Michigan State University Extension
Ornamental Plants plus Version 3.0 - 00001799
11/12/99
Organic Mulches
Because organic mulches are derived from plant material,
they will decompose and affect both the soil and the
plants in several ways.
Physical Effects
If mixed in the upper soil layer, the mulch dilutes the
soil and usually increases root growth. When a mulch
such as sphagnum peat moss, bark or aged sawdust is used,
it affects the soil almost immediately. On clay soils,
bark or aged sawdust increases aeration. Sphagnum peat
moss, increases the water-holding capacity is of sandy
soil.
If the mulch is not well decomposed, it will promote
granulation, or clinging together, of the soil particles.
During decomposition of the organic material, soil micro-
organisms secrete a sticky material that glues soil
particles together to form granules. This is especially
true of heavy soil types. Materials that decompose
slowly, such as sphagnum peat moss, have little effect on
granulation. Straw, fresh leaves or aged manure, which
decompose rather rapidly, do promote granulation.
Mulches improve and stabilize soil structure, the
arrangement of the soil particles. Because of the mulch
layer, the soil structure is not disturbed by pelting rain
or coarse streams or drops of water from irrigation
devices. Some gardeners do not realize that cultivating
when the soil is wet destroys good soil structure. Using
mulches eliminates the danger of cultivation at the wrong
time because very little, if any, cultivation is
necessary. Another way to harm the soil structure is to
walk on the soil when it is wet. A mulch on the soil
serves as a cushion and reduces soil compaction.
Chemical Effects
The pH, the degree of acidity or alkalinity of the soil,
may be slightly affected. Acid sphagnum peat moss will
gradually lower the pH--make the soil more acid--while
most other organic materials will raise the pH
slightly--make the soil reaction more alkaline. Oak
leaves may be acid when reasonably fresh, but as they
finally decompose, the net result is a slightly alkaline
reaction. Continuous use of oak leaves, however, will
keep the soil acid.
Small amounts of plant nutrients become available during
the decomposition of the mulch because it, too, was once
plant material containing these essential elements. The
amount of fertilizer added is usually very small,
however, and may not have much effect on the nutrients
available in the soil. As an example, manure has iron in
it but not enough to correct iron deficiency in an azalea
or a pin oak. Reasonably fresh animal manure could supply
nitrogen but the ammonia in it could damage the foliage
and/or roots. Dried animal manures that are packaged for
sale are relatively high in nitrogen. If used at rates
higher than those recommended on the container these
products can cause problems from too much nitrogen.
Broadcasting gypsum or superphosphate will reduce the
problem from ammonia fumes.
Nitrogen starvation or deficiency may become apparent in
plants mulched with certain materials. If organic mulches
such as crushed corncobs, sawdust, wood chips or straw are
used, micro-organisms decomposing the organic mulch remove
appreciable amounts of nitrogen from the soil. The
removal of this nitrogen, which is used by the
micro-organisms, reduces the nitrogen reserve in the root
zone of the growing plant. If additional amounts of
nitrogen fertilizers are not applied regularly, nitrogen
starvation occurs in the mulched plants, which become
yellowish-green and grow slowly.
Biological Effects
Organic mulches serve as food for many micro-organisms
that maintain and promote soil granulation. The mulch
also helps keep the temperature fairly constant so that
activity of the micro-organisms can occur at a uniform
rate.
Sometimes undesirable organisms--disease-causing fungi,
bacteria and nematodes--may be added to the soil when
mulches are applied. Molds often develop on cocoa bean or
buckwheat hulls when these materials are kept too moist or
are used in shady spots. Stirring them occasionally so
the surface of the mulch will dry eliminates the mold.
Insects and rodents may overwinter in some organic mulch.
Poison baits can be used to eliminate rodents, which may
damage plants.
Weed seeds may be introduced into the garden with hay or
straw or strawy manure. If you use various kinds of hulls
or corncobs, be sure they are free of the grain or seed.
Grain mixed in with crushed corncobs can attract rodents.
To control weeds, mulch needs to be sufficiently deep, or
weeds from the soil below will grow through it.
Organic Mulch Materials
Grass Clippings
Grass clippings are readily available when the lawn is
actively growing. Using grass clippings has several
disadvantages, however. Weeds from the lawn can be
introduced into the mulched bed. If the clippings are
applied too deeply and packed too tightly they will
release heat and a foul odor as they decay. Clippings
from lawns that have been treated with weedkillers can
cause herbicide injury in tender plants.
Leafmold
Leafmold can be made by composting leaves in the fall.
The material should be partially decomposed by the
following spring. It is a good mulch but difficult to
apply evenly and may not be particularly neat looking.
Leafmold collected from wooded areas may contain
nematodes.
Leaves
Leaves, probably the most inexpensive material available,
are used extensively in areas with many trees. Avoid
leaves collected from streets that have been sprayed
with oil or treated with calcium chloride--both oil and
calcium chloride are toxic to plants. Leaves may be
difficult to keep in place in windy locations. Very dry
leaves can be a fire hazard, and packed, wet leaves
interfere with air and water movement into the soil.
Leaves should be mixed with some other light material,
such as straw, to prevent their becoming a compacted,
soggy mass.
Peat
Peat is often called "peat moss" but this is a misnomer.
Moss peat comes from mosses, while other types of peat
originate from cattails, reeds, sedges and other similar
water plants. Sphagnum peat moss is acid and useful for
mulching and amending the soil around azaleas,
rhododendrons and other plants that grow best in acid
soil. For other plants, apply limestone at 3 to 5 pounds
per 100 square feet on top of a 2-inch Sphagnum peat mulch
counteract the acidity. Sphagnum peat is very resistant
to decomposition and has many fibers into which roots
often grow profusely. Hypnum peat is usually neutral or
slightly alkaline, breaks down more rapidly and may have
few fibers.
Most types of peat are brown and serve as a neutral
background in the landscape. If the surface of a peat
mulch becomes very dry, it will shed water and require
stirring. Carelessly tossed cigarettes can ignite peat,
but it smolders rather than flames. Fine peat will often
be blown away in windy weather, but chunky peat will hold.
Self-seeding of some plants may occur in peat.
Pine Boughs or Needles
These are usually recommended for winter protection of
newly set or somewhat tender ornamental plants but can
serve as a mulch as well. Any needle-type evergreen is a
satisfactory source.
Sawdust
Sawdust is very commonly used in areas where it is readily
available. Nitrogen deficiency is almost certain if
fertilizer is not applied regularly. The alleged toxicity
of sawdust is usually nitrogen deficiency in the soil from
the action of micro-organisms decomposing the material.
Sawdust from walnut logs can stunt plants because of toxic
materials in it.
Bark--Shredded, Chunked or Chipped
Bark has become popular in recent years. It makes an
excellent mulch and is very attractive in landscape
plantings. Shredded, chunked or chipped bark lasts as
long or somewhat longer than peat moss and adds organic
matter to the soil. Bark may also be used for walks or
paths in the garden area. To prevent nitrogen deficiency
in plants mulched with bark, apply nitrogen fertilizer.
Straw
Straw is used for winter protection and as a summer mulch.
It is highly flammable, so do not use it where cigarettes
or matches could be carelessly flipped into the material.
Additional nitrogen must be applied to prevent starvation
of the mulched plants. Weed seeds may be introduced with
this mulch. Unless the straw is chopped into short
lengths, it will be difficult to apply between plants
growing close together.
Wood Chips And Shavings
Wood chips and shavings have become available in large
quantities from utility companies. These materials vary
in coarseness. They are sometimes rather fine or may be
largely broken twigs or branches. Wood chips decompose
slowly and may cause nitrogen deficiency if additional
fertilizer is not applied. They make a very
rustic-looking, satisfactory mulch. Wood chips should not
be used in foundation plantings next to the house where
termites are or could be a problem.
This information is for educational purposes only. References
to commercial products or trade names does not imply
endorsement by MSU Extension or bias against those not
mentioned. This information becomes public property upon
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