Unit 10 - Organic Matter, Peatlands, & Soil Erosion
Chapter 2 - Peatlands
Peat is dead and partly decayed plant matter that has
accumulated in a wet environment. Peat ordinarily does not form in an upland
forest (in aspen or oak, for example) because bacteria, fungi, and other
organisms rapidly break down the leaves and twigs on the ground.
In peatlands, stagnant waters inhibit the circulation
of gases, such as oxygen, that are vital to decomposing organisms. Cool
temperatures further retard decomposition. Slow glacial movement across the
landscape created numerous lakebeds and depressions in which decaying mosses,
reed and grass collected. In fact, Minnesota contains the largest development
of patterned
peat lands in the lower forty-eight states. The rate of accumulation in
Minnesota peatlands varies between 2 and 5 inches per 100 years.
Peat deposits are found in 42 states. There are
about 51 million acres of peatland in the U.S. For a peatlands map of the U.S.
go to MAP. The map above and in the lab shows
the extent of the 7.6 million acres of peat deposits in Minnesota. Peat is one
of the state's valuable natural non-renewable (or very slowly renewable)
resources. Minnesota peat lands are approximately six thousands years old.
In Canada there are 270 million acres of Peatlands
Peat is composed by volume of 84% water, 2% ash or mineral material, 8%
organic material, and 6% air. Some important characteristics of peat are:
1) Bulk density increases from 3 to 20 lbs. per cubic foot as the organic
matter becomes more decomposed.
2) Water content in a sample of peat decreases with decomposition. Peats may
contain 95% water when saturated. Non-decomposed peats will drain more easily.
3) Ash content is the percent mineral solids that remain after the organic
elements (carbon, oxygen, hydrogen) have been burned off. Ash content ranges
from less than 2% for fibric peat to 25% and greater for sapric peats.
Peatlands have formed primarily in two ways: either by the filling of small
lake basins (lakefill), or by the spreading outward from flat wet environments
across uplands (paludification).
Paludification, or swamping, refers to
the outward spread of wet, peat-forming environments over adjacent areas. This
process is responsible for the formation of the huge Red Lake peatlands in
northern Minnesota. It began with the onset of a cooler and wetter climate
about 3500 years ago. Because of poor drainage on flat or gently sloping land,
such as old glacial lake beds, reed-sedge peat began to accumulate. Sphagnum
moss began invading the sedge peatlands about 2000 B.C. Decay in sphagnum-rich
areas is particularly slow because both the sphagnum moss and peat are very
acidic and unusually water-absorbent.
See Diagram of Paludification
The Lakefill process begins as sedges
grow towards the center of the lake basin from the shore, forming a floating
mat of vegetation. Expansion of the mat into the lake allows other plants to
migrate onto the older, more stable portions. Ultimately, the floating mat is
on top of the accumulated peat, while the young leading edges continue to grow
outward from the shore and eventually cover the lake surface. The more plant
nutrients supplied to the lake, the faster the process.
See Diagram of Lakefill or this
Lakefill Diagram 2
The first peat type is the Fibric peat. It is mostly undecomposed and is tan to
light reddish brown in color. Fibric peats in Minnesota are primarily acidic
sphagnum-peat moss. Many of the Minnesota's northern peatlands have a shallow
cap of fibric peat, rarely greater than 5 feet thick.
Mosses - After flowering plants and ferns, mosses are the most diverse group
of plants, with more than 10,000 species in 700 genera. This makes mosses
almost twice as diverse as mammals. Mosses don't receive as much attention from
us as flowering plants, ferns, or conifers because most mosses are small and
inconspicuous. They have no vascular tissue or wood to lend them structural
support, nor do they have large leaves or showy cones or flowers. This does not
mean that mosses are not important; in fact, mosses play important roles in
reducing erosion along streams, water and nutrient cycling in tropical forests,
and insulating the arctic permafrost.
The Sphagnopsida are the peat mosses, the
most important mosses today both ecologically and economically. For instance,
peat-burning provides one-third of Ireland's energy requirements, and unlike
fossil fuels, peat is a renewable resource when properly managed.
In addition, peatlands are the habitat of commercial crops such as
blueberries and cranberries. Historically, peat mosses have been an important
resource as well. In addition to serving as a large component of peat fuel,
peat moss has absorptive and antiseptic properties, for which it has been used
for everything from diapers to dressing wounds.
These same qualities make peat important in horticulture as
potting medium and soil additive.
Minnesota fibric peats often occur in a black spruce forest with sphagnum
moss ground cover
. The soil
is classified as a Fibrist. Many areas of northern Minnesota will have similar
forests.
Hemic
(reed-sedge) peat is dark reddish brown. It is derived from
sedges, rushes, reeds, and woody plants. Hemic peat is intermediate between
fibric and sapric peats in degree of decomposition, ash content, and bulk
density. Most of Minnesota's peat is "reed-sedge". Sedges are plants
that grow in saturated soils, they are noted by having a triangular shaped
stem. Harvesting reed-sedge peat for sale requires the drying of the peat and
using a large vacuum to suck up the dry peat.
This is
a landscape view of an open reed-sedge hemic peat bog near McGregor in Aitkin
County. This area at one time was open water that has gradually filled in with
organic debris from the plant material growing around the edge. Eventually, the
lake was completely filled in with the vegetation.
Sapric peat is very dark and is the most decomposed
of the three peat types. It has the highest bulk density and ash content and
the lowest water content. Sapric peat is common as a thin layer at the base of
peat deposits, but occasionally it is found up to five feet in thickness. The
white material below the peat is marl (soft calcium carbonate and mud) that was
deposited in the lake bottom before the peat began to accumulate.
This is a view of sugar beets on Histosols near Hollandale in Freeborn
County (southern Minnesota).
The sapric
peats are often used for cultivated crops due to their higher pH and lower
water-holding capacities. These peats are often referred to as muck, or muck
farms. Many crops have been grown successfully on peat soil, even though the
growing season can be short and unpredictable.
Sod,
blueberries, wild rice, and vegetables are some examples of crops which thrive
on sapric peat soils. Most of the peatlands in Minnesota are presently used for
forestry, with black spruce being one of the major pulpwood species.
After
the peat soils are dry, care is needed to avoid wind erosion, as seen here,
since the peat is very light. Also, peat will oxidize slowly once the water
table has been lowered, and in some cases it can even catch on fire. After peat
has been farmed for a number of years, the fields will be considerably lower
due to subsidence. Notice this peat field's elevation compared to the road.
Minnesota's peat may be used for nearly every lawn, garden and
horticultural application. Minnesota producers harvest peat deposits about six
months a year, but stockpiles of this natural resource are bagged year round.
The beneficial effects of organic matter in the soil have long been recognized
as essential or good plant growth systems. In terms of quality, Minnesota peat
is comparable to any European or Canadian product. Shorter distances between
Minnesota producers and the marketplace help reduce transportation costs.
Peat is an important component of potting soil
mixes, soil conditioners, rooting medium, and packing medium for shipping.
Peats increase the water storage capacity of sandy soils and the permeability
of heavy clay soils. Added to any soil, peat will increase the nutrient holding
capacity of the soil. Peat Inc.-Aitkin
County, Minnesota
Peat can also be molded into peat pots or containers that can be
used to germinate seedlings. When they are ready for transplanting, they can be
put directly in the soil with minimal root disturbance. The peat will then
gradually decompose.
Jiffy Pots Inc.
Peat is a resource that will become even more valuable in the future as our
fossil fuel resources become more scarce. It can be mined directly as a
non-renewable fuel, or it can be used to grow renewable woody or non-woody (see
cattails) fuels.
How we use the peatlands of Minnesota
is yet to be decided.
In
Canada they have re-established the native vegetation after harvesting the
peat. This area was last harvested for peat in 1968.
For more information, see Peatlands around the World.
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Chapter 3 Soil Erosion
Lab Units
© Terence H. Cooper & Regents of the University of
Minnesota, 2007 The University of Minnesota is an equal opportunity educator
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