is a microbial process that converts plant materials such as grass
clippings and leaves to a usable organic soil amendment or mulch.
Gardeners have used compost for centuries to increase soil organic
matter and supply some of the essential nutrients for plant growth.
Mulching refers to the practice of applying a substance such as
compost or grass clippings to the soil surface with the purpose
of modifying soil temperature and moisture as well as controlling
weeds and soil erosion.
With the ban
on outdoor burning and with laws which soon will limit dumping of
leaves and grass clippings into landfills, composting and mulching
have become attractive alternatives for managing yard waste and
recycling natural materials. Grass clippings and leaves can be hauled
to city composting areas as one means of disposal. However, many
homeowners may find it more convenient and economical to compost
these materials in their own backyards. In either case, the finished
compost can be used as a mulch or as a soil amendment to improve
most garden soils. This bulletin should help you learn how to build
and maintain a compost pile as well as how to use the compost in
the yard and garden.
an efficient method of breaking down organic materials into an end
product that is beneficial to the soil and growing plants. Adding
undecomposed materials directly to the soil without first composting
may initially have some undesirable effects. For example, if large
quantities of uncomposted leaves are incorporated into the soil,
microbes will compete with plant roots for soil nitrogen during
leaf decomposition. This competition for nitrogen can result in
nitrogen deficiency and poor plant growth. Adding composted material
reduces the competition for nitrogen. Another benefit of composted
material is that it is much easier to handle and mix with soil than
uncomposted material. Furthermore, improvement of soil physical
properties, such as infiltration drainage, and water holding capacity,
will usually be faster if composted materials are added.
FOR EFFICIENT DECOMPOSITION
of organic material in the compost pile is dependent on maintaining
microbial activity. Any factor which slows or halts microbial growth
will also impede the composting process. Efficient decomposition
will occur if the following factors are used to fullest advantage.
Oxygen is required
for microbes to efficiently decompose the organic wastes. Some decomposition
will occur in the absence of oxygen (anaerobic conditions); however,
the process is slow and foul odors may develop. Mixing the pile
once or twice a month will provide the necessary oxygen and significantly
hasten the composting process. A pile that is not mixed may take
three to four times longer before it can be used. A well mixed compost
pile will also reach higher temperatures which will help destroy
weed seeds and pathogens.
is essential for microbial activity. A dry compost will not decompose
efficiently. If rainfall is limited, it will be necessary to water
the pile periodically to maintain a steady decomposition rate. Enough
water should be added to completely moisten the pile, but overwatering
should be avoided. Excess water can lead to anaerobic conditions
which slowdown the degradation process and cause foul odors. Water
the pile so that it is damp, but does not remain soggy.
the size of organic wastes, the faster the compost will be ready
for use. Smaller particles have much more surface area that can
be attacked by microbes. A shredder can be used before putting material
in the pile, and is essential if brush or sticks are to be composted.
A low cost method of reducing the size of fallen tree leaves is
to mow the lawn before raking. If the mower has an appropriate bag
attachment, the shredded leaves can be collected directly. In addition
to speeding up the composting process, shredding will reduce the
initial volume of the compost pile.
is affected by the carbon to nitrogen ratio of the organic waste.
Because microbes require a certain amount of nitrogen for their
own metabolism and growth, a shortage of nitrogen will slow down
the composting process considerably. Material high in carbon relative
to nitrogen such as straw or sawdust will decompose very slowly
unless nitrogen fertilizer is added. Tree leaves are higher in nitrogen
than straw or sawdust but decomposition of leaves would still benefit
from an addition of nitrogen fertilizer. Grass clippings are generally
high in nitrogen and when mixed properly with leaves will enhance
decomposition. Manure or blood meal can be used as organic sources
of nitrogen. Otherwise, a fertilizer with a high nitrogen content
should be used. Other nutrients such as phosphorus and potassium
are usually present in adequate amounts for decomposition.
During the initial
stages of decomposition organic acids are produced and the pH drops.
In the past, small amounts of lime have been suggested for maintaining
and enhancing microbial activity at this time. However, high rates
of lime will convert ammonium-nitrogen to ammonia gas which will
lead to the loss of nitrogen from the pile. Research has shown that
although lime additions may hasten decomposition, the loss of nitrogen
from the pile often offsets the benefits of lime. In general, lime
is not necessary for degradation of most yard wastes. The pH of
finished compost is usually slightly alkaline without the addition
of lime. In many areas, the water used to moisten the compost pile
is sufficiently alkaline to increase the pH of the compost. If large
quantities of pine needles or fruit wastes are composted, some additional
lime may be necessary.
materials are suitable for composting. Yard wastes, such as leaves,
grass clippings, straw, and nonwoody plant trimmings can be composted.
The dominant organic waste in most backyard compost piles is leaves.
Grass clippings can be composted; however with proper lawn management,
clippings do not need to be removed from the lawn (see below). If
clippings are used, it is advisable to mix them with other yard
wastes. Otherwise, the grass clippings may compact and restrict
air flow. Branches, logs, and twigs greater than 1/4 inch in diameter
should be put through a shredder/chipper first. Kitchen wastes such
as vegetable scraps, coffee grounds, and eggshells may also be added.
be added in moderate amounts if additional nitrogen is applied.
Approximately 1 lb of actual nitrogen (6 cups of ammonium nitrate)
is required for 100 lbs of dry sawdust. Wood ashes act as a lime
source and if used should only be added in small amounts (no more
than 1 cup per bushel). Excessive amounts of wood ashes will result
in loss of nitrogen from the pile. Ordinary black and white newspaper
can be composted; however, the nitrogen content is low and will
consequently slow down the rate of decomposition. It is recommended
that newspaper be recycled through appropriate community paper recycling
centers rather than through backyard composting.
other organic materials that can be used to add nutrients to the
pile include: blood and bone meal, livestock manure, alfalfa hay,
and lake plants.
may pose a health hazard, or create a nuisance, certain organic
materials should not be used to make compost. Adding human or pet
feces cannot be recommended because they may transmit diseases.
Meat, bones, grease, whole eggs, and dairy products should not be
added because they can attract rodents to the site. Most plant disease
organisms and weed seeds are destroyed during the composting process
when temperatures in the center of the pile reach 150F. However,
in most compost piles, it is impossible to mix efficiently enough
to bring all wastes to the center. Consequently, adding large amounts
of weeds with seeds or diseased plants may create problems if the
compost is used in the garden.
that have been treated with herbicides or pesticides should be avoided
for composting, small amounts of herbicide-treated plants (e.g.,
grass clippings) may be mixed in the pile as long as you are careful
to allow thorough decomposition. Clippings from lawns recently treated
with herbicides should be left on the lawn to decompose (see section
on "Alternatives to Composting Crass Clippings").
both pesticides and herbicides, are degraded at varying rates. A
list of herbicides commonly used on the home lawn and their persistence
in soil is provided in Table 1 (page 5). Even if some treated grass
clippings are used, the degradation of these chemicals in a properly
maintained compost pile should be at least as fast as that in the
To save space,
hasten decomposition, and keep the yard looking neat, it is recommended
that the compost pile be contained in some sort of structure. Composting
structures can consist of a variety of materials and can be as simple
or complex as desired. There are many options available that can
be tailored to individual needs. Listed below are a few suggestions
for containing the compost.
Use of plastic
garbage bags is perhaps the simplest way to make compost. The bags
are easy to handle, and require minimal maintenance. To make compost
using this method, 30-40 gallon plastic bags should be alternately
filled with plant wastes, fertilizer, and lime. About one tablespoon
of a garden fertilizer with a high nitrogen content should be used
per bag. Lime (one cup per bag) helps counteract the extra acidity
caused by anaerobic composting. After filling, add about a quart
Set aside for six months to a year. Bags can be set in a basement
or heated garage for better decomposition during winter months.
Using garbage bags requires no turning or additional water after
closing. The main advantage of composting in garbage bags is that
it requires little maintenance; however, because oxygen is limited,
the process is slow.
A barrel or
drum composter generates compost in a relatively short period of
time and provides an easy mechanism for turning.This method requires
a barrel of at least 55 gallons with a secure lid. Be sure that
the barrel was not used to store toxic chemicals. Paint barrels
are a good choice, as the inside already has a protective coating.
Drill several rows of 1/2 inch holes over the length of the barrel
to allow for air circulation and drainage of excess moisture. Place
the barrel upright on blocks to allow bottom air circulation, and
fill it 3/4 full with organic waste material and about one quarter
cup of a high nitrogen-containing fertilizer. If needed, apply water
until moist. Every few days, turn the drum on its side and roll
it around the yard to mix and aerate the compost. The lid can be
removed after turning to allow for air penetration. Ideally, the
compost should be ready in two to four months. The barrel composter
is an excellent choice for the city dweller with a relatively small
For larger quantities
of organic waste, bin-type structures are the most practical. As
an example, a circular bin can be made by using a length of small-
spaced woven wire fencing and holding it together with chain snaps.
The bin should be about three to five feet in diameter and at least
four feet high. A stake may be driven in the middle of the bin before
adding material to help maintain the shape of the pile and to facilitate
adding water. With this design, it is easiest to turn the composting
material by simply unsnapping the wire, moving the wire cylinder
a few feet, and turning the compost back into it.
A very efficient
and durable structure for fast composting is a three-chambered bin.
It holds a considerable amount of compost, and allows good air circulation.
The three-chambered bin works on an assembly line idea, having three
batches of compost in varying stages of decomposition. The compost
material is started in the first bin and allowed to heat up for
three to five days. Next, it is turned into the middle bin for another
four to seven days, while a new batch of material is started in
the first bin. Finally, the material in the middle bin is turned
into the last bin as finished or nearly finished compost.
To make this
structure, it is best to use rot resistant wood such as redwood,
wood treated with a preservative such as "copper green," or a combination
of wood and metal posts. Unless the wood is treated or rot resistant,
it will decompose within a few years. Each bin should be about five
feet by three feet, and about three to five feet high. Using removable
slats in the front offers complete access to the contents for turning.
There are many
other structures for composting, and no one structure is best. Invent
your own, or for a more thorough description of different structures,
refer to J.I. Rodale's Complete Book of Composting (3). If you don't
want to build a structure, there are several commercial composting
units available through local garden stores or mail-order catalogues.
Most of these are similar to the barrel composter described previously
and are for the city dweller who desires an easy method to make
small amounts of compost quickly.
pile should be located close to where it will be used and where
it will not interfere with activities in the yard or offend neighbors.
Examples of good locations for the pile include: near the garden
or kitchen, or between the garage and house. The pile will do best
where it is protected from drying winds and in partial sunlight
to help heat the pile. The more wind and sun the pile is exposed
to, the more water it will need.
THE COMPOST PILE
pile should be prepared in layers. This will facilitate decomposition
by insuring proper mixing. Ideally, each pile should be about five
such as leaves, grass, and plant trimmings are put down in a layer
eight to ten inches deep. Coarser materials will decompose faster
if placed in the bottom layer. This layer should be watered until
moist, but not soggy. A nitrogen source should be placed on top
of this layer. Use one to two inches of livestock manure, or use
a nitrogen fertilizer such as ammonium nitrate or ammonium sulfate
at a rate of one third of a cup for every twenty five square feet
of surface area. If these nitrogen sources are not available, one
cup of 10-10-10 fertilizer per 25 square feet of surface area will
also suffice. Do not use fertilizer that contains herbicide or pesticide.
Other organic sources of nitrogen that can be used are grass clippings,
lake plants, alfalfa hay, or blood meal. Crass clippings tend to
mat and should either be mixed well with other materials, or placed
in layers only two to three inches thick.
About a one
inch layer of soil or completed compost can be applied on top of
the fertilizer layer. One purpose of adding soil is to ensure that
the pile is inoculated with decomposing microbes. The use of soil
in a compost pile should be considered optional. In most cases,
organic yard wastes such as grass clippings or leaves contain enough
microorganisms on the surface to effect decomposition. Studies have
shown that there is no advantage in purchasing a compost starter
or inoculum. Microbes multiply as rapidly from the soil and/or added
organic wastes as from the inoculum. Those microbes already in the
soil and on organic materials are just as efficient in decomposing
the waste as those provided by the commercial inoculum. Adding soil,
however, will help reduce leaching of mineral nutrients such as
potassium released during decomposition. Repeat the sequence of
adding organic waste, fertilizer, and soil (optional) until the
pile is completed, remembering to water each section.
(C/N) ratio will determine how long decomposition will take. A C/N
ratio of about 20 is needed for rapid composting without nitrogen
being tied up. If the C/N ratio is above 50, the process will be
considerably slower. The C/N ratio of common organic yard wastes
is provided in Table 2.
THE COMPOST PILE
To prevent odors
and hasten decomposition, the pile must be turned occasionally.
Turning also exposes seeds, insect larvae, and pathogens to lethal
temperatures inside the pile. Turning may be done by inverting segments
of the compost, or by shifting the pile into another bin. The compost
pile should be kept moist but not waterlogged. Odors may arise either
from the addition of excessive amounts of wet plant materials like
fruits or grass clippings, or from overwatering. A properly mixed
and adequately turned compost pile will not have objectionable odors.
An actively decomposing pile will reach temperatures of 130F in
the middle. Reasons for the pile not heating up may include too
small a pile, not enough nitrogen, lack of oxygen, too much moisture,
or not enough moisture. The pile should be turned when the temperature
in the center begins to cool. This will introduce oxygen and undecomposed
material into the center and subsequently regenerate heating. The
composting process is essentially complete when mixing no longer
produces heat in the pile.
of fresh materials may be added provided that the pile is occasionally
turned. Vegetable wastes should be buried inside the pile to avoid
attracting rodents. If enough material is available, it is best
to make a new pile instead of combining with old compost.
well managed compost pile with shredded materials under warm conditions
will be ready in about 2 months. A pile left unattended and material
not shredded may take over a year to decompose. Piles prepared in
the late fall will not be ready for use the following spring. When
the compost is finished, the pile will be about half its original
size and have an earthy smell to it.
1. PERSISTENCE OF HERBICIDES IN SOIL (2)
TABLE 2. APPROXIMATE CONCENTRATION OF NITROGEN AND CARBON TO
RATIO OF VARIOUS MATERIALS USED IN MUNICIPAL AND BACKYARD COMPOSTS(4)
| Wood (pine)
*See text for further explanation
COMPOSTING OF YARD WASTES
your own compost pile is impractical, there are municipal composting
sites available in many counties for disposal of leaves and grass
clippings. Depending on the location, leaves only or leaves and
grass clippings may be dropped off at the composting sites. Some
city compost programs also have curbside pick-up in the fall. Completed
compost is also available free of charge from these sites, and some
sites have free delivery of quantities over 10 cubic yards. For
information on the nearest composting site, contact your local county
In the past
there has been some concern about using municipal waste compost
because of contamination with lead and other trace metals. Possible
modes of yard waste contamination may be due to direct exposure
of leaves and grass to automobile exhaust or to inclusion of street
sweepings (which might contain high levels of lead from automobile
exhaust) in the compost pile. A study at the University of Minnesota
(5), characterized elemental composition of yard waste composts
from 11 different sites in the seven-county metropolitan area. The
mean and ranges of elemental concentrations in the compost piles
over two years are presented in Table 3. There was a wide range
in lead values from the different sites. The highest concentrations
were found in composts produced at sites in the most urban areas.
Generally it has been considered safe to use garden produce grown
in soils with total lead levels less than 500 mg/kg (parts per million).
The lead levels in the yard waste composts are considerably less
than this suggested limit. Other trace metals such as cadmium, nickel,
copper, chromium, and zinc are also present in compost in small
quantities. The allowable levels in milligrams per kilogram on a
dry weight basis are:
of these elements found in municipal yard waste composts are many
times less than the allowable levels.
TABLE 3. CHEMICAL CHARACTERISTICS OF MUNICIPAL YARD WASTE COMPOSTS:
MEAN OF 11 COMPOST SITES OVER 2 YEARS (5)
(dry weight basis)
0.04 - 2.71
0.70 - 8.04
| Iron %
| Zinc mg/kg
| Lead mg/kg
*Ratio of carbon to nitrogen (See text for further explanation).
< means "less than."
= milligrams per kilogram, which is the same as parts per million
USE OF COMPOST
AS A SOIL AMENDMENT
Compost is used
as an organic amendment to improve physical, chemical, and biological
properties of soils. Adding compost will increase the moisture holding
capacity of sandy soils, thereby reducing drought damage to plants.
When added to heavy clay soils, compost will improve drainage and
aeration, thereby reducing waterlogging damage to plants. Compost
will increase the ability of the soil to hold and release essential
nutrients. The activity of earthworms and soil microorganisms beneficial
to plant growth will be promoted with compost additions. Other benefits
of adding compost include improved seed emergence and water infiltration
due to a reduction in soil crusting.
Over time, yearly
additions of compost will create desirable soil structure, making
the soil much easier to work. For improving soil physical properties,
add and incorporate 1 - 2 inches of well decomposed compost in the
top 68 inches of soil. Use the lower rate for sandy soils and the
higher rate for clay soils.
To a limited
extent, compost is a source of nutrients. However, nutrient release
from compost is slow and the nutrient content is often too low to
supply all the nutrients necessary for plant growth. As noted in
Table 3, there is a wide variation in nutrient content of municipal
leaf compost. Differences are probably due to several factors including
age of the compost, amount of water added, plant species, and the
amount of soil that becomes mixed into the pile during turning.
It is usually
necessary to supplement compost with some fertilizer, particularly
nitrogen. If the C/N ratio of the compost is less than 20 to 1,
nitrogen will tend to be released rather than tied up (5). For the
majority of municipal yard waste composts, the C/N ratio is less
than 20 to 1 (Table 3). Thus, while composts may not supply significant
amounts of nitrogen, especially in the short run, nitrogen tie-up
should not be a major concern with most yard waste composts. Approximately
1 cup of ammonium nitrate (0.15 lb actual nitrogen) per 3 bushels
(100 lbs compost) is required to provide the additional nitrogen
needed by most garden plants.
Have your soil
tested every few years to determine whether supplemental phosphorus
and potassium are required. The pH of most yard waste composts is
usually between 7.0 and 8.O.This slightly alkaline pH of compost
should not pose any problems when diluted by mixing into the soil
and in fact is beneficial to plants growing on acid soils. Because
of the alkaline pH, yard waste composts do not appear well suited
for use on acid loving plants such as azaleas and blueberries.
USE OF COMPOST
IN POTTING SOILS
can be used as a component of potting mixes. Generally, no more
than one quarter to one third by volume of the potting mix should
be compost: over time some of the compost is likely to decompose
and the volume of the potting soil will be reduced. In addition,
high levels of compost in a mix may cause waterlogging and poor
aeration for roots.
composting destroys most weed seeds and disease organisms, some
may still survive due to incomplete mixing. To obtain a completely
pasteurized leaf compost, it is necessary to heat the material in
an oven until the temperature of the center reaches 160 degrees
F and is maintained for 30 minutes.
USE OF COMPOST
AS A MULCH
used in gardens to suppress weeds, reduce soil erosion, modify the
soil temperature (cooler in the summer, warmer in the winter), and
conserve soil moisture. Yard waste compost makes an ideal mulch
for annual and perennial gardens. All that needs to be done is to
apply a 3-6 inch layer of compost around the base of the plant.
Periodically throughout the summer, you may need to add more compost
over the old layers to maintain the benefits of the mulch.
The soil environment
beneath the mulch is favorable for promoting earthworms which in
turn are valuable for aerating the soil. Organic matter is gradually
added to the soil as the mulch decomposes. For annual gardens, the
mulch can be worked into the soil at the end of the season to further
improve soil physical properties. For perennials, it may be beneficial
to remove the mulch in the spring to allow the soil to thaw out
faster. As discussed above, a well decomposed yard waste compost
will not tie up nitrogen. Therefore, additional nitrogen beyond
that recommended for plant growth is unnecessary. If uncomposted
or partially composted leaves are used, one tablespoon of a high
nitrogen fertilizer should be applied to each bushel of mulch.
TO COMPOSTING GRASS CLIPPINGS
lawn maintenance is used, there is no need to collect grass clippings
(6). The question of whether to collect or not to collect grass
clippings has been around for a long time. However, it is now agreed
that as long as the grass is not excessively long and clippings
do not thickly cover the lawn surface after mowing, there is normally
no need to collect the clippings. Aside from reducing the work involved
in lawn maintenance, leaving the grass clippings benefits the turf
by returning nutrients and organic matter to the soil. If evenly
distributed, clippings left on the lawn can be equivalent to one
fertilizer application per year.
In order to
keep your lawn looking healthy and to control the amount of clippings
generated, several maintenance practices should be followed. It
is important that mowing height be properly adjusted. The height
of the cut will depend on the turfgrass varieties present in the
lawn and whether the lawn is in sun, full shade, or a combination
of both. Lawns in full sun have the greatest potential for quick
recovery after mowing and can therefore be cut shorter. Those in
the shade need all the available leaf surface possible for photosynthesis.
Thus, grass in the shade should be cut slightly higher than grass
in the sun.
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