Nutrient Management for Ornamentals

Introduction

Plants in their wild or 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 grow where they are best adapted or have a competitive advantage.

Ornamental 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 soil beneath ornamentals are sometimes needed to replenish essential mineral elements and to promote healthy growth.

However, fertilizer application should be viewed as one part of a nutrient management program for trees and within the context of a Plant Health Care program. For example, in landscapes and urban settings, it is important to select plant 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 to maintain soil fertility. These practices might include incorporating compost into soils at the pre-plant stage and applying organic mulches. Proper maintenance of soil fertility and attention to plant nutritional needs is at the heart of an effective Plant Health Care program.

Soil pH

A fertility program for trees begins with an analysis of soil pH, or level of acidity. Acidity reflects the concentration of hydrogen ions in the soil. Soil pH is measured on a scale of 1 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 because soil pH enhances the optimal availability of all essential nutrients for plant existence, and facilitates the binding of some elements that may be toxic to plants. At extremes in pH, many nutrients occur in forms unavailable for uptake by plant roots. Figure 1 shows the relationship between pH and the availability of elements essential to plant growth.

Soil pH also 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. Mineralization is the decomposition of organic matter by soil microbes with the subsequent release of mineral nutrients, including nitrogen. Sustaining a large microbial population in soils can help reduce disease problems due to antagonism between beneficial microbes and pathogenic fungi and bacteria. From a Plant Health Care perspective, reducing disease problems decreases the number of pesticide applications during the growing season.

Because New England soils are generally acidic, lime is an essential amendment for improving soil and plant health. Soil pH levels should be tested prior to any new plant installation. Typically, lime in the form of pulverized limestone is required to adjust pH upward, while sulfur is used to lower pH. These materials should be incorporated into soils prior to planting since surface applications are slow to affect pH levels. Most lime 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.

The rate at which limestone affects change in pH is largely a factor of the fineness of the limestone, (i.e. the finer the limestone particles, the faster is the liming reaction). The fineness of pulverized limestone is measured by the percentage of crushed limestone that passes a mesh screening. A mesh screen with 100 small openings in one square inch would be considered a 100 mesh screen. Almost all pulverized limestone will pass a 10 mesh screen and at least 50% will pass a 100 mesh screen. To facilitate ease of application, fine limestone particles are sometimes bound into pellets and sold as pelletized limestone.

Testing of soil pH is the only way to accurately determine the amount of limestone needed to bring pH to desired levels. Most soil testing laboratories measure liming needs in terms of liming index or buffer pH. Buffer pH differs from soil pH in that buffer pH takes into account the reserve acidity in a given soil, that is, the hydrogen ions that are held in reserve on clay and humus particles in the soil. Soil pH is simply a measure of the free hydrogen ions in the soil solution. The buffer pH tells us how much lime is needed to change the pH of a given volume of soil. Soils laboratories make liming recommendations using standard tables which correlate the buffer pH and the amount of lime (pure calcium carbonate) needed to change the soil pH to a target pH.

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 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 (Figure 4).

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 trees can consist of applications of nitrogen sources alone. Under normal conditions, complete fertilizers as mentioned above may be used every 4 or 5 years to ensure a supply of the other essential nutrients.

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 degrees F. 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.

Nitrogen in slow-release form may also be obtained from natural organic fertilizers. 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 the National Organic Farmers Association (NOFA). The term natural is used here to indicate fertilizers that are not synthesized, but which are derived from naturally occurring materials, whether they are organic or inorganic, in the chemical sense.

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 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 general 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 (Figure 5). 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 natural organic fertilizer would be almost 100% slow-release.

Compost, well-rotted manures, and sewage sludge may be used to fertilize 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. However, that is not always true when buying bulk quantities of compost. As such, always request a nutrient analysis of the product. These types of 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 fertility management program. As a guideline, apply finished compost at a rate of no more than 4 cubic yards per 1000 square feet (3/4 inch thick layer of compost).

Know How to Read the Fertilizer Bag

Knowing how to read a fertilizer label is as important as understanding a pesticide label. A horticulture professional should be able to find the necessary information from the guaranteed analysis on the bag.

Every bag or container of fertilizer is required by law to carry a label which shows the composition of the material. By understanding the fertilizer label you can recommend proper materials and accurate rates for different plants and different size areas. Following are two typical formulas, complete with source of materials, as they would appear on the label of a commercial fertilizer.

Some commercial fertilizers provide only one or two of the primary elements. Their formulas would be expressed as follows:

• Urea 45-0-0
• Ammonium Nitrate 33.5-0-0
• Ammonium Sulfate 21-0-0
• Ammonium Phosphate 16-20-0
• Superphosphate 0-18-0
• Potassium Chloride 0-0-60
• Potassium Sulfate 0-0-50

These generally are used when it is necessary to supplement other fertilizers, although by the application of some arithmetic and common sense, the user can make up almost any analysis of a complete fertilizer. Fortunately, this is seldom necessary as the most needed combinations are already available in packaged form.

Following is a typical guaranteed analysis, complete with source of materials, for a commercial 6-8-8 fertilizer.

The fertilizer label provides a wealth of information. First, the analysis is obvious. This example is a 6-8-8 fertilizer. From the label, you can determine how much plant food is available. A 100 pound bag of 6-8-8 would contain 6 pounds of N, 8 pounds of phosphorus (available P2O2) and 8 pounds of potash (soluble K2O). The remaining 78 pounds would consist of the other components and inert filler materials.

The label also tells what kinds of nitrogen are present, the chlorine content, secondary plant foods, and the source of the nutrients (what they are derived from). The form of nitrogen is very important. Nitrate, ammoniacal, and water soluble organic nitrogen are fast acting chemical forms of nitrogen. They produce quick "green-up" and growth in plants but also cause burning if used at high rates. The slow acting, long lasting organic nitrogen will be listed under water-insoluble nitrogen.

Fertilizing Trees and Shrubs in the Landscape

Fertilizing the soil beneath trees and shrubs replenishes the necessary mineral elements, and results in faster growth and increased vigor. Some trees do not need to be fertilized. However, trees in poor soil or restricted growing area or of poor vigor need additional nutrients. Most shrubs will also benefit from periodic applications of fertilizer.

Fertilizer applications must be made with thought and care. Excessive fertilization can stimulate lush, weak growth, which makes trees susceptible to wind and ice damage, as well as certain diseases.

Rates of Application, Pre-planting Application

Pre-plant 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 trees 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 should be paid to phosphorus levels in soils planted to needled evergreens, since their growth response to nitrogen is greatest when phosphorus levels are high.

Pre-plant incorporation of potassium can provide sufficient reserves to support plant growth for several years in soils 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 pre-plant nutrient should be in a slow-release form or natural organic form.

Post-planting Applications

Rates of fertilizer application are typically based upon the amount of nitrogen in the fertilizer, since nitrogen is the mineral element most responsible for tree and shrub growth. For annual maintenance in New England, the following rates of nitrogen application (pounds of actual nitrogen per thousand square feet of surface area per year) are recommended:

Reduce the amount of fertilizer applied at any one time on sites with shallow or coarse soils so as not to burn the plant’s roots. Using fertilizers with slow-release forms of nitrogen will help reduce the possibilities of root injury in such situations. Rates of nitrogen application should be also 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 potential for leaching.

Rates of nitrogen application should also be adjusted according to levels of soil organic matter since the natural mineralization of soil organic matter will release nitrogen. 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 general, at a pH between 6 and 7, it can be assumed that 1/4 to 1/2 pound of nitrogen per 1000 square feet is being made available per year for each 1% of organic matter in the soil. Therefore, a soil with 4% organic matter can contribute from 1 to 2 pounds of nitrogen per 1000 square feet per year. That is typically enough nitrogen to support healthy growth of established plantings.

In coastal areas where organic matter content of sandy soils is often in the range of 1 to 2%, addition of fertilizer may be necessary. In such cases, use fertilizers with at least 50% of the nitrogen in water-insoluble (WIN) or slow-release form. However, be cautious in applying nitrogen fertilizer to soils low in organic matter. Applying high rates of nitrogen on 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.

Fertilizer Math

Calculating the amount of a given fertilizer formulation to apply per 1000 sq. ft. is based on both the results of a soil test and the % nitrogen 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

Determination of the correct amount of fertilizer to apply to trees and shrubs is best done with the root area measured in square feet. 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.

A. Area of a square or rectangle. To measure the root area of a tree or shrub growing in a confined area that is a square or rectangle, measure the length and width of the area to be fertilized and multiply the two to get the area in square feet.

Example:
length x width = square feet
60 ft. x 50 ft. = 3,000 square feet

B. Area of a circle. To measure the area of root coverage for a tree or shrub in a non-confined site, calculate the area of a circle. Measure the radius in feet from the trunk out to the drip line, or beyond for larger specimens.

Example: (For a circle 62 ft. in diameter)
3.14 x r2 = square feet of a circle
3.14 x (31 x 31) = 3017 square feet

C. Area of a triangle. To measure the root area of a tree or shrub growing in a confined area that is triangular in shape, measure the length (b) of one side and the height (h) of the triangle as pictured below. or

Example:
Area of the triangle is 1/2 × b × h
For a triangle having base length of 52 ft. and a height of 42 ft.
1/2 ×52 × 42 = 1092 square feet

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 is the method most often used by landscape professionals today 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 to 3 feet apart depending upon pressure and about 6 inches deep. Slow-release forms of liquid injection fertilizers are also available.

Drill hole. Rarely used on a commercial scale, this technique requires drilling holes into the soil and distributing granular fertilizer evenly among the holes. Air spades may also be used by a professional to create the holes.

Holes are drilled to depths of 6 to 8 inches and are spaced 2 to 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 to 3 feet beyond the drip line. Only fertilizers with a high percentage of water insoluble nitrogen should be applied to the holes in order to prevent injury to roots wounded during the drilling. The holes may be left open or filled with compost, peat or other organic material. These porous materials should be firmly packed to reduce potential for leaching of fertilizer to ground water. The drill hole method should be used where high fertilizer rates 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. This method is quick, easy and inexpensive, and recent studies have shown it 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.

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 are 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 ornamentals. Woody plants growing in rich soils with continual replenishment of nutrients from decomposition of organic matter may not need regular fertilizing. However, plants that show abnormal leaf size and color, little or no annual growth, or significant amounts of dead wood within the plant, should be fertilized annually until growth and appearance of the plant appears normal.

Timing of Application

Recent studies have shown that nitrogen uptake by ornamental plants increases after flushes of shoot growth are completed. During shoot growth, more energy in the form of carbohydrates is directed to shoots rather than roots. When shoot growth slows, carbohydrates are redirected to plant roots to support root development and uptake of nitrogen. Furthermore, nitrogen uptake is least when trees are leafless. This latter observation would argue against early spring and fall applications of nitrogen. Almost all of spring growth is supported by nitrogen that was taken up during the previous growing season and stored by the tree. Based on the results of these studies, it can be concluded that nitrogen fertilizers are best applied after flushes of growth and before leaf drop. Not surprisingly, this pattern of nitrogen uptake corresponds with the maximum levels of nitrogen release via mineralization in forest habitats.

Summary

A nutrient management program for plants is more than just the application of fertilizer. A sustainable program of providing an adequate supply of mineral nutrients for plant uptake includes managing soil pH and levels of soil organic matter both in the pre-plant and post-plant stages. When applications of fertilizer are necessary, horticulturists should consider the specific nutrients required, the various sources of these nutrients, quantities to be applied, methods of application, the timing of application.

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Revised: 08/2011