Using Compost in Landscape Beds and Nursery Substrates
T.E. Bilderback and M.A. Powell
Extension Horticulture Specialists
Published by: North Carolina Cooperative Extension
Publication Number: AG 473-14
Last Electronic Revision: March 1996 (JWM)
Compost has been used by gardeners in
backyard landscaping and gardening for
many years. The benefits are substantial,
especially when compost is used on the
compacted, nutrient-deficient soils common in many urban areas. Commercial
growers and professional landscape contractors, on the other hand, have usually
relied on peat moss or pine bark as a soil
amendment or a medium in which to
propagate or pot plants. The main deterrent to the commercial use of compost
has been the lack of a reliable supply of a
consistent, fully stabilized product. The
nutrient content of compost and its air
and water relationship can vary widely.
If cllrrent technical composting guidelines are followed and the process is
closely monitored, however, it is possible
to produce uniform compost that the
horticulture industry will find acceptable. That industry represents a tremen-
dous market for properly composted
materials in North Carolina.
Incentives for Composting
Recent federal and state laws required
that the amount of material deposited in
North Carolina landfills be reduced by 25
percent by June 30, 1993. To help meet
that requirement, North Carolina passed
l law that prohibits depositing organic
yard wastes such as leaves, grass clippings, or tree trimmings in the state's
landfills. (Prior to passage of the law an
estimated 20 percent of all landfill deposits consisted of organic yard wastes). To
comply with this law, many municipalities have been forced to develop
composting and recycling facilities.
These facilities represent a significant
cost to the town or county, some or all of
which must be recovered by the marketing of the composted products.
To ensure a market for these products,
alternative uses must be developed for
composted yard wastes, municipal solid
waste, agricultural wastes, animal
wastes, food processing wastes, and any
other industrial material that can no
longer be sent to a landfill but that can be
composted. If the compost is reasonably
priced and of uniformly high quality, it
may find widespread use in the horticulture industry.
Mulch or Soil Amendment?
Some yard wastes, such as wood chips,
are very difficult to compost fully and
are therefore not suitable for incorporation into the soil. However,
these materials can be used as mulches to modify soil
temperature and reduce moisture loss in
natural areas and informal landscapes.
Shredded leaves can also be used in this
manner. These materials should not be
incorporated into the soil like stabilized
organic matter often creates problems
such as depletion of soil nitrogen, leading
to nitrogen deficiencies in plants.
What is "Good" Compost?
Most of the compost currently being used by the
landscape and nursery industries is classified as unrestricted grade. This material includes yard,
agricultural, and silvicultural wastes plus untreated
wood products that have been treated by pathogen-free requirement procedures (PFRP)-that is, maintained at 131°F (55°C) for 48 to 96 hours. This
treatment ensures that the compost is free of disease-causing organisms.
Municipal solid waste (MSW) compost or any
compost that contains sewage sludge must also meet
the requirements for unrestricted grade. These requirements are stated in Section .1407 of the North
Carolina Solid Waste Compost Rules, "Classification/
Distribution of MSW Compost Products." All MSW
composts must be subjected to the pathogen-free requirement procedures. In addition, MSW compost is
classified by its physical and chemical properties and
degree of stabilization. The compost must be marketed as fresh if the organic matter content has been
reduced only 20 to 40 percent during composting. It is
considered semimature if it has undergone a 40 to 60
percent reduction in organic matter and mature if the
organic matter reduction is greater than 60 percent.
For all three classifications, the maximum particle size
is 1.0 inch, and the maximum proportion of inert solids is 6 percent of the dry weight.
Another requirement for unrestricted grade MSW
compost is that its electrical conductivity, often referred to as its soluble salts level, must measure
less then 10 millimhos (mmhos) per centimeter
(mmho/cm) dry weight unless the product is marketed as a fertilizer with a
guaranteed fertilizer analysis. A mho is a measure of conductivity. It is the
reciprocal of an ohm, which is a measure of electrical
resistance. A millimho (mmho) is one thousandth of a
mho. Conductivity is measured by an instrument
called a solubridge or conductivity meter. More
information can be obtained from the Solid Waste Section of
the North Carolina Department of Environment,
Health, and Natural Resources.
Nutrient Content of Compost
As one might expect, the nutrient content of compost
is quite variable. It is affected greatly by the raw materials that are composted. Typical yard wastes, such
as leaves or tree bark, may contain less then 1 percent
nitrogen and phosphorus (dry weight), whereas animal wastes may contain nearly 2 percent nitrogen and
even higher percentages of phosphorus and potassium.
Most compost also contains as much as 2 percent calcium. The amount of time that the compost is
allowed to "cure" after the initial composting process
also affects the final nutrient content. It is a good
idea to have compost analyzed before using it. This
precaution ensures that landscape plants will not be
injured and that there will be no negative environmental
effects related to overapplication of nutrients.
Screening and Testing for Stability
Compost should be screened before use. Passing the
compost through l/2-inch mesh removes hazardous
materials such as glass or metal objects that are sometimes collected
with the organic matter. It is also imperative that the compost be stable before it is applied
in a commercial landscape or nursery. If the producer
has had the compost tested for stability, the results of
those tests will indicate the odor potential. If the compost
has not been tested, three simple tests can be
applied in the field to gauge its stability:
- Put a handful of the material in a plastic bag
and seal it for 24 hours. If it does not have an
offensive smell when opened, it is probably
- None of the materials in the compost should be
identifiable. The compost should appear and
smell like rich, organic earth.
- If a small pile (3 cubic yards) of compost does
not heat more than 20°F above the temperature
of the surrounding air in 24 hours, it is probably
Before using compost, be sure to study a copy of any
soil or waste chemical nutrient analyses, pesticide and
heavy metal analyses, and stability tests that the
producer of the compost performed. Items to look for in
the results of soil and waste nutrient analyses are pH,
soluble salts level, and percentages of all elements,
especially nitrogen, phosphorus, potassium, calcium,
and magnesium. Most stabilized composts have a pH
of about 7.0. If the compost contains animal manure,
the soluble salts level can be relatively high (4.0
mmhos/cm or higher when measuring a solution of
two parts water and one part compost). This reading
would indicate that the compost should be leached
(rinsed) before use until the conductivity measures no
more than 2.0 mmhos to prevent damage to ornamental plants.
Information on the amounts of metals such as zinc,
iron, lead, nickel, and cadmium in the compost
should also be studied. Metals content should be well
below the threshold values shown in Table 1.
Table 1. Maximum Allowable Chemical Content of Marketable Unrestricted Grade Code 1 Composts
Application Rates for Landscape Use
As a soil amendment, compost is thought to enhance
the physical, chemical, and biological properties of
soils. In clay soils, organic materials such as compost
and pine bark increase drainage and air space. Compost tends to raise pH and increase biological activity
in the soil. In sandy soils, compost increases the ability to hold moisture and nutrients. Landscapers find
that all of these characteristics help ornamental plants
become established and develop a good root system.
Table 2 gives suggested application rates. For landscape use as a soil-incorporated amendment, animal
waste compost should be applied in a layer 1/2 to 3/4 inch
thick on the soil surface, then incorporated. This
amount is approximately equal to 880 to 1,300 pounds
per thousand square feet. Yard waste compost can be
applied at a higher rate because it contains fewer nutrients. A 2- to 3-inch layer can be spread over the soil
surface, then incorporated into the root zone. This
amount equals approximately 2 to 3 tons per thousand square feet. The weight of the finished compost
is influenced by the source of the material and the
moisture content. After the material has been incorporated, it is always advisable to sample the soil again
to determine future nutritional needs.
Use in Nursery Potting Substrates
Organic materials that have been properly composted
are very acceptable as components of nursery potting
mixes. Several nursery crops have shown favorable
responses to compost. For container use, be cautious
and use approximately 10 percent compost by volume
in pine bark mixes. The compost should be considered a substitute for peat moss and sand. It is good to
subject the medium to a soil test before adding fertilizer. Addition of minor elements will probably be
recommended. For container production, the use of
slow-release fertilizers is also recommended. Dolomitic limestone should be omitted or reduced to no
more than 3 pounds per cubic yard of potting mix.
The soluble salts level and pH should be monitored
during the growing season. Before using compost on
any nursery crop, it is wise to establish a small test
area to determine the material's suitability for the
particular ornamental species.
Table 2. Suggested Application Rates for Compost
Because North Carolina has a large poultry industry, large amounts of turkey litter is composted, and
the compost is readily available. North Carolina State
University tests have found that turkey litter compost
has high electrical conductivity (soluble salts level)
and a pH range from 6.0 to 6.2. These values are
slightly higher than those of commercial potting
Mixes containing approximately 10 percent (by
volume) of turkey litter compost initially contain
higher amounts of nutrients and particularly higher
phosphate levels than currently available potting
mixes. The greatest advantage of using animal waste
composts may be that phosphate levels remain within
the suggested range longer during the growing season. This factor may
be important in flower bud development in late summer and fall for nursery crops
such as hybrid rhododendron.
As water quality and solid waste disposal regulations
become stricter, the amount of organic material being
composted will increase. The landscape and nursery
industries in North Carolina are prime candidates for
using these products, but only if the materials produced meet high quality standards and can be used in
an environmentally responsible manner. The incorporation of composted organic materials into soil and
nursery media will provide environmental and economic benefits.
For more information on composting, obtain the following North Carolina Cooperative Extension publications from your county Extension Center.
Soil Facts: Using Municipal Solid Waste Compost. North
Carolina Cooperative Extension Service publication
Compostingfor Home Gardens. North Carolina Cooperative Extension Service publication AG-467.
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