Fertilizing Trees and Shrubs
Trees and shrubs growing in their natural habitats rarely display symptoms of nutrient deficiency. This is due not only to the natural recycling of nutrients that occurs in nature, but also to the fact that plants in the wild typically grow only where they are best adapted or have a competitive advantage.
Nursery, street tree, and landscape plantings are, for the most part, an artificial habitat. Soils may be vastly different from those of the native habitat of a given plant, and nutrient recycling systems may be altered or diminished as a result of planting schemes (planting in turf areas) or maintenance practices (collection of fallen leaves). For these reasons, periodic applications of fertilizer to the soil beneath ornamental trees and shrubs are sometimes needed to replenish essential mineral elements and to promote healthy growth.
In landscapes and field nurseries, it is important to select species that are best suited to the site. A program of cultural practices that sustains or replenishes soil organic matter and nutrients should also be established. These practices might include incorporating compost into soils at the preplant stage, applying organic mulches, and cover cropping. Proper maintenance of soil fertility and attention to plant nutritional requirements is at the heart of an effective IPM or Plant Health Care program.
A fertility program for woody plants begins with obtaining an analysis of soil pH (or level of acidity). Soil pH is measured on a scale of 0 to 14. Soils with a pH below 7 are acidic while those above 7 are alkaline. Adjusting pH levels is important not only because specific plants grow best within a certain range of pH, but also soil pH affects the availability of both major and minor nutrient elements. Furthermore, soil pH influences the level of microbial activity in soils. Microbes involved in mineralization of organic matter are most active between a pH of 6 and 7. At extremes in pH, many nutrients occur in forms unavailable for uptake by plant roots. Figure 1 at right shows the relationship between pH and availability of elements essential to plant growth.
Analysis of soil pH levels should be routinely made prior to any planting in nursery soils or at landscape sites. Typically limestone is required to adjust pH upward while sulfur is used to lower pH. It is best if these materials are incorporated into soils prior to planting, since surface applications are slow to affect pH levels. Most liming and sulfur recommendations are based on the assumption that the material is worked in to depths of 8 inches. Deeper incorporation of either limestone or sulfur will require adjustments in rates to accommodate larger volumes of soil.
What to use?
Basic plant nutrition involves the uptake of sixteen mineral elements essential to plant growth. In addition to carbon, hydrogen and oxygen, which are obtained from air and water, the elements nitrogen (N), phosphorus (P) and potassium (K) are required in greatest abundance. Research in woody plant nutrition has shown however that nitrogen is the element that yields the greatest growth response in trees and shrubs. For this reason, high nitrogen fertilizers with N-P-K ratios of 4-1-1, 3-1-1 or 3-1-2 are generally recommended for feeding established woody plants. These include fertilizers with analyses such as 8-2-2, 15-5-5, 24-8-16 and similar formulations. The analysis refers to % nitrogen, % phosphorus (as P2O5) and % potassium (as K2O) in the fertilizer.
Phosphorus, potassium and essential elements other than nitrogen are slow to be depleted from soils. Provided these nutrients are at recommended levels, a fertilizer program for established woody plants can consist of applications of nitrogen sources alone. Complete fertilizers should only be used for trees and shrubs if a soil and/or leaf tissue test demonstrates need. Typically, in New England soils there are sufficient amounts of phosphorus (P) and potassium (K).
Application of slow-release forms of nitrogen provide the most efficient use of this nutrient because root growth and nutrient absorption can occur anytime soil temperatures are above 40° F. Slow release nitrogen fertilizers are preferred over quick release water soluble fertilizers because they provide nitrogen more slowly resulting to more uniform growth. They also have lower potential impact on the environment. Quick release nitrogen fertilizers should only be used if the objective of fertilization is to restore leaf nitrogen content and green-up the plants.
On fertilizer labels, slow-release nitrogen is represented as Water Insoluble Nitrogen or WIN. Isobutylidene diurea (IBDU), ureaformaldehyde, sulfur-coated fertilizers (e.g. Sulfur Coated Urea) and resin-coated fertilizer are commonly used sources of slow-release nitrogen or WIN.
Fertilizer Math: Calculating the amount of a given fertilizer fomulation to apply per 1000 sq. ft. is based on both the results of a soil test and the % nitrogen (N) in the bag. Use the following method:
Example: Assume the fertilizer to be used is a 30-10-10 formulation with 30% nitrogen.
Area Method: In the past, determination of the correct amount of fertilizer to apply was based on the DBH (Diameter at Breast Height) of the tree or on the root area measured in square feet. Today, only the square foot method is recommended, since this reduces the risk of over-fertilization. When calculating the area of a tree or shrub bed, only measure the area where fertilizer can actually be applied. Do not include areas such as the driveway or sidewalk.
Nitrogen in slow-release form may also be obtained from natural organic fertilizers. Because of a lack of industry standards for the definition of "organic" and "natural" a great deal of variability exists among these products in terms of their composition and analysis. For those adhering strictly to "organic" methods, the label of a given product should be examined for organic certification either by the state agriculture department or organizations such as NOFA (National Organic Farmers Association). The term "natural" is used here to indicate fertilizers that are not synthesized but are derived from naturally occurring materials.
Before applying natural fertilizers, the user must be aware of the nutrient analysis, i.e. the amount (by percent) of N, P and K, and the rate of release of the nutrients. Often mineral elements in natural materials, whether organic or inorganic, are released very slowly. This can benefit plants if nutrient release is steady and continuous over a long period of time. However, these materials may be of little immediate value in correcting nutrient deficiencies. Generally, slow-release materials must be applied in large amounts so that a balance exists between the rate of release and the amount of nutrients available at a given time for absorption by plant roots. Unfortunately, objective information on rates of release of mineral elements from natural materials is often lacking, in part because rate of release is a function of highly variable environmental factors.
Fertilizer labels do contain information on how fast the nitrogen will be released. The WIN (Water Insoluble Nitrogen) number will list the percent of nitrogen that is insoluble or slow-release. The WIN number is compared to the percent of total nitrogen in the fertilizer. As an example, a fertilizer with a total of 30% nitrogen and a WIN percent of 15 (50% of the total nitrogen) would be considered slow-release. That is, when the WIN is equal to or more than 50% of the total nitrogen, the nitrogen is considered to be slow-release. If WIN is less than 50% of total nitrogen, the nitrogen is considered to be fast-release. A true organic fertilizer would be almost 100% slow-release.
Compost, well-rotted manures and sewage sludge may be used to fertilize woody plants, although their nutrient composition is quite variable. Those forms of compost, manure, or sludge that are sold commercially as fertilizers will have nutrient analyses listed on the product package. When buying bulk quantities of compost materials, always request a nutrient analysis of the product. These materials can supply some nutrients and contribute significant amounts of organic matter to improve soil structure and fertility and should be a part of a soil and fertility management program. Compost guidelines for the Northeast suggest applying finished compost at a rate of no more than 4 cubic yards per 1000 square feet (3/4 inch thick layer of compost).
Rates of application
Preplant incorporation of phosphorous and potassium into soils should be based on soil test results. It is advisable to incorporate these nutrients so that they will be in the root zone when woody ornamentals are planted. This is especially important for those mineral elements that are not very mobile in soils. Phosphorus, for example, moves very slowly, as little as one inch per year from the site of application. Superphosphate (0-20-0), triple superphosphate (0-40-0), ammonium, and potassium phosphates are commonly used forms of phosphorus fertilizer. Rock phosphate is a natural source of phosphorus, but rates of application should be adjusted to accommodate the very slow rate of release of the nutrient. Particular attention must be paid to phosphorus levels in soils planted to needled evergreens since their growth response to nitrogen is greatest when phosphorus levels are high.
Preplant incorporation of potassium can provide sufficient reserves to support plant growth for five years in soils that are high in organic matter or clay content. When dissolved in soil water, potassium is a positively charged chemical (cation) and binds to particles of clay and organic matter. With high levels of clay and organic matter, potassium can be added in a single application. More frequent applications of this nutrient are necessary in sandy soils because they have less ability to bind potassium. Common fertilizer forms of potassium include potassium chloride (muriate of potash), potassium sulfate, potassium nitrate, and natural materials such as kelp meal, greensand and alfalfa meal.
Rates of application of phosphorus, potassium, and nutrients other than nitrogen should always be based upon soil test results. Any nitrogen applied as a preplant nutrient should be in a slow-release form or natural organic form.
Rates of fertilizer application are typically based upon the amount of nitrogen in the fertilizer since nitrogen is the mineral element most responsible for vegetative growth. For annual maintenance, it is recommended that a tree receive 1 to 3 pounds of actual N per 1000 sq. ft. of surface area (see Fertilizer Math above). The actual amount of a fertilizer to apply for maintenance of woody plants may be determined by the area method (see Area Method above).
Reduce the amount of fertilizer applied at any one time to trees on shallow, sandy, or poor sites, so as not to burn the plant's roots. Using fertilizers with slow-release forms of nitrogen will also help reduce the possibilities of root injury in such situations. Rates of nitrogen application should be adjusted on sites where there is a high potential for ground water contamination from nitrate leaching. On such sites, nitrogen application rates of 1 lb N/1000 sq. ft. or less would be advisable. Several applications at these reduced rates may be made during the growing season if needed for improving plant health. Again, use of slow-release forms of nitrogen can reduce the potential for leaching.
Rates of nitrogen application should also be adjusted according to levels of soil organic matter. Applying high rates of nitrogen to soils low in organic matter will accelerate depletion of the organic matter and in the long run reduce the fertility and structural integrity of the soil. Analysis of organic matter levels may be requested when submitting soil samples for testing. Soil organic matter levels of 4% or greater are desirable. In coastal areas where organic matter content of sandy soils is often in the range of 1-2%, use fertilizers with at least 50% of the nitrogen in water-insoluble (WIN) or slow-release form. In general, at a pH between 6 and 7, it can be assumed that 1/4-1/2 pound of nitrogen per 1000 square feet is being made available per year for each one percent of organic matter in the soil. Therefore, a soil with 4% organic matter can contribute from 1-2 pounds of nitrogen per 1000 square feet per year. That is typically enough nitrogen to support healthy growth of woody plants.
Methods of application
There are several methods of applying fertilizers to trees and shrubs. The method selected depends upon soil characteristics, site factors, cost, and type of nutrients to be applied.
- Liquid soil injection: This is the method most often used by professional arborists because it is quick, easy, and also leads to rapid uptake of nutrients. It utilizes high pressure injection of liquid fertilizer into the soil. Injection points should be 2-3 feet apart depending upon pressure and about 8-12 inches deep. Slow-release forms of liquid injection fertilizers are also available.
- Drill hole: This technique requires drilling holes into the soil and distributing granular fertilizer evenly among the holes. Holes are drilled to depths of 8-12 inches and are spaced 2-3 feet apart in concentric circles around the tree, beginning at a point about 1/3 the distance from the trunk to the drip line and extending 1-3 feet beyond the drip line. While rarely used today on a commercial scale, this method is effective in opening heavy compacted soils, allowing fertilizer, water and air to reach the root zone. The holes may be left open or filled with compost, peat or other organic material. The drill hole method should be used where high fertilizer rates or fertilizers with a high salt index create a potential for injury to fine turf.
- Surface application: Granular forms of fertilizer may be spread by hand or mechanical spreader over the surface of soil around trees and shrubs. This method is quick, easy and inexpensive, and recent studies have shown this method to be as effective in supplying nutrients to plant roots as other techniques. It is particularly appropriate for applying fertilizers to mulched areas and shrub borders. A tree growing in a lawn area will utilize nutrients from surface applications of fertilizer made to the lawn and may not need additional fertilizer.
- Fertilizer spikes/stakes: With this method, solid rods of a pre-measured amount of fertilizer are placed in holes in the soil around woody plants. Wide spacing of holes and slow lateral distribution of nutrients limit the effectiveness of this technique. It is not recommended.
- Foliar fertilization: This technique entails spraying liquid fertilizers onto the foliage of plants. It is used primarily as a "quick fix" for minor nutrient element deficiencies. Foliar feeding is not effective in supplying essential nutrients in quantities necessary for satisfactory growth. The most effective time to spray foliage with micronutrient solutions is just before or during the growth period.
- Tree trunk injections: Injections of nutrients directly into a tree is used almost exclusively to correct minor element deficiencies, e.g. iron, manganese and zinc. This technique may also be used in urban settings where root or surface applications of fertilizers are not practical.
Frequency of application
Frequency of application depends on the general vigor and growth of the plant, with the exception of newly planted trees and shrubs. Woody plants growing in rich soils with continual replenishment of nutrients from decomposition of organic matter may not need regular fertilizing. However, plants that are in a nursery production cycle, as well as landscape plants that show either abnormal leaf size or color, little or no annual growth, or significant amounts of dead wood within the plant, should be fertilized annually.
Time of application
Fertilizers are best applied in late August through September. Root absorption of nutrients is very efficient in late summer and remains so until soil temperatures approach freezing. Nitrogen that is absorbed in fall will be stored and converted to forms used to support the spring flush of growth. The next best time to fertilize woody plants is early spring prior to initiation of new growth.
Trees and shrubs should not be fertilized during times of drought stress or when they are showing signs of water stress unless irrigation is available. Plants do not absorb nutrients without adequate water. Some fertilizers may also damage roots if water is lacking.
Written by: Ron Kujawski and Dennis Ryan