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Fertilizing WOODY Plants

Text and photos by Dr. Curtis E. Swift in American Nurseryman, August 1, 2005

It might not be easy, but soil tests could be well worth the time and effort in determining how to effectively fertilize woody ornamentals.

Very few landscape managers do soil tests before they apply fertilizer. This is true whether we are talking about trees and shrubs, flowers or turf. If a soil test is not performed, how can a decision be made on which nutrients are needed by the plant?

I realize that taking a soil sample from every property you maintain is difficult, if not impossible, but taking and submitting soil samples on occasion can help fine-tune your nutrient applications and help you avoid problems caused when the applied nutrients were not needed.

Sampling the complete rooting area of a large tree would be difficult, but samples can be taken from several spots in the root zone and mixed together, and then this composite sample could be sent to the lab. This will give you a better understanding of the needs of the tree.

The area to test can be daunting, as the diameter of the spread of most trees and shrub roots is three to five times the height of the plant. Thus, a tree with a height of 60 feet, even a columnar poplar, has roots that spread an average of 180 to 300 feet from side to side. In dry areas, root spread can be even more extensive. The old concept of roots staying within the drip line of the tree or shrub is totally inaccurate.

Because most of the nutrient- and water-absorbing roots are in the top 8 inches of soil, samples should be taken from this depth. In heavy clay soils, roots are most likely shallower, but collecting samples from the top 8 inches still is recommended.

The reasons for determining the level of nutrients already in the soil are well documented. Most industry professionals know that applying a nutrient already at a high or excessive level will result in a nutrient imbalance. Even phosphorus, the nutrient supposedly necessary for the stimulation of root growth, can cause problems when it is applied to soil that does not need it.

Striking a balance. Phosphorus is a constituent of protein, coenzymes, nucleic acids and metabolic substrates, and it is critical for energy transfer within the plant. While the level of sufficiency for phosphorus varies for different types of plants - from as low as 7 parts per million (ppm) for woody plants to 40 ppm for turf - many of the soil test results I see have phosphorus levels of 400 ppm or higher. So, why the concern? It is well known that high levels of phosphorus can cause iron and manganese deficiencies. High levels of phosphorus cause plant-available iron to precipitate out in the soil solution and plant sap as iron phosphate. This changes plant-available iron to a form (iron sulfate) that the plant cannot use. Levels above what is needed by the plant also can shut down the plant's ability to excrete phytochelates, organic compounds used by the plant to turn unavailable forms of iron into those that can be taken up by the plants's roots. Excess soil levels of phosphorus can also reduce the effectiveness of mycorrhizal-forming fungi and even cause the death of these important symbiotes. As a result of this complication, some companies now are recommending the application of phosphorus be avoided when applying mycorrhizae.

Mycorrhizae are an integral part of a plant's root system and occur on 83 percent of dicotyledonous plants and 79 percent of monocotyledonous plants investigated. Infection of a plant's root system by these fungi creates a symbiotic (beneficial) relationship between the plant and fungus. Upon root infection and colonization, mycorrhizal fungi develop an external mycelium, which is a bridge connecting the root with the surrounding soil. One of the most dramatic effects of infection by mycorrhizal fungi on the host plant is the increase in phosphorus uptake mainly caused by the capacity of the mycorrhizal fungi to absorb phosphate from soil and transfer it to the plant's roots. In addition, mycorrhizal infection results in a beneficial increase in the uptake of copper, zinc, nickel, chloride and sulfate. Mycorrhizal-forming fungi also are known to reduce problems with pathogens that attack the roots of plants.

The benefits listed above are greatest in phosphorus-deficient soils and decrease as soil phosphate levels increase. Very high and very low phosphorus levels may reduce mycorrhizal infection/colonization. It is well established that:

  • Infection by mycorrhizal fungi is significantly reduced at high soil phosphorus levels.
  • The addition of phosphate fertilizer results in a delay in beneficial root infection by mycorrhizae, as well as a percentage decrease in the infection level.
  • An increase in the level of soil phosphate results in a reduction in chlamydospore production by the fungus. These spores are involved in beneficial root infection and spread of the fungus through the soil profile.

The development of mycorrhizal relationships has been found greatest when soil phosphorus levels were at 50 ppm or below, and development significantly decreases when soil phosphorus levels were 133 ppm or greater depending on the fungi involved. Therefore, including phosphorus in a fertilizer mix when sufficient available soil phosphorus is present is detrimental to the plant. Some soild need more phosphorus, especially sandy soils, but how do you know if you don't do a soil test first? One last thing I find interesting about phosphorus is the common belief that adding phosphorus increases root growth of woody plants. While this is true if soil-available phosphorus is deficient, adding more phosphorus when phosphorus is sufficient does not increase root growth. Adding phosphorus when levels are suffieicent, because of the problems noted earlier, actually can reduce root growth.

The pros of potassium. Potassium is another major nutrient often applied to cocktail fertilizer applications. When sufficient soil potassium levels are present, plants have reduced levels of disease, faster callusing of wounds and increased resistance to frost injury. Potassium regulates protein and starch formation and controls more than 60 enzyme systems within the plant. Potassium also is responsible for the opening and closing of the stomates, those pores responsible for gas exchange and transpiration in the leaves. Consequently, potassium is a nutrient critical to the growth of plants. As with phosphorus, sufficient soil potassium levels vary based on the plant type and excessive levels should be avoided. Again, a soil test is the only way to determine if soils in your area have sufficient levels of this nutrient.

The objectives of any fertilization program are: to overcome a visible nutient deficiency; to eliminate a deficiency not yet visible, but identified by a soil or tissue test; to increase vegetative growth, flowering or fruiting of the plant; or to increase the plant's vitality. Remember, a plant that lacks proper nutrients will be more susceptible to disease and be less able to heal itself. While phosphorus and potassium soil levels may already be sufficient or excessive in the soils of your area, nitrogen is most often insufficient. Another nutrient that is often available in sufficient quantities in our soils is iron.

Nutrient application. Chlorosis often is thought to be caused by a deficiency in plant-available iron. However, a deficiency of manganese, magnesium or nitrogen can also cause chlorosis. In many areas, even though iron deficiency is considered the problem, nitrogen deficiency is the main cause of chlorosis (emphasis provided by Rail City Garden Center). Without a soil test, you'll never know the actual cause.

There are a number of different ways to apply fertilizer, with broadcast applications of liquid or granular forms of fertilizer being more common. When soil injection is used, injection holes should be between 4 to 8 inches deep. If less than 4 inches, the fertilizer bubbles to the surface and, if the trees are in turf areas, burning of the grass will most likely result. If injected more than 8 inches deep, the fertilizer is placed deeper than the fine roots and is lost to the tree. While trunk injections are available for both macro- and micronutrients, one must keep in mind that injection holes wound the tree and can lead to internal decay. When the fertilizer should be applied also is important.

The roots only absorb nutrients when a plant is in active growth, from the time buds break in the spring until leaves change color in the fall. Applicators in most parts of the country avoid applying nitrogen to woody plants after midsummer due to the potential of increased winter damage, resulting from succulent growth that does not properly harden off (acclimate) for winter. While this is not true with all wood plants, it is a good recommendation to follow unless you have knowledge of the metabolism of the specific plant during the fall and early winter months.

Soil-applied fertilizer, when broadcast or injected, is not effective if the soil is lacking in sufficient moisture. Watering prior to and after fertilizing is recommended. Remember that fertilizers are salts, which can dehydrate and kill roots unless diluted by water. It is always best to avoid fertilizing prior to warm, dry weather or other droughty conditions. The plant will be under enough stress at this time without the added stress of the applied fertilizer.

Fertilizing the plant with nitrogen can stimulate pests, such as adelgids, aphids, mites, psyllids, scale and whiteflies. When these pests are present, it is best to treat for them prior to or just following the application of fertilizer. Some diseases may increase in severity when nitrogen is applied, while other disease problems become less of an issue when the plant is stimulated into growth by the added nutrient. Check out the plant before you fertilize and advise owners of the proper maintenance of the trees based on the problem(s) you notice. This could result in the development of a long-term business relationship as a result of your professionalism and concern for the health of their plants.

It is very difficult to find individuals who consistently conduct soil tests prior to fertilizing trees, shrubs, flowers or other ornamentals. As a green industry professional, you could conduct random soil tests throughout the area or check with the local cooperative extension office or soils lab at your land-grand college to obtain the necessary information to fine-tune your fertility applications. This will help avoid problems and allow you to provide your customers with a more professional approach to plant care.

Dr Curtis E. Swift is a Colorado State University area extension agent for horticulture, based in Grand Junction, CO. He can be reached at

For more information on the care of woody plants, visit


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