Tips on Fertilizing Woody Ornamentals
It's fall and time to consider your fertility program. Testing field soils or container media prior to planting reveals nutrient deficiencies or imbalances and provides an accurate basis for their correction. Application rates for all nutrients except nitrogen should be based on a soil test. This goes for both preplant and postplant fertilizer applications.
If you have not tested your soil, then the first step prior to fertilizing is to sample and test your soil. Testing your soil in the fall avoids the spring rush of agronomic farmers and homeowners which results in backlogs at both private labs and the Penn State Soil Analysis Lab. Soil testing during the busy spring season can take several weeks to get results. Testing in the fall allows time for preplant fertilizer incorporation prior to winter. Nursery soils should be tested every two to three years with corrective action taken the year prior to planting.
Soil testing kits and instructions for sampling your soil or soilless media are available from your county cooperative extension office. The standard soil test measures soil pH, available Phosphorus (P), exchangeable Potassium (K), Calcium (Ca), and Magnesium (Mg); cation exchange capacity (CEC); and percent base saturation. The final report will also provide lime and fertilizer recommendations. The Greenhouse Soilless Media test includes all of the above plus soluble salts and nitrates. Nitrogen is not regularly measured because it is unstable in soil. Nitrogen readily leaches from the root zone. For this reason, N applications are usually needed annually by field-grown plants and continuously for container grown plants.
Timing Fertilizer Applications
Preplant incorporation of fertilizer is the best
approach for improving soil fertility. Ideally, you would
incorporate the fertilizer during the fall subsequent to a
spring planting. Your preplant fertilizer program should be
based on soil test results. Fertilizing at this time assures
a thorough distribution of less mobile nutrients (i.e.,
phosphorous) throughout the root zone. Potassium, calcium,
and Magnesium reserves can be built up in the soil and are
only limited by the cation exchange capacity. These cations
(K, Ca, and Mg) are positively charged and bind to the
negatively charged soil particles. Clay and highly organic
soils can be charged with sufficient K, Ca, and Mg to
support five years of crop growth. Sandy soils have lower
CECs and cannot hold as many nutrients. Liming or
acidifying soils should also take place during the season
prior to planting. This assures a proper pH correction by
planting time.
Nitrogen levels can and should be built up prior to planting. Inorganic nitrogen sources are usually unacceptable as a preplant fertilizer because of their ready leaching and high levels may "burn" the fine roots of newly planted stock. Slow release organic materials are the best source of nitrogen for preplant incorporation. In the landscape, fish emulsion and well-composted manure are good sources of organic nitrogen.
Postplanting applications of nitrogen provide the greatest improvement in plant growth. The general recommendation is to test and amend soils subsequent to planting and to apply a high nitrogen fertilizer after planting. Roots absorb nutrients most efficiently when they are actively growing in the spring, at bud break, and late summer to early fall. The best time to apply fertilizer is about two weeks prior to these periods so that the nitrogen is leached into the root zone in time to be available to the roots.
Plants do not respond with growth immediately after fertilizer applications. Plants absorb both the nitrate and ammonium forms of nitrogen; however, the plants do not directly use these ions. The nitrogen is converted into organic compounds, primarily amino acids. For this reason, fall applications of nitrogen provide a greater and more rapid growth response from plants than a spring application. Nutrients applied in the spring are either used later in the season or stored for the following year.
Some plants can have repeated growth spurts throughout the season for example Forsythia. They go through repeated cycles of active root growth followed by active shoot growth. To maximize growth of plants like forsythia, sufficient nitrogen must be present each time the roots are actively growing. In the landscape or field, this is accomplished by applying slow-release forms of nitrogen in the spring. In containers, the slow-release forms of nitrogen are usually supplemented by applying liquid fertilizers throughout the growing season.
Application Rates
There are no general fertilizer rate recommendations
without soil or tissue testing. Recommendations can range
from 2 to 6 pounds of nitrogen per 1000 square feet (about
90 to 260 pounds per acre) per year. The nitrogen rate for
individual small trees or shrubs depends on several factors
including stage of development, location, soil texture,
species, and fertilizer formulation.
At planting time, nitrogen availability should be low, but sufficient to support the anticipated new root and shoot growth. High nitrogen rates may force excessive top growth that cannot be supported by the plants limited root system.
For landscape plants and established nursery stock, the early stages of development are targeted to ensure maximum growth. High nitrogen rates are required at this time to achieve maximum growth. As the plant matures and rapid growth is no longer needed or desired, lower levels of nitrogen are sufficient to maintain the vigor of the plant. A moderate application of nitrogen fertilizer sometimes help plants recover from stress factors such as insect or disease attacks or physical damage to the roots or trunk. In contrast, over fertilization of stressed plants can result in further stress and enhance susceptibility to insect and disease attack.
Landscape plants located in areas with restricted root zones (e.g., paved areas) should receive lower fertilizer rates, than plants in unrestricted sites. High nitrogen rates in restricted locations can encourage unsupported excessive top growth. Similarly, coarse textured soils (silty/sandy soils) have a low cation exchange capacity and should receive low rates of fertilizer at increased frequency rates. Fine textured soils (for example, high in clay or organic matter) have a high cation exchange capacity. These types of soils can be charged with higher fertilizer rates.
Plants that naturally grow fast, producing excessive growth and resulting in increased maintenance require little or no nitrogen. These plants include; Siberian elm, silver maple, willow, forsythia, honeysuckle, and privet. Plants that produce well-structured, strong growth and require little maintenance care may be heavily fertilized. These plants include; red oak, sugar maple, arborvitae, and viburnum. Plants like azalea, rhododendron, hemlock, and dogwood have shallow, sensitive, and fibrous roots that are easily burnt by high fertilizer rates.
Fertilizer types also affect application rates. Slow-release nitrogen fertilizers can be applied at higher rates than readily available soluble nitrogen formulas. A slow-release form of nitrogen means that less of the nitrogen is in the soil solution at any one time. Root injury is reduced under these conditions.
In looking longingly outdoors for a chance to get ahead, check your soil test report, review your goals, consider plant location and species, soil textures, and the types of fertilizers you want to use. Weigh your options for this year and plan for next year by putting into place a soil testing and preplant fertilizer program.
| For more information on fertilizing
woody ornamentals, consult the Penn
State Extension publication circular #353 Fertilizing Woody Ornamentals available from your local county cooperative extension office. |
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Information provided by Rob Berghage and Jim Sellmer Penn State Department of Horticulture, Ornamental Horticulture Extension Specialist, 103 Tyson Building, University Park, PA 16802, (814) 863-2571
This publication is available in alternate media upon request
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