertilizer can hurt trees if it’s not needed.
Soil conditions, especially pH and organic matter content, vary greatly, making the selection and use of fertilizer a somewhat complex process. But when you consider the benefits and value a mature tree can provide for your landscape, it is worth spending a little time and money to have the nutrient content of your soil tested.
Testing 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. Most quality garden centersn also can arrange to have your soil tested at a laboratory. With test results in hand, you can consult your local garden center staff, qualified arborist, or a plant care professional for advice on application rates, timing, and the best blend of fertilizer for trees and other landscape plants.
Plant Nutrient Needs
- Nitrogen – Promotes green leaves and stem.
- Phosphorus – Used in the production of roots, flowers, and fruit.
- Potassium – Aids in flowering and fruiting, sturdiness, and disease and stress resistance.
- 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.
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. 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 should be added when a deficiency exists.
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 also affect the health of a tree.
Before beginning any fertilization program, soil should be tested to determine what is needed. A soil acidity (pH) test can also help determine if the pH level of the soil is preventing the tree from taking up adequate nutrients from the soil.
Types of Fertilizer
Fertilizers are available in numerous forms and combinations. A “complete” fertilizer is one that contains significant amounts of the three primary nutrients: nitrogen, phosphorus and potassium. The relative percentages of these three nutrients is listed on the fertilizer 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 or two of the primary nutrients. These are not lesser fertilizers, but are used to correct specific deficiencies.
Fertilizers are 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, 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.
Factors that Affect Fertilizer Needs
- Higher concentrations of plants increase competition for nutrients.
- Different species create different demands on the soil. 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, moving quickly into and through the root zone.
- Clay soils have greater ability to hold nutrients until plants can absorb them.
- Younger, more vigorous trees use more nutrients than do older, slower growing trees
Understanding the actual size and extent of a tree’s root system, before you fertilize, helps to determine how much, what type, and where to best apply fertilizer.
Fertilizer needs to be placed so that the tree can absorb it. Mature trees have expansive root systems that extend from two to three times the size of the leaf canopy. (This is also important to keep in mind if you fertilize your lawn, as many lawn fertilizers contain weed and feed formulations that could harm your trees if spread too near them. The same herbicide that kills broadleaf weeds in your lawn is picked up by tree roots and can harm or kill your broadleaf trees if applied incorrectly.)
How to Apply Fertilizer:
There are many ways to apply fertilizer. 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.
- 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.
- Advantage: Easiest, least expensive, uses simple nutrients taken up by grass and other plants and thus not available to the trees.
- Disadvantage: Smaller and more frequent applications need to be applied when grass is present.
- Spraying a liquid, water soluble fertilizer: This method requires a tank and spray mechanism, which can be as simple as a hand sprayer or as complicated as a large tank sprayer mounted on a trailer.
- Advantage: easy and quick to apply after the desired formulation is mixed.
- Disadvantage: Can require expensive equipment.
- Soil injection: Applying fertilizer directly into the root zone to keep nutrients from being absorbed by grass and other shallow-rooted plants. Fertilizers are mixed in a tank, and a soil probe is pushed into the soil to a depth of 6-12 inches.
- Advantage: fertilizer is injected directly into the root zone where it is needed; applying water at the same time and adding air space by breaking up compacted soils.
- Disadvantage: requires expensive equipment and has the potential of rapid leaching.
- 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.
- Advantage: Helps to eliminate the patchy effect visible when grass is present. This method also has the advantage of aerating the soil.
- Disadvantage: Requires the location of utility lines before starting
- The Fertilizer Spikes method: Fertilizer spikes, two to three inches long and made of a compacted fiber impregnated with fertilizer, are inserted into the ground. The fertilizer is released slowly as the spike disintegrates in the soil.
- Advantage: This method requires very few tools.
- Disadvantage: It is expensive when treating large areas and slightly compacts the soil where the spike is inserted.
- Foliar applications: For minor nutrient deficiencies, fertilizer can be applied directly to the foliage. Foliar application sprays are not adequate to provide all necessary nutrients and can be performed once or twice per year. They are most effective when performed just prior to the onset of active growth.
- Disadvantage: Requires expensive equipment and spray may drift to non-desired locations
- Direct application into tree tissue: Tree implants and injections introduct the fertilizer 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 with have minor nutrient deficiencies.
- Advantages: Very effective at treating specific deficiencies and deliver nutrients directly into the tree with minimal waste.
- Disadvantages: Correct application is critical, and repeated applications cause coalesces and overdoses, which can severely damage the tree’s cambium and xylem.
- Mulching: Often overlooked as a method of fertilization, mulching can help provide your tree with the nutrients it needs. The decomposition of the plant materials returns the nutrients to the soil in a manner similar to that which naturally occurs.
- Advantages: Reduces competition for nutrients with other plants, moderates soil temperatures, reduces water loss, and reduces the use of equipment around the base of a tree. Can be used in combination with other methods when just a few nutrients are deficient.
- Disadvantage: Does not deliver nutrients at a known rate.
- Never put any type of fertilizer containing a herbicide into holes around shade trees or large shrubs.
- Do not use a fertilizer containing a herbicide unless those plants are specifically listed on the label of the product. Herbicides may have specific toxicities for desirable plants, too.
- Do not use any lawn fertilizer containing 2,4-D, dicamba, or MCPP formulations under shade trees or other ornamental plants.
- Fertilizer applications are not good without moisture. If conditions are dry, irrigation will be necessary.
- Do not apply dry fertilizers to wet leaves of turf, shrubs, or trees.
- Fertilizers containing water-insoluble, organic nitrogen sources may take 3 to 8 weeks to break down to a usable form. Time the applications accordingly.