1998 Iowa Turfgrass Research Report
Fertilizing Trees and Shrubs
Jeff Iles
You would think after decades of research, thousands of anecdotal reports from the field, and the endless procession of articles in print, there would be unanimous agreement regarding the importance of tree and shrub fertilization. Instead, confusion, controversy, and heated words exchanged in "letters to the editor" sections of trade journals indicate the apparent lack of consensus among landscape maintenance professionals when it comes to applying fertilizers.
Some use fertilizers as preventive medicine, slinging mineral elements around the landscape in an effort to ward off marauding insect and disease pests. Others use fertilizer as a rescue treatment for plants besieged by any number of biotic and abiotic stresses. Still others adopt a policy of nonintervention, avoiding all together the application of fertilizer. So, who's right? As is often the case, everybody and nobody. Yes, vigorous plants supplied with optimal levels of mineral elements are better equipped to handle stress, but fertilizer cannot remedy poor growth resulting from causes other than nutrient deficiencies such as compacted soils, poor drainage, restricted rooting areas, mechanical injury, or competition from turfgrass. In fact, the only valid reason to fertilize landscape plants is to correct nutrient deficiencies.
Why Fertilize?
Have you ever been asked this one? "If trees in the forest are able to thrive without the addition of fertilizer, why should I pay you to fertilize the trees and shrubs in my yard?" The difference is, trees in their natural setting benefit from a layer of decayed leaves, twigs, and other organic matter that accumulates on the forest floor. Small, absorbing roots of trees and other woody plants colonize this rich, well-aerated, nutrient-laden soil, and derive everything they need from the cycling of organic matter production and decomposition. But in urban and suburban landscapes where the environment has been drastically altered from its original state, mineral cycling does not occur and supplemental fertilization often becomes necessary.
What a Plant Needs
Fertilizers often are incorrectly referred to as "plant food." More precisely, fertilizers provide essential mineral elements that trees and shrubs need to produce their own food (sugars) through the process of photosynthesis, and to carry out other important biological functions.
For proper growth and development, woody plants must obtain 17 essential mineral elements from their environment. The macronutrients, so called because plants require them in large amounts, include hydrogen, carbon, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. Micronutrients or trace elements are needed in smaller amounts and include iron, chlorine, manganese, zinc, boron, copper, molybdenum, and cobalt. Throughout a plant's life, mineral elements are required for growth and maintenance; however, all woody plants do not have the same mineral requirements. For example, a beech tree requires more calcium, potassium, and phosphorus than most pines.
Importance of Soil pH
The availability of a number of mineral elements, particularly phosphorus and the micronutrients, is directly influenced by soil pH. Soil reaction, expressed as pH, refers to the acidity or alkalinity of a soil or the relative proportion of hydrogen (acid) and hydroxide (alkaline) ions. Equal concentrations of the two produce a neutral reaction (pH 7.0). As a soil becomes more acid, its pH decreases; as it becomes more alkaline, its pH increases.
Certain trees (pin oak, red maple, river birch, etc.) growing on high pH soils frequently display iron or manganese deficiency symptoms (yellow leaves with green veins). Most woody plants tolerate a wide range in pH (5.5 to 7.5) particularly if the soil is well-drained, but a soil pH between 6.0 and 6.5 is thought to be best. Soils with pH levels above 7.0 can be successfully treated with elemental sulfur (96%) to lower soil pH and improve plant growth; however, a better solution might be to use plants that grow naturally on alkaline soils. Trees like hackberry, sycamore, catalpa, honeylocust, blue ash, Kentucky coffeetree, bur oak, and chinquapin oak are known to tolerate soils having a pH of 8.0 or higher!
Knowing Which Plants to Fertilize
Soil tests can help determine when to fertilize trees and shrubs. Unfortunately, researchers have not been able to determine optimum mineral element levels for many landscape plants. Therefore, the landscape manager must rely on deficiency symptoms to identify nutritional problems. A few common deficiency symptoms include, (1) unusually small leaves, (2) abnormal leaf color (light green or yellow), (3) poor annual shoot elongation, (4) dead twigs at ends of branches, and (5) general lack of vigor. If these symptoms are present and are not the result of other factors such as drought, disease, root injury, herbicide damage, or mechanical injury, then fertilization is probably warranted.
How Much Fertilizer to Apply?
The soil around woody plants is most commonly supplemented with nitrogen, phosphorus, and potassium. But because nitrogen is so important to plant growth and is usually the element most likely to be deficient, most fertilizer recommendations are keyed to this vital element.
Recently transplanted and/or poorly established plants have minimal fertilization needs. In fact, the fate of recently installed landscape plants is more closely tied to proper water management than to fertilization. But once established (the establishment period can last from months to years), fertilization can stimulate growth and improve the appearance of landscape plants. A rate of 3 pounds of actual nitrogen/1,000 square feet/year will satisfy the nutritional needs of most trees and shrubs. Dwarf and slow-growing trees and shrubs require only half this rate. Mature plants and those compromised by root injury will benefit from fertilization, but rates are usually in the neighborhood of 1 to 2 lbs. N/1,000 square feet/year. But remember, fertilizer is not a rescue treatment for seriously injured or declining plants.
When to Apply?
Fertilizers should be applied when environmental conditions favor root activity. Of all the environmental conditions, soil moisture and temperature have the greatest influence on root growth. The most intense root growth occurs when soils are moist and when temperatures are between 68 and 84 F.
Of course, correct timing of fertilizer application varies with geographic region. But in general, late summer or early autumn is usually the preferred time to apply fertilizer. At this time, vegetative growth has ceased, the soil is warm, carbohydrate supplies are at their highest, daytime high temperatures are moderating, and moisture is usually not limiting growth.
Another benefit to fall fertilization is early shoot growth in the spring depends almost entirely on nitrogen absorbed the previous summer and autumn. Nitrogen can still be applied in the spring, however, it must be in the root zone about one month before growth begins for it to have any effect on early season growth. Fertilizer applied later in the spring isn't necessarily wasted, but it may not produce the desired growth until the following year.
To prevent loss or leaching of nitrogen from the root zone of plants in sandy, well-drained soils, split applications in spring and fall can be used. But if leaching is not a problem, no benefit has been shown from applying the same total amount of fertilizer in more than one application.
Other macronutrients can be applied when deficiencies are discovered, regardless of calendar date. But when treating micronutrient deficiency symptoms (for example, iron or manganese), water-soluble chelates of iron or manganese should be applied to the soil in the fall to have a positive impact on early season shoot growth.
Application Methods
Fertilizers can be, (1) broadcast on the soil surface, (2) placed in holes in the soil, (3) dissolved in water and injected into the soil, (4) sprayed on the foliage, (5) driven into the soil in fiber impregnated spikes, or (6) implanted or injected into the plant. The method chosen will depend on the nature and slope of the soil surface, mineral elements being applied, presence of competing vegetation (turfgrass most commonly), and equipment available. No single method is best suited for every situation, but for the fertilizer to be effective, it must reach, or be placed near the root zone.
Because the root zone of most trees and shrubs lies just below the soil surface (4 to 12 inches deep), surface application (broadcasting) is the easiest, least expensive, and most effective method for applying mobile elements like nitrogen. But when applying immobile elements like phosphorus, working on severe slopes, or trying to place mineral elements below a highly competitive groundcover like turfgrass, granular or liquid formulations applied 8 to 12 inches deep are useful.
Fertilizers sprayed on the foliage are effective for correcting micronutrient imbalances (primarily iron and manganese). Plant response is quick, but applications must be repeated, at the very least, every season. Fertilizer stakes, spikes, and tablets driven into the soil at regular intervals around plants are popular with homeowners because they are easy to apply, but they are expensive, cause compaction where the spike is inserted, and provide poor lateral distribution of mineral elements in the root zone.
Trunk implants and injections have become the method of choice for correcting micronutrient deficiencies in trees over 4 inches in diameter when they have not previously responded to soil treatments. These methods deliver mineral elements directly into the vascular system of the tree and provide a quick and positive response. Implants and injections should be reserved for high value trees because of the time and expense associated with these methods. Drilling holes also may cause decay around the injection or implantation site.
A fertilization program tailored to the specific needs of every tree and shrub in the landscape is only one piece of the maintenance puzzle for landscape technicians. Without proper plant selection and installation techniques, timely watering, mulching, appropriate pruning, weed control, and a host of other maintenance activities, the functional and aesthetic benefits of the landscape deteriorate. Finally, fertilizers should be applied only when a nutrient deficiency has been identified. Fertilizing "to be safe" wastes time, money, and could contribute to contamination of surface and groundwater supplies.
Further Reading
Darr, B. 1996. Tree fertilization: A world of options. Arbor Age 16(2):14-17.
Ferrandiz, L.S. 1990. Tree fertilization techniques. Grounds Maintenance 25(6):10-14.
Gilman, E.F. 1990. Tree root growth and development. I. Form, spread, depth and periodicity. J. Environ. Hort. 8(4):215-220.
Harris, R.W. 1992. Arboriculture: Integrated management of landscape trees, shrubs, and vines. Prentice-Hall, Englewood Cliffs, NJ.
Lanphear, L.S. 1996. Stimulating healthy growth. Arbor Age 16(9):12-14.
Shaw, M. 1998. Tree nutrition 101. Arbor Age 18(3):16-19.
Ware, G. 1990. Constraints to tree growth imposed by urban soil alkalinity. J. Arboric. 16(2):35-38.
Warren, S. 1995. Nitrogen fertilizer and beyond. Arbor Age 15(4):10-12.
Watson, G.W. 1992. Trees and shrub
fertilization. Grounds Maintenance 27(1):42-46.
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