Experts don't agree on all aspects of fertilization of urban trees in landscapes, but they do agree on a few basic principles, which apply to all trees. There are a few key things to look for and observe, such as leaf color. Leaves that are “off-color” indicate a nutrient deficiency.

  1. Determine if your trees are in need of fertilization. This can be accomplished by noting how much growth is put on annually. Healthy trees should grow an average of 6 inches or more annually. If growth is between 2 and 6 inches, fertilizer may be necessary, and if less than 2 inches, fertilizer is necessary.

  2. Determine how and when to apply the fertilizer for your growing conditions and soil type. In some soils, fertilizers move in and out faster than the plants are able to utilize them; you may need to apply fertilizer more often. Late fall or early spring fertilizations are most often recommended.

  3. If nitrogen is the only limiting nutrient, broadcasting may be sufficient, especially if the trees are growing in a landscape bed.

  4. If phosphorus, potassium and/or micronutrients are lacking, soil injection under the sod may prove most beneficial for trees. Phosphorus and potassium move slowly through the soil, if at all. There have been many guides written on the drill method of creating holes and filling them with granular fertilizer. Measure the area under the tree canopy. Prepare enough fertilizer to cover the area, calculated at 0.1 to 0.2 pounds of nitrogen per 100 square feet. Divide the fertilizer equally among holes drilled into the soil approximately 2 feet apart and 6 to 8 inches deep. Do not drill too deeply as you could put the fertilizer below the tree's absorbing root system. This method is time consuming and can be expensive to perform.

  5. Liquid fertilizer injection into the soil is treated similarly. Once the fertilizer is in the soil, it performs much the same way as the granular fertilizer after watering. In porous soils, there may be an advantage because of the pressure moving the fertilizer solution from the hole and out into the surrounding soil. In tight soils, there is little lateral movement. Because the fertilizer can move out, increase the spacing to 3 feet apart between injections. With both methods there is the risk of injecting/drilling too much fertilizer, which could lead to root and leaf burn. Err on the side of too little rather than too much.

  6. Liquid injection into the tree trunk is used when micronutrients are deficient. The most common is a shortage of iron or magnesium, leading to iron chlorosis. Trunk injection is the most successful at correcting this problem; however, the process needs to be repeated every few years. With each application, there are holes drilled into the cambium of the trees, providing quick access for diseases or insects. New holes must be drilled each time. In addition, there must be adequate moisture for the roots to absorb water needed to push the fertilizer up into the canopy of the tree. The Mauget system, which uses a hypodermic-type needle, causes minimal damage when inserted into the trunk, but there is still damage. Follow label directions for application rates and mixing instructions.

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