Steinegger, Extension Horticulturist
Susan D. Schoneweis, Extension Coordinator
This Guide discusses
the advantages of compost, the compost heap, ingredients, uses,
and instructions for making compost.
Compost, a mixture
of partially decomposed plant material and other wastes, is used
in the garden to amend soil and fertilize plants.
The chief advantage
of compost is its ability to improve soil structure. Cod garden
soil is loose, has a high water-holding capacity and adequate drainage.
Addingcompost to heavy clay soil improves drainage by improving
soil structure. Compost also absorbs water and improves the water-holding
capacity of sandy soils. If one intends to conserve moisture or
develop a xeriscape landscape requiring little water-it is essential
to improve the water holding capacity of the soil.
to improving the structure of "problem" soils, decomposing compost
also slowly releases plant nutrients. Unless applied in very large
amounts, compost will not provide all the nitrogen that highly productive
crops require. Organic gardeners can supplement generous compost
applications with manure to produce good yields without the addition
of other fertilizers. Making and using compost also allows the gardener
to recycle garden wastes and reduce the trash disposal burdens of
The decay which
creates compost is the same process which naturally occurs at the
soil surface in a field or on the forest floor. However, certain
composting practices encourage this decomposition to occur more
hastened by gathering compost ingredients together in a heap. A
heap allows for the large buildup of the organisms which cause decomposition.
The organisms' growth is encouraged in a number of ways. Materials
are shredded to provide a large surface area for decomposition.
The organisms also require the proper balance of different types
and sizes of ingredients in the heap. Lastly, the growth is encouraged
by providing oxygen and sufficient moisture.
materials together in a heap encourages the growth of microorganisms
which decompose the residues. A compost heap of the proper size
and ingredients can heat up to 150 F. The heap should be at least
4 feet wide across the bottom and 3 to 4 feet high. Smaller compost
heaps may dry out too fast and slow decay. If a heap is very large,
air may not get to the center, giving the compost a bad odor.
Heaps can be
freestanding or made in more permanent compost "bins". Various materials
can be used to make compost bins, including: concrete blocks, scrap
lumber, lath fencing and wire mesh. Bins should be partially open
on the sides to allow air to enter the heap. They should be constructed
to allow the compost to be aerated by turning and to be easily removed.
Locate compost bins or heaps in the shade to prevent the compost
from drying out too fast.
be made from any organic refuse such as leaves, grass clippings,
weeds, kitchen scraps, or animal wastes. Some organic materials
however, should not be added to compost heaps. To be safe, do not
use grass clippings from the first two cuttings after applying lawn
Do not add
cat manure to compost heaps because it may contain parasites which
can infect humans.
Do not add bones,
meat scraps or greasy substances to compost; they may attract rodents,
and the fats in them will slow decomposition.
Do not add diseased
plants or weeds that have formed seeds to the compost heap since
disease organisms or weed seeds may not be destroyed during decomposition.
as "bacterial activators" are not necessary for successful composting.
like branches or cornstalks should be shredded into smaller pieces
to decompose faster. Strive for a mixture of differently-sized ingredients
as you construct the compost heap. This ensures plenty of air in
the heap since the microorganisms require oxygen to decompose the
ingredients. A heap made of only one ingredient like grass clippings
or leaves will tend to mat down. Decomposition will occur slowly
or the decomposition may be undesirable and produce ammonia or sulphur-like
odors. These odors indicate that the compost heap requires more
air inside it.
is also encouraged by using the right balance of types of plant
refuse. Two nutrients, carbon and nitrogen, are particularly important
in building a compost heap. Green materials like fresh grass clippings,
weeds, and kitchen scraps are high in water and relatively high
in nitrogen, while dry materials like leaves and straw are high
in carbon. The balance between the amount of carbon and the amount
of nitrogen in a material is called the carbon-nitrogen ratio (C/N
ratio). See Table 1 for the C/N ratio of some materials commonly
used in compost heaps. Materials with high C/N ratios are high in
carbon; low C/N ratios indicate high nitrogen content.
1. Carbon/Nitrogen Ratios of Some Organic Materials
Food wastes (tables scraps)
range of 80/1 to 40/1
only of materials with low C/N ratios may decompose too quickly,
and much of the nitrogen will be lost as gas (ammonia). Heaps made
of materials with high C/N ratios like straw or pine needles will
decompose very slowly. The ideal C/N balance for effective composting
is around 30. For example, combine similar amounts of leaves (50/1)
with manure (20/1) to obtain a ratio of 35/1.
To ensure a
proper balance between high nitrogen and high carbon materials,
compost heaps are often constructed in layers. Layers are a convenient
way of measuring how much of each ingredient goes into the heap.
The layers disappear as the compost is mixed during decomposition.
A general guideline
is to use 1/3 green material and 2/3 dry material. Form a heap by
alternating 4- to 6-inch layers of dry carbon materials with 2-
to 3-inch layers of green nitrogen material or animal manure. (If
you are low on nitrogen materials, sprinkle 1/2 cup of 5-10-10 or
10-10-10 over the dry materials.)
Add a shovelful
of garden soil between each layer to introduce the necessary microorganisms
to the heap. If the materials in the heap are not moist, sprinkle
each layer with water as you build the heap.
Cover the finished
heap with a plastic tarp to keep it from drying out and to prevent
rain from washing away nutrients.
need water and oxygen to do their work. Check the heap as it decomposes
to make sure it is moist enough. A compost heap should feel as moist
as a squeezed-out sponge. If necessary, add water by sprinkling
the heap as you turn it. Overwatering the heap will reduce the oxygen
the microorganisms require.
It is important
that air be added to the heap by regularly turning and mixing the
materials. The first turning should be three or four days after
the heap is formed, and subsequent turnings once a week. Use a shovel
or pitchfork to fluff and mix the ingredients. Large heaps can be
turned by removing the top layer and using it as the base for a
new heap. As ingredients are moved from one heap to the new one,
air is incorporated.
Failure of the
heap to heat up in a few days indicates the decay process has not
started. Add more nitrogen materials and be sure that the heap is
sufficiently moist. Compost that smells strongly of ammonia or is
slimy requires more frequent turnings and the addition of high carbon
materials. How soon compost is "finished" will depend on the size
and balance of the ingredients used, the air temperature, and how
fast decomposition was encouraged by keeping the heap moist and
turning it regularly.
is dark brown and crumbly and smells like potting soil. Small pieces
of the leaves or other ingredients may be visible. If the compost
contains many materials which are not broken down, it is only partly
decomposed. Adding partly decomposed compost to the soil can reduce
the amount of nitrogen available to plants. The microorganisms will
continue to decompose but will use soil nitrogen for their own growth,
restricting the nitrogen's availability to plants growing nearby.
decomposed compost to break down further before using it around
growing plants. Or add extra nitrogen (such as in manure or commercial
fertilizer) to ensure that growing plants will not suffer from a
has many uses. It can be spread over and incorporated into garden
soil in the spring or fall. Generally, a layer of compost I to 3
inches deep is applied. Compost can be used as an ingredient in
potting mixes. For a fine texture, sift compost through a 1/4-inch
wire mesh screen. Mix with equal parts of soil and sand, or scratch
lightly into the soil surface of potted plants.
Top dress growing
plants like vegetables, flowers or shrubs with compost by spreading
a 1- to 2-inch layer around the plants. half-decomposed compost
can be applied more thicKly as a mulch around trees or shrubs. It
will conserve soil moisture and slowly release nutrients to plants.
compost when establishing a new lawn or when rejuvenating patches
of turf by reseeding. Mix 1 inch of compost into the top 2 to 3
inches of soil. Compost should not be applied to established turf
because its high organic matter content can encourage thatch formation.
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