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Copyright 2005, Board of Trustees, University
Comparison of Several Peat Moss Products
C. A. Conover and R. T. Poole
University of Florida, IFAS
Agricultural Research and Education Center Apopka
AREC-A Research Report RH-85-10
Some controversy has always existed about baled and other peat mosses,
and the value provided by various brands and sources. Some growers buy only
Canadian peat moss because they feel it grows the best plants, while others
purchase native American peats. Observation of the crops that various
growers produce from these widely divergent peat sources indicates that good
quality plants can be produced in most peats, provided the grower
understands good cultural principles.
Potting medium components have certain inherent physical and chemical
properties that determine their value for plant production. Peat moss,
probably the most widely used amendment, provides high cation exchange
capacity (fertilizer retention), water holding capacity (ability to hold
water against the pull of gravity) and at the same time provides aeration
(oxygen to the roots). Peat moss is usually mixed with other components
such as pine bark, perlite, styrofoam or vermiculite to obtain potting media
with desired physical and chemical properties, but some growers
use 100% peat moss.
Potting Mix is an important factor not fully understood by some
growers. Growth of foliage plants is often reduced and sometimes plants are
killed because of inadequate aeration caused by overwatering in combination
with improper potting mix ingredients. Roots cannot readily absorb
nutrients except in the presence of oxygen and roots emit carbon dioxide
which can become toxic unless it escapes from the growing medium. Potting
mixtures must allow for rapid air exchange between the medium and the
atmosphere so that carbon dioxide is removed and oxygen supplied to roots.
Factors in the media which affect aeration include particle size, uni-
formity, compaction and irrigation practices. Small pore spaces hold water
against the force of gravity and air will be unable to move into such spaces
until water is removed by plant absorption or evaporation. Plants obtain
most of their water from small pores, but larger pores provide necessary
aeration. When the percentage of small pores is too high, potting media
will Femain wet for long periods and reduce aeration, while if the per-
centage of large pores is too high, poor growth may result because of lack
of water. Non-capillary pore space of good quality foliage potting mixtures
varies between 5 and 20% by volume after removal of gravitational water. A
measurement of non-capillary pore space is an indication of aeration
existing after drainage, while oxygen supplied from-capi-ll-ary pore space
exists only after water in these pores is 'u"ed by t8~e ianhtor evaporates.
Professor and Center Director, and Professor of Plant Physiology,
respectively, Agricultural Research and Educiti6in Center,' 2807 Binion
Road, Apopka, FL 32703. ------. ---
Cation Exchange Capacity (CEC) refers to the ability of soil to retain
nutrients against leaching effects of water and to release them for plant
growth. Peat mosses are usually high in CEC and provide buffering action to
prevent rapid changes in nutrient availability and pH. Potting media with a
high CEC have a high nutrient reservoir, whereas, potting mixtures or
components with low CEC retain smaller amounts of nutrients and, therefore,
require more frequent applications of fertilizer.
Rapid changes in acidity or alkalinity (pH) can be prevented by using
potting media with high CEC, but if pH correction is necessary, more
corrective materials will be required in high CEC media (that is why so much
dolomite or limestone is necessary to raise the pH of peat moss).
CEC is measured by the number of units of nutrients held by a given
quantity of soil (millequivalents [meq] per 100 grams [g] of soil). CEC of
5-40 meq/100 g or 20-50 meq/100 cc is satisfactory for good plant growth.
Very high CEC levels are not desirable either, because this will reduce the
ability to leach excessive soluble salts from peat moss or other media. The
use of meq/100 g as a unit of measurement can be misleading when discussing
potting mixtures with high organic content, and meq/container or unit volume
gives a more realistic indication of the character of the mixture.
Water Holding Capacity (WHC) is the amount of water held in a potting
medium after free drainage has occurred and is expressed as percent dry
weight or percent volume. This water is the water most readily absorbed and
used by plants. The amount of water held against the pull of gravity (free
drainage) will depend upon the surface area of medium particles per unit
volume, the absorptive properties of ingredients, attractive forces acting
between water and the particles and size of container. Small containers
require a more open potting mixture than larger sized pots because of
frictional forces of pot walls and height (pull) of gravity. Small size
pots containing potting mixtures having high water holding capacities are
often completely saturated because the gravitational pull is not strong
enough to remove water. However, the same potting mixture in a larger pot
will have better aeration because gravity will pull some water from the
upper. portion of the pot. Available water held, rate of water movement and
speed water enters the root zone are all influenced by potting mixture. A
water holding capacity of 40-100 percent by weight or 20-50 percent by
volume is satisfactory for foliage plant potting mixtures, but it is higher
in most peats, and this is the primary reason peat moss is mixed with other
components with lower water holding capacities so that the final average is
within the desired range.
Data that follow on 6 peat mosses were obtained from unopened bales in
the case of the 4 baled peat mosses, and by measurement of a full load of
the Florida peats (Table 1). All the baled peat mosses were shredded
through a Lindig Model -M4 shredder to obtain volume measurement, and each
of the physical and chemical samples listed is an average of 4 samples.
Cost Although baled peat mosses list specific contents in number of
cubic feet of material, there has always been some contention that there was
variation in yield after shredding.
Reasons variation may exist in yield of "fluffed" peat volume may
relate to compression pressure utilized in packaging, moisture level,
texture and particle size. No matter how the differences occur, they are
real, and yield has a direct relationship with final3cost. Data in Table 2
show the true cost of 6 peats in relation to cost/ft of usable material.
Quality Measurements of quality relate to cation exchange capacity
(CEC), water holding capacity (WHC), pore space, bulk density, pH and
soluble salts levels. Data in Table 3 show that CEC and WHC are adequate
for all peats tested. Low pH levels are normal for peat moss, and those
observed are within acceptable ranges. However, higher levels of dolomite
and/or limestone would be required to raise the pH of Florida peats due to
their slightly lower initial pH and higher CEC. Soluble salts levels were
low for all peats tested and would present no problem for crop growth.
Capillary and non-capillary pore space are important measurements of
peat quality, with non-capillary pore space the most important. In table 4,
peat No. 2 with 4.8 percent non-capillary pore space is slightly below the
suggested level, but if amended with coarser ingredients such as pine bark,
perlite or other materials could be a satisfactory peat source. All the
other peats had non-capillary pore space percentages above 5%, indicating
they would be satisfactory if combined with other amendments.
Relatively low levels of non-capillary pore space in this test compared
with a previous test (1) are the result of a change in testing procedure.
In this test media was compacted to a standard of 1.5 Ibs/in to simulate
potting pressures as compared to noncompacted peat in the previous test.
In summary, selection of a peat moss depends on personal preference,
cultural practices, cost and availability. All peats tested have physical
and chemical attributes that would indicate ability to grow excellent
foliage crops with normal cultural practices.
Table 1. Sources of peat moss used in these tests.
Fafard Peat Moss
ASB TORF Peat Moss
Weaver Black Forest
Peace River Peat
New Brunswick Canada
New Brunswick Canada
New Brunswick Canada
Polk Co. Fla. U.S.A.
Clay Co. Fla. U.S.A.
Table 2. Peat moss costs per cubic foot of product obtained after shredding
6 peats (FOB Apopka, Fl).
Table 3. Cation exchange and water holding
levels of 6 peats.
CEC % WHC Soluble
capacities, pH and soluble salts
Table 4. Capillary and non-capillary pore space found in 6 peats.
pore space %