Fertilizing Landscape Plants

Fertilization is a very important component of plant health care in the landscape. Fertilization is necessary to supplement naturally occurring essential mineral elements in the soil to maintain an optimum supply for plant growth. Soil analysis (testing), combined with observations of plant growth, are the keys for the home gardener to develop the most effective nutrition program for the landscape. The mineral elements critical for optimum growth and development of landscape plants must be present in the soil and plant at proper levels.

The objective of this fact sheet is to help the gardening public make informed decisions regarding the nutrition of their landscape plants. Included is a brief review of soil analysis, soils, pH, essential elements, fertilizers and fertilizer rates, timing, and methods of application.

Soil Analysis

Prior to planting, one of the first priorities is to have the soil tested, simply because it is much easier to correct nutrition imbalances at this stage. Additional soil tests every 2 to 3 years are highly recommended to monitor the fertilizer program and prevent mineral element deficiencies that could result in abnormalities or a decrease of optimum plant growth.

Samples should be taken from a minimum of 6 to 8 sites per area (tree and shrub beds, vegetable garden, annual beds, etc.). The samples should be combined and thoroughly mixed to provide uniformity. Dry the soil sample at room temperature and place it in a self-mailer available from your local Extension offices in most counties or private soil testing laboratories listed in the classified section of the telephone directory.

Results from the testing laboratory will include corrective recommendations for soil pH, phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg). Nitrate nitrogen (NO₃N) and soluble salts (EC, electrical conductivity) are not tested regularly by most laboratories; however, these tests can be requested.

Soil Properties

The physical and chemical properties of soils significantly influence the growth of landscape plants. Fertilizer applications are dependent on organic matter, soil texture (size of soil particles), and drainage.

Organic matter in soil may be a slow-release source of nutrients, may contribute to desirable soil structure (arrangement of soil particles), and increases total water available to crops. Organic matter increases the water-holding capacity of sandy loam soils while increasing aeration of silt and clay loam soils. As organic matter decomposes into humus, it becomes colloidal in nature and cation exchange occurs (positively charged ions, such as calcium and magnesium, are adsorbed on to negatively charged particles). Incorporation of sphagnum peat moss, composted municipal sludge, composted yard waste, pine bark chips, among other sources, is recommended at planting if tests indicate less than five percent organic matter in the soil.

Soil texture is determined by the relative amount of sand, silt, and clay in the soil. Common soil textural classes are sandy loam, silt loam, and clay loam. The surface area of soil particles is important and varies with the size of these soil particles. Clay particles have 100 times the surface area as the same volume of sand particles; therefore, clay that is negatively charged has a greater capacity to attract positively charged soil nutrients. Sandy loam soils must be fertilized more often than clay loam soils because of their lower capacity to attract and hold (adsorb) positively charged mineral elements.

As stated above, clay has a negative charge that can be measured to indicate the exchange capacity for cations such as Ca++, Mg++, K+, and others. This is called cation exchange capacity (CEC) and its determination is included in many soil test results. The CEC is an indication of the soil's capacity to provide nutrients for plant use, and is a measure of nutrient leaching potential.

Soil drainage is critical to survival and growth of most landscape plants, especially evergreen trees and shrubs. When the rate of water movement through soil is restricted by fine-textured clay soils, sub-soil, hard pan, or other material difficult to penetrate, a saturated zone may develop in the root zone of plants. Spaces in the soil normally containing air are filled with water, resulting in saturated soil. Wet soils cause more problems to landscape crops than any other single cause. When drainage is poor, roots are injured from the lack of oxygen, fertilizer uptake is limited, and plant growth is reduced. Soil moisture problems can be solved by installing surface and/or internal drainage.

Adjusting Soil pH

Mineral soil pH values between 6.0 and 7.0 result in the greatest number of mineral elements to be available for uptake by plants. Several plants such as certain conifers, most broadleaf evergreens, maples, oaks, sourgum, and sweetgum should be grown in acidic soils with a pH from 5.5 to 6.0. Other plants such as viburnum, hydrangea, and lilac grow best at neutral (7.0) to slightly alkaline soil pH values. In most situations, mineral element deficiencies can be avoided by proper soil pH management.

When the pH of a mineral soil drops below 4.5, aluminum (Al), iron (Fe), and manganese (Mn) are very soluble. When this occurs, these elements are absorbed in large quantities and may become toxic to certain plants, while nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and/or magnesium (Mg) may become limiting for plant growth.

As the soil pH increases, ions of Al, Fe, and Mn precipitate (settle out of the soil solution) and the availability of these elements decreases to a point where nutrients may become deficient for normal plant growth.

It becomes evident that a soil pH of 6.0 to 7.0 is generally desirable, although slight adjustments are needed for specific plants. A soil test will indicate the amount of lime needed to increase the pH of acidic soils or the amount of sulfur needed to lower pH of alkaline soils.

Essential Elements

Nine essential elements required in relatively large amounts for plant growth are called macronutrients or major elements. Included are nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, carbon, hydrogen, and oxygen. The last three are readily available in air and water. Seven other essential elements required in small amounts by plants are called micronutrients or minor elements and include iron, manganese, zinc, boron, molybdenum, copper, and chlorine.

If an insufficient amount of any of these 16 essential elements is lacking or in excess, plants will not grow properly. More or less distinct symptoms occur for individual nutrient element deficiencies or excesses because each element has its own role in the growth and development of the plant. Once a deficiency or toxicity symptom is visible, plant growth has been and will continue to be reduced until corrected.

Fertilizer

The analysis or grade of a fertilizer refers to the minimum amounts of nitrogen (N), phosphorus (as P2O5), and potassium (as K₂O) in the fertilizer, and is always printed on the bag, can, or bottle. A 10-10-10 fertilizer would represent 10 percent nitrogen, 10 percent P₂O₅, and 10 percent K₂O. Therefore, in 50 pounds of 10-10-10, there are 5 pounds of N, 5 pounds of P₂O₅, and 5 pounds of K₂O.

Fertilizers may be divided into two broad groups, organic and inorganic or chemical. An organic fertilizer is derived from a living plant or animal source. Nitrogen in an organic fertilizer is slow to become available for plant use because the organic nitrogen (NH2) must be reduced (converted) by micro-organisms to ammonium (NH4) or nitrate (NO3). The NH4 and NO3 forms are useable by plant roots.

Inorganic or chemical fertilizers are either mixed or manufactured and have the advantage of lower cost. High analysis, rapid solubility, and availability necessitate some caution when applying these fertilizers.

Slow-release fertilizers may be either inorganic or organic. They are characterized by a slow rate of release, longer residual, low burn potential, low water solubility, and higher cost.

There are several fertilizer categories of slow-release nitrogen fertilizers commercially available in garden centers including urea-formaldehyde (UF) and related urea based formulations, isobutylidene diurea (IBDU), sulfur coated urea (SCU), plastic coated (various formulations such as Multicote^TM and Nutricote^TM, salts (MagAmp^TM), and natural organics such as composted sewage sludge.

Water soluble or liquid fertilizer is applied either to the soil or on the foliage. Many water soluble formulations are available for almost any specific need from plant starter, high nitrogen fertilizers, to minor element formulations. Chelated iron is used extensively for prevention and control of iron deficiency of azalea, rhododendron, oak, and sweetgum, among others.

Rates of Application

Studies have shown that approximately three pounds of actual nitrogen per 1,000 square feet per year is needed to maintain the health of woody plants in most landscape situations. If foliage color, annual growth, or general vigor is not normal, the application rate should be increased to five pounds of nitrogen per 1,000 square feet per year. Certain plants such as broadleaf evergreens, dwarf conifers, and alpine plants should be fertilized with one-half the above rates. If soil and foliar test results are available, follow the recommendations provided, otherwise the suggested rates given above could be used as a guide. Woody plants respond well to fertilizers with a 4-1-2, 3-1-2, 4-1-1, or 3-1-1 ratio such as 24-6-12, 18-6-12, 20-5-5, 12-4-4, respectively. Landscape plants respond to 3 to 4 times as much nitrogen as phosphorus, and twice as much potassium as phosphorus. An application of three pounds of actual nitrogen per 1,000 square feet using a 3-1-2 ratio would include one pound of P2O5 and two pounds of K₂O.

To convert from actual nitrogen to fertilizer, divide the amount of actual nitrogen desired per 1,000 square feet by the percentage of nitrogen in the fertilizer analysis or grade. Example: How much 18-6-12 is needed to apply three pounds of nitrogen per 1,000 square feet? Answer: 16.6 pounds (3 / 0.18 = 16.6 pounds).

Timing Fertilizer Treatments

In the landscape, fertilizing once a year is preferable to less frequent applications, especially with newly planted materials. Applications twice a year in light sandy soils or in seasons of excess rainfall are suggested.

The best time to fertilize in the northern United States is autumn, generally after the first hard freeze in October and before the soil freezes in December.

The next best time to fertilize landscape plants would be prior to growth in early spring, between February and early April again in the northern United States. If fertilizer was not applied during the autumn or spring season, applications may be made up to July 1. Fertilizer applied after this midsummer date is not recommended, as it could delay acclimation to winter weather conditions.

Methods of Application

Fertilizer can be applied in the landscape via 1) liquid soil injection, 2) drill or punch bar holes in the soil, 3) surface application, 4) fertilizer stakes or spikes, 5) foliar sprays, and 6) tree trunk injection or implantation. Each serves a specific role depending on the site and plant health. Regardless of the method selected, the soil should be moist at the time of fertilizing to prevent fertilizer injury.

Liquid Injection into Soil

Liquid injection of soluble fertilizer into the soil is rapidly absorbed by the roots, and is an excellent method of correcting deficiencies quickly. Injection sites should be 2 to 3 feet apart, depending on pressure, and 6 to 9 inches deep. Fertilizing deeper than nine inches may place the fertilizer below the feeder roots. The addition of water to dry soil is desirable in summer or during periods of drought.

Drill Hole or Punch Bar

A major advantage of the drill hole system is the opening of heavy, compacted soils which allow air and fertilizer to penetrate the soil. This technique and liquid injection prevent excess growth of grass in turf areas. The drill holes should be placed in the soil in concentric circles or in a grid system around the main plant stem beginning 2 to 3 feet from the main stem and extending 3 to 6 feet beyond the dripline. Space the holes two feet apart and drill them 6 to 9 inches deep. The recommended rate of fertilizer for the area should be uniformly distributed among the holes.

After the fertilizer is applied, the holes can be filled with either organic materials such as peat moss or compost or inorganic materials such as gravel or calcined clay. The selection of organic or inorganic material will depend on the greater need for either water or air after the fertilizer is applied.

Fetilizer Placement- Drill Hole Method

Surface Application

Fertilizing via the surface of the ground is as effective as most other methods. However, this method should not be used in good quality turf, as injury could occur, particularly if more than two pounds of actual nitrogen per 1,000 square feet is applied at any one time. In turf areas, apply fertilizer with either liquid injection or drill hole techniques.

Fertilizer Stakes or Spikes

Fertilizer stakes or spikes that are driven into the soil contain satisfactory fertilizer materials. Unfortunately, the spacing of spikes is such that very little fertilizer comes in contact with the root system. One or two stakes per inch of trunk diameter does not represent adequate fertilizer distribution because lateral fertilizer movement is limited in soil.

Foliar Spraying

Spraying liquid or water soluble fertilizer on the foliage should be a consideration to correcting minor element deficiencies, especially of iron or manganese. This method should not be considered adequate as a means of providing all the macronutrients required by plants. To correct chlorosis, several applications may be necessary during any given growing season.

Tree Trunk Injection or Implants

The infusion of liquid or implants of fertilizer is often the most satisfactory method of correcting iron or manganese problems. In areas of adverse soil pH, high moisture relationships, or locations where other means of application are not practical, this method is often the most satisfactory in obtaining desired results. Holes must be placed in the trunk root flare which causes a wound that will close within a growing season.

¹This factsheet was originally produced by Elton M. Smith, Professor Emeritus, The Ohio State University for the Ohio Florists' Association who has granted permission for its use and distribution. Dr. Rose is the person currently responsible for the contents of this factsheet.