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PRINCIPLES OF
HOME LANDSCAPE FERTILIZATION

M.L. Witt, A.J. Powell, J.R. Hartman, W.O. Thom, W.M. Fountain

     Plants growing in a home landscape coexist with one another in a non-native environment. The urban landscape includes combinations of trees, shrubs and turf with most of their feeder roots intermingled in the top 3 to 4 inches of soil. During home construction, many urban soils are compacted or topsoil is removed. The remaining soil, which often is higher in clay, is spread over the surface to make an almost impenetrable material. This situation often reduces oxygen and nitrogen within the root zone, lowers available water and reduces root penetration into the soil.
     It is easy to visualize competition for light, nutrients and water when one observes grass attempting to grow under a shade tree. However, this competition may be just as intense over the entire lawn. Tree roots from large trees compete with turf in unshaded as well as shaded lawn areas, since they may extend horizontally at least two times the tree's height. Even neighbors' trees in an adjacent lawn may be competing with your turf.
     The question is, "Can one fertility program promote growth and/or quality for all landscape plants in a managed lawn?" The answer is "No." Each situation is unique and several choices exist for home landscape fertility programs. Consider the following aspects when deciding on the best program for a specific landscape.

SOIL TESTING

What is Measured?
     Soil testing is the basis for many fertility recommendations. A soil test can be obtained for a nominal fee through your County Extension Office. An agricultural soil test provides information about levels of potassium, phosphorus and soil pH. Although nitrogen is the most important component of many fertilizers, the soil test does not provide information about nitrogen because it can be rapidly lost through leaching (carried away by soil water) or removed by plants. Specialized soil tests that reveal levels of calcium, magnesium or some micronutrients can be obtained for an extra fee. However, such tests are not normally used unless a specific plant nutrient problem is suspected and needs verification. Perhaps the most valuable information revealed through the soil test is soil pH. See AGR-57: Soil Testing and AGR-16: Taking Soil Test Samples, available at your County Extension Office.

When Do You Test?
     Begin with a soil test before trees are planted and the lawn is seeded. In an established landscape, take a soil test any time. Collect soil from 8-10 scattered locations in the front yard, put them in a large container and combine them by mixing. Remove a pint of soil from this mixture for testing. Repeat for the backyard. Take a soil sample only 4 inches deep. Often, samples are taken 6 to 8 inches deep, which dilutes the important surface sample and may lead to erroneous fertility recommendations. In a landscape situation most of the fertilizers will be scattered evenly over the soil surface with very little tillage, so most of the important fertility reactions will take place in the top 2 to 3 inches of soil.
     This publication first discusses each aspect of soil fertility and then assesses different types of fertilizers.

ACIDITY (pH)

    Soil acidity is measured as "pH." Soil pH is an index of the amount of acidity present. The pH scale ranges from 0 to 14. At pH 7, soil is neutral. At pH levels below 7, soil is acid (sour) and at pH levels above 7 it is alkaline (sweet).

Acid Soil
     Most turfgrass species will perform best at a pH between 6 and 7. A pH as low as 5 does not present severe problems to turf except that it encourages Kentucky bluegrass thatch development. Since tall fescue accumulates no thatch, it does very well at low pH levels. A soil pH between 5 and 7 is within the range of adaptation for most tree and shrub species, although azaleas and rhododendrons, for example, may prefer a pH of 4.5 and should certainly be maintained at pH levels below 6.0.
     For acid-loving plants, you may have the problem of increasing the soil's acidity. If the soil is above 6.5 the most practical thing to do is to remove it, say to about 20 inches for rhododendrons and azaleas, and replace it with naturally acid surface soil from the woods or with peat moss. But for soils with pH below 6.5 sulfur can be added according to the amount shown in Table 3.

Alkaline Soil
     A soil pH just above 8, which is the maximum found in Kentucky, will not detrimentally affect turf although it will be a serious problem for such woody plant species as azaleas, rhododendrons, dogwoods, hollies, oaks and blueberries. Other species such as butterfly bush, beech and viburnums may do best at a pH of 7 or above.
     Plants should be matched to a soil's natural pH as much as possible, since permanently adjusting soil pH to a radically different level is difficult. However, even if the pH is not optimum for a particular species, plants rarely die because of this problem alone. Improper soil pH will reduce plant growth rate and cause yellowing (chlorosis) especially between veins of new leaves. This stress, if severe, will allow otherwise harmless microorganisms to attack the affected plant and cause death of roots, branch tips or even whole plants.

Adjusting Soil Acidity Levels
What to Use--If soil pH correction is needed, agricultural limestone can be applied to raise pH (make less acid), and finely ground, elemental sulfur or aluminum sulfate can be applied to lower pH (make more acid). Since these materials are normally surface-applied in landscape situations, they only adjust the pH in the top few inches of soil. Changing soil pH may take several months or years and repeat applications are often needed. Lime or sulfur can be applied any time of year. Tables 1-3 provide rates of these materials to use to make soil pH changes. If most of your woody plants need a soil pH below 5.5, select either tall fescue or one of the fine fescues for the lawn instead of Kentucky Bluegrass.
     For most urban landscapes, maintaining a soil pH between 5.5 and 6.0 is the best compromise. Reducing the pH beyond this range would not be necessary except for azaleas and rhododendrons. Since these woody plants are normally grown in beds, pH could be adjusted by applying sulfur around the plants' roots.
    How Much to Use--The amount of lime or sulfur needed to adjust pH depends on how much change is needed, soil texture, and what fertilizer is used. Tables 1, 2, and 3 give general guidelines but soil tests should be taken periodically to evaluate pH. Soil test results often give two figures for soil pH: water pH and buffer pH value. The buffer pH provides a better estimate of how much lime or sulfur is needed to change soil pH because it takes into account the soil's buffering capacity. A soil with high buffering capacity is more resistant to changes the pH. Clay loam soils usually have more buffering capacity than sandy loam soils, for example.
    Before planting, lime or sulfur should be broadcast evenly on the soil and worked into the top several inches. If woody plants and turf are already established, a surface broadcast application is the only alternative. Immediate irrigation after applying sulfur will help prevent foliage burn.
    Besides sulfur, the use of acid mulches (such as pine needles, sawdust and acid peat) and continued use of ammonium sulfate as a nitrogen fertilizer tend to increase soil acidity.

Table 1. Amount of Agricultural Limestone Needed (Silt-Loam Soil) as Related to Water pH Measurement.

Table 2. Amount of Agricultural Limestone Needed as Related to Buffer pH Measurement.

Table 3. Suggested Application of Ordinary Powdered Sulfur To Reduce the pH of an 8-Inch Layer of Soil, as Indicated in Pt/100 Sq Ft**

**Some pH is usually reduced on a per plant basis, pt/100 sq ft is useful. Although aluminum sulfate often is recommended to gardeners for increasing the acidity of the soil, it has a toxic salt effect on plants if it is used in large amounts. Small amounts are not very effective. About 7 lb. of aluminum sulfate are required to accomplish the same effects as one lb. of sulfur.

NITROGEN (N)

     Nitrogen is the nutrient most responsible for plant growth and vigor and yet is the nutrient most often deficient or misused in urban landscapes. Before deciding whether or not to use nitrogen, the owner or manager must decide on objectives for landscape appearance.

Why Do You Want to Fertilize?
    Fertilization objectives for shade trees could include:

1. Promoting rapid growth so young trees quickly become large and functional.
2. Maintaining health and appearance of mature trees.
3. Attempting to rescue declining trees.

Turfgrass fertilization objectives could include one of these three:

1. Promoting thick, lush, green growth even if it needs a lot of maintenance.
2. Maintaining a healthy and persistent turf with less mowing and maintenance needed.
3. Having a grassy area for erosion control or utility requiring minimum upkeep.

     Furthermore, by examining the landscape periodically, the owner or manager can tell by the appearance of plants whether or not nitrogen is needed. For example, if mature trees are consistently adding 6 inches of new twig growth annually, no additional nitrogen would be needed. On the other hand, if turfgrass or plants are growing slowly and appear off-color (pale green) or weak nitrogen may be needed. Finally, a landscape is a complex system and a single program, while adapted for some plants in the yard may be disastrous for others. One may need to either compromise or use nitrogen in different ways in different parts of the landscape.

Factors Influencing Nitrogen Use
    The following factors will influence how nitrogen might be used in the landscape.

    • In many urban environments, low organic matter content of disturbed soils makes it necessary to apply nitrogen annually for maintaining turf and ornamental growth and quality.
    • In many mature and natural landscapes, trees and turf live harmoniously without added nitrogen because nutrients are recycled from decaying plant residue or from original soil organic matter.
    • In most home lawns, if a tree is big enough to be aesthetically pleasing, adding nitrogen fertilizer to force rapid growth would be senseless.
    • Rapid turfgrass growth due to heavy nitrogen fertilization increases stress on the grass, which then requires more mowing, irrigation, pest control and often thatch control.
    • Turf growing in heavy shade will only survive when it has low nitrogen fertility during late spring and summer months. High levels of nitrogen on such areas are harmful because the grass is forced to metabolize nitrogen, requiring more energy (light). As light becomes the limiting growth factor, shaded turf is soon depleted of energy and will deteriorate. As this happens, moss, algae and vining weeds become established. However, you can still fertilize the tree under these circumstances. Since most active tree roots are growing beyond the tree canopy, tree fertilization can be accomplished by reducing the nitrogen rate in the most shaded turf area and increasing the rate on unshaded turf nearby.
    • Many shade trees can tolerate and respond to high rates of N (3 lb of N/1,000 sq ft) in infertile soils; conifers and broadleaf evergreens should not be over-fertilized (maximum of 3 lb of N/ 1,000 sq ft).
    • Lush, succulent nitrogen-stimulated growth may make landscape plants and turfgrass more susceptible to fire blight and patch diseases and those caused by obligate parasites such as powdery mildew and rust.
    • Weak, nitrogen-deficient growth may make landscape plants more susceptible to canker and decay diseases and turfgrass more susceptible to red thread and dollar spot diseases and weed infestations.

When Should Fertilizer Be Applied?
    Fall and winter are the best time to fertilize most landscape plants and grasses that grow in Kentucky. The annual amount of nitrogen should be split into 2 or 3 applications approximately 6 weeks apart, to benefit both turf and woody plants. (Nitrogen applied to turf between April and September can promote excessive top growth and thus decrease drought, disease and heat resistance.)
    Do not apply nitrogen to woody plants between July 1 and November 1 since some of the less winter-hardy plants may not harden in time for winter weather.
    Fall and winter fertilization benefits turfgrass because it promotes root and stolon growth needed for improved health and density. Woody plants and turf can absorb nitrogen any time in late fall and early winter that soil temperatures are above 32 F.

How Much Nitrogen Should Be Applied?
    Refer to landscape maintenance objectives to determine needs that may exist. Nitrogen is normally applied at rates expressed as pounds actual nitrogen (N) per 1000 sq ft. If a fertilizer contains 10% nitrogen and 2 lb N are needed, then 20 lb of fertilizer would be spread over 1000 sq ft. The percentage nitrogen contained in a package of fertilizer is stated on the package and is expressed by the first number in a series; i.e., 10-10-10 has 10% N, 33-0-0 has 33% N, etc. The consequences of applying nitrogen at different rates in the landscape are presented in Table 4.

Table 4. Sample Programs for Nitrogen Fertilizer in the Landscape**

** "Landscape" means an established landscape having turfgrass, shrubs, and small, medium and large shade trees; disturbed, low organic matter soil; tree leaves, but not grass clippings removed, and fertilizer applied to the soil surface.

PHOSPHOROUS (P)
and POTASSIUM (K)

    All plants need a favorable phosphorous (P) and potassium (K) level. Kentucky soils usually have adequate P and K for tree and grass survival except for the P level in many western Kentucky soils. However, Kentucky soils may not have optimum rates of P and K that are needed for maximum growth, color, heat and cold tolerance and maximum resistance to drought. (Note that if grass clippings are removed regularly during mowing, soil K may decrease more rapidly than if the clippings are not removed.)
    To be sure of the fertility level, have soil tested every 3 to 4 years. Table 5 gives the amounts of P and K that should be applied according to soil test levels.

Table 5. Phosphate and Potash Levels for Lawn and Ornamentals

CALCIUM (Ca)
and MAGNESIUM (Mg)

    Calcium and magnesium are important components of plant structure, metabolism, and photosynthesis. They are seldom deficient in Kentucky soils to the extent that they inhibit growth. In a landscape situation, as grass clippings and leaves decompose, much calcium is recycled. A soil test is the best method of determining calcium status.
    When soil becomes too acid, agricultural lime can be applied to add calcium while dolomitic lime adds both magnesium and calcium. If soil needs to be maintained at acid pH levels, Ca and Mg can be applied as soluble fertilizer. Soluble Ca is available in gypsum or calcium nitrate. Soluble Mg is available in epsom salts (magnesium sulfate) or sulfate of potash-magnesia.
    In other states, magnesium has been found to be deficient for some landscape plants, but this situation has not been confirmed in Kentucky. Magnesium deficiency is usually described as "interveinal chlorosis" or yellowing of tissue between the veins of leaves. Other limitations within the plants' root zone, such as soil compaction, soil oxygen, heat, cold, drought, root damage, air pollution, diseases, insects and certain chemicals may also cause leaf chlorosis. Although careful observation and plant/soil testing can greatly narrow the possible causes, most people will still have difficulty determining the exact cause of chlorosis.

MINOR NUTRIENTS

    Minor nutrient deficiencies are uncommon in Kentucky soils. Kentucky's soils are very high in iron although most of it is only slightly available to plants. Treatment of some chlorotic trees with iron, e.g. iron sequestrene foliar sprays, iron chloride injections, or iron sequestrene soil applications, help leaves become greener, at least temporarily. In addition, when soil pH is decreased, chlorosis symptoms have abated. Iron applied to the surface of soils with a pH of 6.5 or higher is chemically tied up and not available for immediate plant root uptake. Therefore, soil and foliar iron treatments may have to be made repeatedly to correct an iron deficiency in certain species growing in near neutral or high pH soils. Certainly, reducing soil pH will provide better long-term control of chlorosis where high iron-requiring species like pin oaks are desired.

TYPES of FERTILIZER

    Nutrient deficiencies can be corrected by applying single ingredient fertilizers such as ammonium nitrate (N), urea (N), triple super phosphate (P), and muriate of potash (K). They can also be corrected by making appropriate applications of a combination fertilizer such as 10-10-10, 5-10-10, or 20-20-20. Specialty fertilizers marketed for certain ornamentals and grasses can also be used effectively but may not be more beneficial than more economical farm-type fertilizers. However, specialty fertilizers are convenient to use because the package often gives specific rate and calibration information and the fertilizer often contains some slow release (organic) nitrogen that reduces foliar burn potential.

Dry vs. Liquid Fertilizers
    There is essentially no difference between the nutrient availability of fertilizers if applied in dry or liquid form. Liquid fertilizers do not move deeper into the soil than dry fertilizers. Both require water from rainfall or irrigation to make nutrients available for root uptake.

Foliar vs. Soil-Applied Fertilizers
On Foliage--Foliar applications of fertilizers give only temporary response and do not give a long-term solution to a fertility problem. To prevent serious foliage burn, foliar fertilizers must be very dilute. 
On or In the Soil--Broadcast applications of fertilizer on the soil are effective since most roots of woody plants and turf grow near the soil surface. Liquid fertilizers can also be applied below the soil surface with a lance or water needle. Similarly, applying dry fertilizers by holes drilled into the soil or made by a punch bar, may benefit woody plants, but probably only part of the root system will be in contact with the fertilizer. The more holes or injections made, the more uniform and responsive woody plants will be to fertilizer. For those woody plants whose roots reach greater depths and do not have to compete with grass roots, place the fertilizer deep, below most grass roots but not below tree or shrub roots (6-12 in. deep).

ISSUED: 6-86