A Gardener's Guide to Fertilizing Trees and Shrubs
| Fertilizer Terms | Types of Fertilizer
| Natural Fertilizers | Fertilizing
New Plants | Fertilizing Established Plants
| Soil Testing | General Requirements
| Time and method of application
Addition of the correct amount of fertilizer can promote healthy flower production and foliage growth while an excessive fertilizer application can decrease plant health and can lead to decline and death. Over application or incorrect application of fertilizer can contribute to polluting our rivers, streams, lakes, and estuaries. Excess fertilizer can increase the likelihood of some plant diseases. Fertilizing plants that have already outgrown their allotted space can only lead to more pruning. A moderate rate of growth and good, green color is desired for most woody plants. Excessive vigor, which is evident by lush, green leaves and long shoot growth is often undesirable. Such plants are more susceptible to injury by cold in winter, are more likely to be broken during wind and ice storms, and usually will require more pruning than plants making moderate growth.
All too often gardeners assume
that if a plant is not doing well they should fertilize to correct the situation.
Fertilization may be helpful but only after the problem causing poor growth
has been corrected. Plants which are growing poorly will exhibit one or more
of the following symptoms:
These symptoms of poor growth may be caused by inadequate soil aeration, moisture, or nutrients; adverse climatic conditions; incorrect pH; or disease. Recently transplanted trees and shrubs often will not resume a normal growth rate until the original root system is reestablished. Plants disturbed by construction within the past five to ten years may be in shock and exhibit limited new foliage growth. Do not assume that an application of fertilizer will quickly remedy any problem which is encountered, in many cases it can make existing problems worse. You should attempt to determine the specific cause in each situation and apply corrective measures.
Plants require 17 elements for normal growth. Carbon, hydrogen, and oxygen are found in air and water. Nitrogen, potassium, magnesium, calcium, phosphorous, and sulfur are absorbed from the soil. The latter six elements are used in relatively large amounts by the plant and are called macronutrients. There are eight other elements that are used in much smaller amounts; these are called micronutrients or trace elements. The micronutrients include iron, zinc, molybdenum, manganese, boron, copper, cobalt, and chlorine. The nutrients that are most likely to limit plant growth are nitrogen, phosphorus, and potassium.
Nitrogen is a primary component
of proteins and is a part of every living cell. This nutrient is usually more
responsible for increasing plant growth than any other nutrient. Shortages can
cause slow growth, reduced leaf size, yellowing, short branches, premature fall
color and leaf drop, and increases the likelihood of some diseases. An over
abundance can cause excessive shoot and foliage growth, reduced root growth,
low plant food reserves, and increased susceptibility to environmental stresses
and some plant diseases.
Nitrogen is a mobile nutrient it is in constant motion. Nitrogen applied to the soil can be used by plants, washed off the soil surface, lost to the air as a gas, or leached through the soil). Nitrogen from granular fertilizer can enter streams from surface runoff. Nitrogen loss is higher when a heavy rain immediately follows a surface application of fertilizer, especially on sloped areas. Incorporating fertilizer into the soil or lightly watering (1/4 to 1/2 inch) after making a surface application will reduce the amount of nitrogen loss.
Phosphorus (P) plays a role in photosynthesis, respiration, energy storage and transfer, cell division, and cell enlargement. It promotes early root formation and growth, and the production of flowers, fruits, and seeds. Many of our urban soils are low in phosphorus. Cultivated farm land often has a high phosphorus level from years of fertilization. In these cases, the addition of more phosphorus is not going to increase yields and can potentially harm the environment.
When applied as fertilizer, phosphorus is quickly bound by soil particles. Phosphorus is extremely immobile in soils (except sand); it moves about 1 inch from its original placement. Unless phosphorus is incorporated into the soil, watered in, or applied as a band, plants may not be able to use it.
Potassium in involved in many plant growth processes; it is vital to photosynthesis and helps regulate water in plants. Potassium fertilization helps plants overcome drought stress, increases disease resistance, and improves winter hardiness. Potassium can be leached through the soil by water, but not as quickly as nitrogen.
Nutrient uptake by plants
Nutrients in the soil can be in a solid form (granular fertilizer, organic matter), attached to the soil particles or dissolved in soil water. For an element to be absorbed by plants it must be in a form the plant can use, and present (dissolved) in soil water. Water and oxygen are required for nutrient movement into plant roots. Without adequate oxygen in the soil, there is limited nutrient absorption.
Anything that lowers or prevents the production of sugars in the leaves can lower nutrient absorption. If the plant is under stress due to low light or extremes in temperature, nutrient deficiency problems may develop even though adequate nutrients are available in the soil solution. Diseased or damaged roots, improper soil pH, water logged sites, and plantings that are too deep can result in inefficient nutrient absorption. Adding fertilizer under these conditions will not enhance plant growth, and may actually cause plant damage.
The stage of growth or how actively the plant is growing may also affect the amount of nutrients absorbed. Many plants go into a rest period, or dormancy, during part of the year. During this dormancy few nutrients are absorbed.
Fertilizer: Any material used to supply one or more of the essential plant nutrient.
Fertilizer ratio: The relative proportion of N, P, and K. 16-4-8 fertilizer has a ration of 4:1:2 or 4 parts nitrogen to 1 part phosphorus to 2 parts potassium.
Balanced fertilizer: A fertilizer containing equal parts of each major element, i. e. 10-10-10.
Complete fertilizer: A fertilizer containing nitrogen, phosphorus, and potassium. Examples of commonly used complete fertilizers are 10-10-10, 16-4-8, and 12-4-8.
Incomplete fertilizer: A fertilizer missing one or two of the major elements, i. e. 0-20-0.
Fertilizer analysis: The minimum amount of each element in a fertilizer as stated on the label.
Weed and feed fertilizers: A combination of fertilizer and herbicide. They are often used on lawns to prevent certain weeds from germinating, or to kill existing broadleaf weeds.
Chelate: Chemical compounds that help hold metal ions (such as iron) in solution so the plant can adsorb them more readily.
High analysis: A fertilizers containing 30 percent or more active nutrients, i. e. ammonium nitrate 33-0-0. The cost per bag is usually more but the cost per pound of nutrient is less (therefore the cost for fertilizing a given area is less).
Types of Fertilizers
All fertilizers are labeled with three numbers, giving the percentage (by weight) of nitrogen (N), phosphorus (P2O5), and potassium (K2O). A 100 pound bag of fertilizer labeled 0-20-10 has 0 pounds of nitrogen, 20 pounds of phosphorus, 10 pounds of potassium, and 70 pounds of filler. Filler is added to make the fertilizer easier to spread and to reduce the likelihood of burning plants with too much fertilizer. A fertilizer may contain secondary nutrients or micronutrients not listed on the label.
The diversity of fertilizer types and brands can be overwhelming when you go to make a purchase. Generally speaking, the timing and rate of application are more critical than which fertilizer you purchase. In regards to water quality, the most important issue is the type of nitrogen in the fertilizer you purchase. Most fertilizers contain nitrogen in a quick release form (10-10-10 and 8-8-8), others are in a slow release form (16-4-8, 12-4-8) some are a combination of both. Quick release forms of fertilizer are immediately available, but do not last as long and can cause plant damage if a large application is made.
Ammonium nitrate, ammonium sulfate, calcium nitrate, and potassium nitrate are all water soluble, quick release forms of nitrogen. The nitrogen becomes available as soon as it come in contact with soil water. Urea is an organic form of nitrogen but it is also quickly converted to nitrate nitrogen. High application rates combined with high irrigation or rain fall can result in large amounts of nitrogen being leached below the root zone.
A slow-release fertilizer
releases nutrients at a rate that makes them available to plants over a long
period. Slow-release fertilizers need not be applied as frequently as other
fertilizers and there is less potential of leaching into ground water.
The initials W.I.N. and W.S.N. on fertilizer labels stand for water insoluble nitrogen and water soluble nitrogen, respectively. The water soluble nitrogen dissolves readily and is usually in a very simple form, such as ammonia nitrogen or nitrate nitrogen. Water insoluble nitrogen is referred to as a slow release nitrogen source. Nitrogen which will not dissolve readily is usually in an organic form of nitrogen (with the exception of urea) that must be broken down into simpler forms by soil microorganisms before it can be used.
Sulfur-coated urea is a slow-release fertilizer with a covering of sulfur around each urea particle. Different thicknesses of sulfur control the rate of nitrogen release. Watering does not affect its release rate. Sulfur-coated urea applied to the soil surface releases nitrogen more slowly than if incorporated into the soil. This material generally costs less than other slow-release fertilizers, and it supplies the essential element sulfur.
Some fertilizer products are coated with multiple layers of resin. When they come into contact with water, the layers swell and increase the pore size in the resin so that the dissolved fertilizer can move into the soil. Release rate depends on the coating thickness, temperature, and water content of the soil. There is often a large release of fertilizer during the first 2 or 3 days after application. Release timing can be from 0 to 12 months, depending on the coating.
Natural fertilizer materials can serve as effective fertilizers, but only if their nutrient contents are known and their mineralization rates are estimated closely. Tables 1 and 2 provide information on the amount of nutrients provided by various natural materials and manures. The advantages and disadvantages of natural and synthetic fertilizers relate to the consumer, not to the plant.
Natural fertilizers are mostly complex chemical substances containing carbon. In general, natural fertilizers release nutrients at a slow rate over a fairly long period. This can be advantageous for perennial crops, since only one large application may be needed.
Most natural materials are far
less predictable in nutrient content, nutrient release, and nutrient efficiency
than commercial grade fertilizers. Before most organic nutrients can be absorbed
by plants, they have to be broken down to an inorganic form by soil microorganisms
through a decaying process called mineralization. This process is affected by
moisture, temperature, and the microbial species and populations in the soil.
Most organic materials are effective only when the soil is moist and soil temperature
is warm enough for the soil organisms to be active. They may not release enough
of their principal nutrient at a time to give optimum plant growth.
When packaged as fertilizers, natural fertilizers will have the nutrient analysis stated on the labels. Some organic materials, particularly composted manures and sludges, are sold as soil conditioners and do not have a nutrient guarantee (small amounts of nutrients may be available). Most natural fertilizers are high in only one of the three major nutrients; many are low in all three.
Natural fertilizers can be expensive if applied in amounts adequate to supply nutrients for good plant growth; however the value for improved soil structure should be considered. The nutrient content may need to be supplemented with other organic or inorganic materials to achieve a balanced ration of nutrients.
How much to use varies with the nutrient content of the material. The age of the material is also a factor; when organic material decays and is rained on, they lose nutrients, especially potassium and to some extent nitrogen. Fresh manures may cause plant injury, especially when large quantities are used.
Examples of Natural Fertilizers
Cottonseed meal is a byproduct of cotton manufacturing. As a fertilizer, it is somewhat acidic in reaction. Nutrient content varies slightly but generally contains 7 percent nitrogen, 3 percent phosphorus (P2O5), and 2 percent potash (K2O). Nutrients from cottonseed meal is readily available to plants in warm soils, and it poses little danger of burning. For general garden use, apply 2 to 5 pounds per 1,000 square feet. The nitrogen is slowly made available. Cottonseed meal is frequently used for fertilizing acid-loving plants such as azaleas, camellias, and rhododendrons.
Dried blood is a byproduct
from beef processors. As blood is drained from recently killed animals, it is
dried and ground into a powder. It is a rich source of nitrogen and supplies
certain essential trace elements including iron. The gardener must be careful
not to use more than the amount recommended on the label.
Fish emulsion is a partially decomposed blend of finely pulverized fish. The odor is intense, but dissipates within a day or two. Fish emulsion contains up to 5 percent readily available nitrogen and is a source of several trace elements. Contrary to popular belief, too strong a solution of fish emulsion can burn plants, particularly those in containers.
Animal manure is also a complete fertilizer but is low in nutrients. Manures vary in nutrient content according to the animal source and what the animal has been eating. A nutrient ratio of 1-1-1 is typical. The amount of nitrogen in manure varies but is normally around 0.5 percent (poultry litter is higher in nitrogen). This is 1/20 of the nitrogen found in an equal amount of 10-10-10 fertilizer. Commonly available manures include horse, cow, chicken, and sometimes sheep. The actual nutrient content varies widely: the highest concentration of nutrients is found when manures are fresh. As it is aged, leached, or composted, nutrient content is reduced. Fresh manure should not be used where it will contact tender plant roots. Manures are good soil conditioners. Manures should be incorporated into the soil to conserve nitrogen. If left on the soil surface, up to 25 percent of the ammonia nitrogen can be lost within 2 days and 75 percent or more can be lost within 1 month after application.
Sewage sludge is a recycled product of municipal sewage treatment plants. Two forms are commonly available: activated and composted. Activated sludge has a high concentration of nutrients (approximately 6-3-0) and may contain significant amounts of lime. Composted sludge is usually sold in a dry, granular form for use as a general purpose, long lasting, nonburning fertilizer. Composted sludge is used primarily as a soil amendment and has a lower nutrient content (approximately 1-2-0). There is some question about the long term effects of using sewage sludge products. Heavy metals, such as cadmium, are sometimes present in the sludge and may build up in the soil. Possible negative effects vary, not only with the origin of the sludge, but also with the characteristics of the soil where it is used. It would be desirable to have the sludge analyzed for heavy metals.
Rock phosphate is ground rock that contains as much as 30 percent phosphate. It is not in a readily available form and is released very slowly.
Granite dust contains about 5 percent potassium but little of it is available for plant growth.
Greensand contains 6 percent potassium. It is also available very slowly.
Wood ashes are often used as a soil amendment. They contain potash (potassium), phosphate, boron, and other elements. Wood ashes can be used to raise soil pH; use twice as much wood ash as limestone for the same effect as lime. Ashes should not come into contact with germinating seedlings or plant roots as they may cause root damage. Spread a thin layer during the winter and incorporate into the soil in the spring. Check pH yearly if you use wood ashes. Never use coal ashes or large amounts of wood ash (no more than 20 pounds per 1,000 square feet), as toxicity problems may occur.
Soil pH is a measure of the hydrogen (acid-forming) ion activity of the soil solution. The scale contains 14 divisions known as pH units. It is centered around pH 7, which is "neutral." The term neutral means that the acids and alkalines are present in equal concentrations. Values below 7 constitute increasing acidity (acid range of the scale), and values above 7 make up the alkaline range.
The pH scale is not a linear scale but a logarithmic scale. A soil with a pH of 8.0 is ten times more alkaline than a soil with a pH of 7.0. A soil with a pH of 4.0 is ten times more acidic than a soil with a pH of 5.0 (100 times more acidic than a pH of 6.0). A slightly acidic soil pH is generally considered ideal for most plants in North Carolina.
The major impact pH extremes have on plant growth is the availability of plant nutrients. For example, if the pH drops as low as 5.0, the phosphorus combines with soil iron and is not readily available to most plants. A low pH also reduces the activity of soil organisms that decompose organic matter. The ability of legumes to fix nitrogen is also reduced. Low soil pH can be altered by applying lime.
Newly Installed Plants
Wait until spring to fertilize fall planted trees and shrubs. Wait six to eight weeks to fertilize plants installed in the spring. Apply a slow release fertilizer in a light band along the perimeter of the planting hole. Remember that newly installed plants are under stress and should receive only a light application of fertilizer. For 1 gallon container plants, apply 1 teaspoon of a nitrogen fertilizer or 1 tablespoon of 10-10-10. For larger plants apply 2 to 3 tablespoons.
Fertilization rates should be based on soil test results, plant age, current and desired growth rate, plant type, or by using general guidelines. The rate should also be influenced by rainfall and soil type. A wet season will normally increase the need to fertilize especially in sandy soils. During periods of dry weather, reduce the amount of fertilizer. Fertilizer encourages water-demanding new growth and can injure roots of ornamentals under drought stress.
Soil test - The best answer to how much fertilizer to apply is to use the amount recommended by a soil test. Have the soil tested before planting and every two to three years thereafter.
The accuracy of the report depends on the quality of the sample you submit. Collect samples with stainless steel or chrome plated tools. The sample could be contaminated by using brass, bronze, or galvanized materials. The bucket to put the sample in should be clean and made of plastics; even small amounts of residual lime or fertilizer in the bucket can affect the test results. Avoid taking samples from areas that are obviously different; wet spots, compost pile, animal urine spots, brush piles, under eaves, sites where trash has been burned, etc. For trees and shrubs take a sample from around the drip line to a depth of 6 inches.
If you use a trowel or spade, dig a hole then take a slice of soil down one side. Repeat this procedure in five to eight spots for each area to be tested. Mix these cores together to obtain one composite sample. Remove large pieces of organic material such as roots, stalks, and leaves from the sample. If the soil is very wet it could be more difficult to mix, but do not attempt to heat the soil to dry it.
Place about a pint of the composite sample in the soil testing box and enter your name, address, and five digit code on the side of the box. Forms and boxes are available from NCDA or your Extension center.
Soil samples are analyzed by the NCDA Agronomic Division, 5300 Reedy Creek Road, Raleigh. You can submit samples at the lab or through the county Extension Center. Information on soil pH, suggested lime and fertilizer application rates will be sent in the mail. There is no charge for the test.
The soil test report provides recommendations for pounds of lime, and a rate and grade of fertilizer per 1,000 square feet. A 1,000 square feet area is an area 50 feet by 20 feet. Multiply the length of the area by the width of the area to be fertilized to determine the number of square feet. Divide by 1,000 to obtain the number of units to be treated. Multiplying the number of units times the pounds of material to treat 1,000 square feet will give you the amount of fertilizer and lime to apply.
If the area is 500 feet by 20 feet, and the suggested lime or fertilizer treatment is 30M (pounds per 1,000 square feet):
500 x 20 = 10,000 square feet
Divided 10,000 by 1,000 = 10 units
Multiply 30 times 10 units = 300 pounds of material (fertilizer or lime)
Many home gardeners have difficulty determining how much fertilizer to use even when they know the recommended rate and the size of the area. Table 3 can be used to determine the amount to apply for small areas. Table 4 provides information on converting different weights to volumes.
Some fertilizer recommendations pertain to nitrogen only, such as 1 pound of actual nitrogen per 1,000 square feet instead of pounds of fertilizer. To determine the amount of fertilizer to use, divide 100 by the first number in a fertilizer analysis (percent nitrogen). For example if you are using 33- 0-0 fertilizer and want to apply 1 pound of actual nitrogen per 1,000 square feet, you would divide 100 by 33(= 3.3 pounds of actual fertilizer to apply). Unless the soil is deficient in other nutrients, a fertilizer containing only nitrogen is often the best buy. Table 5 provides information on the amount of several fertilizers to apply for various amounts of actual nitrogen.
Growth rate - The amount of new growth can be used as a guide to determine fertilizer needs. When new shoots are more than 6 inches long in one season, fertilizer is normally not required. When new shoot growth is between 2 and 6 inches long, fertilizer is optional. When plants exhibit poorly colored leaves, smaller than normal leaf size, or premature fall color or leaf drop the plants may need additional fertilizer. These signs may also indicate some type of root problem. Deficiency symptoms do not give an indication of how much fertilizer is needed --- only that fertilizer is needed.
Age - Newly installed plants should be given time to reestablish their root system before trying to push new growth with high nitrogen fertilizer. If you are trying to push the growth of a young hedge you may wish to make several light applications of fertilizer per year (March, May, July). As woody plants mature, the need for nitrogen decreases; rapid growth is no longer needed or desired. Most established woody plants perform well with just one application per year.
Type of plant/location -
Plants growing in a restricted root zone will need less nitrogen. Plants with
a fibrous root system, such as azalea, rhododendron, and blueberry, are very
easily damaged by fertilizer. Light applications are recommended. Plant roots
normally grow three times as far as their branches. Ornamentals located near
a lawn that is fertilized regularly may not need additional fertilizer since
many of their roots extend into the lawn area where they will absorb nutrients.
It might be all that an established plant needs.
General guidelines - Normally 2 to 4 pounds of a complete fertilizer per 1,000 square feet per year is recommended for optimum growth. If your are trying to push new growth use the higher rate. To determine how much of a particular fertilizer to apply, divide the percent nitrogen into 100 and multiply times the amount of nitrogen recommended. For example, the amount of 12-4-8 needed to apply 1 pound of actual nitrogen per 1,000 square feet is obtained by dividing 100 by 12 and multiplying by 1. Thus 8.3 pounds of 12-4-8 should be applied per 1,000 square feet.
For small trees and shrubs use 1/2 to 1 cup of 10-10-10 fertilizer. The amount of fertilizer should not exceed 1 tablespoon per foot of height for fertilizers containing 10 or more percent nitrogen. Do not apply more than 3 pounds of actual nitrogen per 1,000 square feet. For large trees measure the diameter 4 feet from the ground and apply 0.1 pound actual nitrogen (0.1 pound of actual nitrogen equals 1 pound of 10-10-10, 0.3 pound of ammonium nitrate (33-0-0), or 0.2 pounds of 5-10-10) for each inch of trunk diameter. If the area under the tree is known, simply broadcast 0.1 pound of actual nitrogen per 100 square feet.
Unless a soil test indicates otherwise use a fertilizer containing 10 to 16 percent nitrogen. Fertilizers such as 16-4-8 and 12-4-8 have the ideal ratio for woody plants, however, fertilizers such as 10-10-10 or 8-8-8 can be used. At least 30 percent of the nitrogen should be in the ammoniacal or urea form. They are slower to release nitrogen than those in a nitrate form.
method of application
A nitrogen fertilizer application will have its greatest effect three to four weeks after application. Woody plants can absorb nutrients as long as the soil temperature is above 40°F. Root growth occurs during cool weather even when the foliage appears dormant. Root growth of woody ornamentals is most active in fall and late winter/early spring but slows during hot, summer weather.
Fertilize trees and shrubs in the spring or fall. Spring fertilizer application should be made before new growth starts. Fall fertilization should be made approximately one month after the first killing frost. Many gardeners are reluctant to fertilize in late fall for fear it will stimulate new growth if a period of unseasonably warm weather occurs. Fertilizer applied in late fall is more effective in promoting plant growth than spring applied fertilizer.
Fertilization in late summer (mid August) should be avoided since it may stimulating late growth that will not harden off before frost. Remember when applying slow-release fertilizers around trees or shrubs to not apply late in the season (after July 15) because they may keep the plant growing rapidly late in the summer. The late season growth may not "harden off" completely, and winter damage may occur.
The fertilizer should be spread evenly over the entire root zone which can extend two to three times the width of the branches. Remember that some of the root zone may have already been fertilized when fertilizer was applied to the lawn or flower bed. Sprinkle the fertilizer on top of the soil or mulch and water lightly. Since the fertilizer will quickly move through the mulch there is no need to remove it or to place the fertilizer below it. Spread the fertilizer evenly under the branches. Dumping fertilizer in one spot can caused the roots below the fertilizer to be burned and die. Apply fertilizer when the foliage is dry so the fertilizer does not stick to plant foliage and cause burning. If fertilizer lodges in the whorls of plant foliage, use a broom to brush the fertilizer off.
The practice of placing fertilizer in holes around mature trees has been tested and research indicates that surface application of fertilizer is sufficient since most of the feeder roots are in the top foot of soil. If the soil is compacted you can bore holes 4 to 6 inches deep, 2 to 3 feet apart using a punchbar or a drill with a 2-inch auger. Start 2 feet from the trunk and continue to 2 feet beyond the branches. Divide the fertilizer into as many equal parts as there are holes and place in the holes. Boring holes in soil can increase soil aeration and water penetration into the root zone.
Fertilizer spikes and stakes that are driven into the ground contain satisfactory amounts of nutrients. Unfortunately, the spacing is such that very little fertilizer comes into contact with most of the root system. Lateral fertilizer movement in the soil is very limited.
Foliar sprays of a liquid or water soluble fertilizer on the foliage can be used for correcting deficiencies of minor elements such as iron or manganese. This method should not be used to provide all of a plant's fertilizer needs. The benefits of foliar sprays are short lived. Since nutrients deficiency are often caused by a disease or improper soil condition (pH, drainage, soil compaction) foliar sprays give only temporary relief and do not correct the main problem.
Tree injections of micronutrients is another method of fertilization that should be used only as a means of last resort. Trees can be permanently injured by drilling holes and the decay that could develop may out weight any benefit the fertilizer might provide. Any benefit from the fertilizer will be temporary at best.
Do not use weed-and-feed fertilizers under trees or shrubs unless the label says it is safe. Some plants, such as dogwoods, are very sensitive to dicamba herbicide that is contained in many weed-and-feed lawn fertilizers.
Prepared by: Erv Evans, Consumer Horticulturist, NC State University
©2000 NC State University