Physical properties of Oregon
substrates
Dr. James Altland
1
Today’s talk
• Important physical properties
• The parent materials
– Douglas fir bark
– Pumice
– Peat
• Substrate mixing
2
A good container media must:
• Provide anchorage for the plant
• Provide a reservoir for water
• Allow oxygen/gas exchange for roots
• Retain nutrients for uptake
These are the 4 functions of a container substrate. Consider these criteria when
selecting or changing a substrate for your container crops.
3
What makes a container media?
• Media…….or……..Substrate
• Media is composed of one or more components
• Components
–
–
–
–
–
Bark
Perlite
Peat
Vermiculite
Etc.
A substrate, media, or potting mix, is a combination of one or more components.
4
• “Making media is similar to making soup.”
• “Learn to shift your thinking from ingredients
and components to properties and parameters.”
– William Fonteno
When you add ingredients to make soup, the ‘components’ meld together so that
often the original ingredients are not even recognizable. Similarly, when you add
components of a container mix, the resulting substrate cannot be described simply
by the additive properties of the original components. The new substrate is a
unique substance.
For example, mix 1 cubic yard of bark and 1 cubic yard of sand, and you will NOT
have 2 cubic yards of the resulting substrate. The sand settles between the pores
of the bark to create mix that will be much less than 2 cubic yards.
Similarly, the physical properties of the resulting substrate are not the additive
properties of the two original components.
5
Physical properties
• Primary characteristics
– Bulk density
– Total porosity
– Air space
– Water holding capacity
• Secondary characteristics
– Moisture retention
These are the primary physical properties that are often measured for container
substrates. Moisture retention, or moisture release curves are also important, but
very difficult to execute accurately. Data from moisture release curves should be
interpreted cautiously.
6
Bulk density
• Bulk density
– Weight per unit volume
• Styrofoam beads – very low bulk density
• Sand – very high bulk density
– Ideal bulk density
• 70-90 lb/ft3
• 0.15 – 0.3 g/cm3
7
Bulk density
• Increased bulk density
– More stable pots, less blow-over
– Heavier pots for moving
– Heavier pots for shipping (freight cost)
– Talstar incorporation based on BD
8
Provide a reservoir for water
• Total porosity (%)
– The percent of a container composed of pore
spaces
– Total porosity (TP) is composed of :
• Air space (AS)
• Water (WHC)
• TP = AS + WHC
9
When bark (or any substrate) is added to a container, a portion of the container is
filled with solids, and a portion is empty spaces. The total of the empty spaces is
called ‘Total Porosity’.
10
One could imagine filling all the empty pore spaces with water.
11
When the water is drained, a portion of the container pore spaces retains water.
This fraction of the container is called the Water Holding Capacity (WHC).
The fraction of the container from which water drains and is subsequently filled with
air is called Air Space.
12
Air space
• The portion of the container filled with air,
after the media is thoroughly irrigated,
then allowed to drain.
13
Air space
• Roots require oxygen for respiration.
– Respiration is a biological process that
converts sugars into energy.
• Roots exude CO2, which must be
diffused away from the root surface.
14
Water holding capacity
• WHC of a media
– The portion of the container that is water, after
irrigated and allowed to drain.
• The amount of water held in a container
depends on 2 things
– Media particle size
– Container height
15
Small pore spaces are completely filled with water. Larger pores are partially filled
with air, with water forming a film around the soil particles.
16
Normal ranges – soil vs. container
• Field soil (typical)
– Solids – 50%
– Total porosity – 50%
• Air space – 25%
• Water holding cap. –
25%
• Container (1 gallon)
– Solids – 15%
– Total porosity – 85%
• Air space – 25%
• Water holding cap. – 60%
WHC
Solid
WHC
Solid
AS
AS
Compare and contrast the porosity, air space, and water holding capacity of a
typical soil and typical container substrate.
17
Water holding capacity
• Unavailable water
– Volume of water in a container not available to
plants
– Water in very small pores (<0.03 mm)
– Water that adsorbs to the surface of soil particles
in large pores.
• Available water
– Water available to plants
– In most container media, roughly ½ the volume of
water is available (so ½ is unavailable).
18
Under pressure!!!!
• Gravitational potential
– Water runs down hill
– Gravity pulls water down, and through drain holes in
the container bottom
• Matric potential
– Water resists gravity by:
• Hydrogen bonding to solid particles
• Capillary action
19
Perched water table (PWT)
• At bottom of container,
matric potential of media
exerts greater pull than
gravitational potential
• A perched water table is
formed at the bottom of
containers
20
151 ml
156 ml
168 ml
180 ml
199 ml
229 ml
261 ml
Notice the volume of water that occurs in each section of a container. From top to
bottom, the amount of water gradually increases and forms a zone of saturation
near the bottom of the container.
21
Factors affecting perched water
table
• Coarse particles
– Large pores
– Lower PWT
• Fine particles in
media
– Small pore spaces
– More capillary action
– High PWT
22
Height of container
• Perched water table will be at the same
height, regardless of container height.
– Assuming the same media
23
Ideal ranges for container crops
• Total porosity
– 50-85%
• Air space
– 10-30%
• Water holding capacity
– 45-65%
These are listed as ideal
ranges for containers in the
southeast U.S.
Should Oregon follow the
same recommendations?
Some Oregon nursery
producers believe our
substrates should have
more Air space and less
WHC to allow for winter
drainage.
24
Substrate mixes
• How does pumice and peat affect
container physical properties?
• How does bark particle size affect physical
properties?
25
Two popular Oregon mixes
• 50% bark
30% peat
20% pumice
• 75% bark
10% peat
15% pumice
• Which has greater porosity?
• Which has greater water holding capacity?
26
Physical properties of 2 substrates
100
15
16
62
63
23
21
50 bark: 30 peat: 20 pumice
75 bark: 10 peat: 15 pumice
80
60
40
Solid
WHC
Air
20
0
These are the measured physical properties of the two previously listed substrates.
Despite drastically different component rates, their physical properties are very
similar.
27
The raw materials
• Douglas fir bark
• Pumice
• Peat moss
28
Where does bark come from?
Chris Schnepf, University of Idaho,
www.forestryimages.org
Doug Page, USDI Bureau of Land
management, www.forestryimages.org
David P. Shorthouse, University of
Alberta, www.forestryimages.org
Trees are harvested by lumber mills virtually year-round. Bark removal is easy
during the spring when water flows readily through xylem. However, during fall and
winter, bark is more difficult to remove thus lumber mills scrape more wood off the
tree in an effort to remove all the undesirable bark. Higher concentrations of wood
in bark supplies is just one way that chemical and physical properties of bark
change throughout the year.
29
Large bark piles at a local (Oregon) bark supplier.
It’s important to note that Douglas fir bark is often ‘aged’, or stored in large piles for
3 to 7 months. It is not composted. Composting would require that it be stored in
piles about 8 feet high, irrigated, turned occasionally, aerated, etc. The process
shown above is not composting.
30
In the foreground is aged bark, in the background is fresh bark. Notice the change
in color. The aging process is poorly understood with Douglas fir bark in Oregon.
One of our goals was to document the differences in fresh and aged Douglas fir
bark with respect to its physical properties.
31
Introduction
• Douglas fir bark
– Primary container component
Aged
Fresh
• Fresh and aged bark are used
• Aged bark
– Large piles sit undisturbed for
several months.
– Not composted.
DF bark is the primary container component in Oregon nurseries.
Aged bark refers to large piles of this material that have been sit undisturbed for
several months.
It is important to note that the aging process is not a true composting process.
32
Particle distribution of raw bark
35
30
Bark (%)
25
20
Aged
Fresh
15
10
5
0
0
0.11
0.18 0.25
0.36
1
1.4
2
2.8
4
6.3
Sieve size (mm)
Particle size distribution:
Along the X axis is the sieve size in mm. Along the Y axis is the percent by weight
that was retained in each sieve size.
Fresh bark has a slightly higher percentage of large particles, while aged bark has a
slightly higher percentage of fine particles. Differences are very minor.
33
Physical properties of raw bark
100
21
18
22
18
32
42
38
40
80
60
55
40
20
25
46
41
40
Solids
Water
Air
42
20
0
Fresh 1
Aged 1
Fresh 2
Aged 2
Recom.
range
Aluminum porometer:
Along the x axis are listed the bark types and the recommended range of physical
properties for container media.
Along the y axis is the percent of the container volume attributed to solids, air
space, and water holding capacity.
When used as the sole substrate, the four bark types have less than ideal water
holding capacity and high air space. However, there is no difference between the
four.
34
Pumice
• Raw volcanic material
– Mined
– Graded to size
• Contains vesicles
– Light weight (when dry)
– Porous
35
A pumice mine in Oregon (near Bend). The grey ribbon of pumice at the bottom is
about 20 feet thick. A layer of volcanic ash sits on top of this ribbon and can range
from 20 to 100 feet thick.
36
Up close to the ribbon layer.
37
Pumice in the mine appeared grey in color, however, when it dries it turns more
white. (Note that this and the previous slides showed pumice from a mine near
Bend, OR).
38
These photos were taken from another mine further south in Chemult, Oregon. This
ribbon of pumice is closer to the surface.
39
Machinery used to grade pumice to a particular size.
40
Pumice and the various particle sizes from the mine near Bend.
41
Notice the slight yellow color in the pumice from the mine near Chemult, OR. This
difference in color is mostly due to its slightly lower SiO2 content.
42
Bulk density of bark + pumice
0.6
Bulk density (g/cc)
0.5
Bark: pumice
0.4
Fine: sifted
Fine: unsifted
Medium: sifted
Medium: unsifted
0.3
0.2
0.1
0
0
10
20
30
100
Pumice (%)
Adding pumice to bark increases the bulk density of the mix, regardless of the bark
type used.
43
Fine bark + pumice
1.00
0.15
0.15
0.20
0.23
0.15
0.15
0.17
0.18
0.61
0.53
0.49
0.54
0.55
0.52
0.51
0.25
0.27
0.29
0.31
0.30
0.31
0.31
10
20
30
0
10
20
30
0.80
0.60
0.54
0.40
0.20
0.31
Solid
WHC
Air space
0.00
0
----Sifted pumice (%)----
---Unsifted pumice (%)---
Adding pumice to bark does not significantly change the physical properties of the
resulting substrate.
44
Medium bark + pumice
1.00
0.22
0.20
0.18
0.38
0.45
0.21
0.21
0.44
0.41
0.16
0.23
0.24
0.44
0.42
0.33
0.34
20
30
0.80
0.60
0.42
0.45
0.40
0.20
0.36
0.42
0.37
0.35
0.38
0.39
10
20
30
0
10
Solid
WCH
Air space
0.00
0
----Sifted pumice (%)----
---Unsifted pumice (%)---
Same data for medium grade bark, again no change in physical properties from
additions of pumice.
45
Pumice
• Adding pumice to Douglas fir bark
– Increases bulk density
• Stability
• Increased weight
– Has little or no impact on container physical
properties (AS, WHC, P)
– Does it reduce compaction over time?
– Does it improve moisture retention?
46
Peat moss
• Peat – organic residues of plants,
incompletely decomposed due to lack of
oxygen
• Peat used in Oregon is primarily
Sphagnum peat moss
– Other types not used
• Hypnum peat, reed peat, sedge peat
47
• Sphagnum peat moss
– Derived from peat bogs, composed of >60%
mosses in the genus Sphagnum
– Considered the highest quality type of peat
moss for horticulture
48
Sphagnum peat moss
• pH: 3.6 to 4.6
• Weed content: 0
• Bulk density: 0.07 to 0.09 g/cc
49
Medium grade bark + peat
0.20
0.16
0.12
Bulk density
0.19
0.08
0.15
0.14
0.04
0.14
0.08
0.00
0% peat
10% peat
20% peat
30% peat 100% peat
Adding peat to medium grade bark lowered its bulk density.
50
Medium grade bark + peat
1.00
0.13
0.11
0.13
0.47
0.55
0.51
0.38
0.39
0.34
0.36
0% peat
10% peat
20% peat
30% peat
0.23
0.80
0.60
0.39
0.40
0.20
Solid
WHC
Air space
0.00
Adding peat to medium grade bark increased the WHC of the resulting substrate.
51
Fine grade bark + peat
0.20
0.16
0.12
Bulk density
0.08
0.17
0.16
0.15
0.14
0.04
0.08
0.00
0% peat
10% peat
20% peat
30% peat 100% peat
Peat also lowers bulk density of fine grade bark.
52
Fine grade bark + peat
1.00
0.15
0.13
0.12
0.09
0.54
0.60
0.66
0.71
0.31
0.27
0.22
0.20
0% peat
10% peat
20% peat
30% peat
0.80
0.60
0.40
0.20
Solid
WHC
Air space
0.00
Adding peat to fine grade bark decreased air space and increases water holding
capacity.
53
Peat moss
• Spongy, fibrous material capable of storing
large amounts of available water.
• Increases WHC of Douglas fir bark
– Decreases air space
• Decreases bulk density
54
Summary
• Physical properties of Oregon substrates
are not well documented
• Future work
– Further investigate interaction of primary
substrate components
– Develop better guidelines for Oregon nursery
growers in selecting substrates.
55
Website
• http://oregonstate.edu/dept/nursery-weeds/
56