Mulching
A mulch is any kind of material applied to the soil
surface for protection or aesthetic improvement of the area
covered. The mulch material may be organic, such as bark or
straw, or inert, such as stones or polyethylene cloth.
A mulch helps maintain favorable soil conditions under
it. Increased plant growth is due primarily to conditions
resulting from the use of a given material rather than to
any growth-promoting substances present in the mulch itself.
Mulches have many beneficial effects on the soil, plants
and the area surrounding the plants. A mulch:
--Conserves soil moisture by reducing the evaporation of
water from the soil.
--Prevents crusting of the soil surface, thus improving
absorption and percolation of water to the soil areas where
roots are growing.
--Maintains more uniform soil temperatures by insulating
the soil, keeping it warm during cool spells and cool during
the warm months of the year.
--Reduces weed problems when the mulch material is weed-
free and is applied deeply enough to prevent weed seed
germination or to smother existing small weeds. Proper use
of mulches considerably reduces time and labor needed to
weed garden areas.
--Adds to the beauty of the landscape by providing a
uniformly colored ground cover that may add an interesting
texture to an otherwise drab surface.
--Prevents plants and fruit from becoming mud splashed
and so reduces losses from soil-borne diseases.
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.