Physical properties of container media
James Altland, Ph.D.
North Willamette Research and Extension Center
Oregon State University
What kind of media are you putting in your containers? Why? Earlier
we held a workshop at the North Willamette Research and Extension
Center, and asked all participating growers what they are using and why.
Most were using Douglas fir bark as the primary component and adding other
components such as field soil, pumice, peat, sand, perlite, and others.
When asked why all these extra components were added, nobody had a definitive
answer, but someone jokingly responded, “Because it makes us feel better.”
This person approached us after the seminar and commented that he really
wasn’t joking, and they had no good reason for adding the extra materials.
So think about it. Do you really know why you’re adding the things
you add? Maybe the answer is, it works, so leave well enough alone.
Maybe that’s a good answer, maybe not.
|Container media is made by mixing several
components. The physical properties of the mixed media are different from
This article addresses the important physical properties of container media
and focuses on the relationship between air and water.
4 functions of media
Container media must perform 4 functions: 1) provide a stable substrate for
root anchorage, 2) provide a reservoir of nutrients, 3) provide oxygen (gas
exchange) for roots, and 4) provide water for roots.
Media that uses bark as the primary component provides adequate anchorage
for plant roots, especially when potting small liners into 1 or 3 gallon
containers. However, when potting tall trees, or any plant with a large
canopy relative to root-ball size, consider adding sand to the mix to enhance
root anchorage. Coarse sand makes the media heavier; which will better
anchor roots in the containers and result in less liner or container blow-over.
But because sand makes medias heavier, moving plants will be more energy
consuming, and shipping plants may be more expensive.
A common misconception is that sand improves drainage. When using bark
as the primary component, adding sand will decrease drainage. Small
sand particles settle between large bark particles, thus decreasing pore
space and drainage. This concept is discussed more thoroughly below.
When using bark-based media, providing nutrients for plant uptake is not
a major concern. Micronutrients are supplied in special micronutrient
packages (MicroMax, STEM, Apex Micronutrient Package, and others) which bind
readily to organic matter. Controlled release fertilizers (CRFs) provide
nitrogen, phosphorus, and potassium over an extended period of time.
CRFs generally release more nitrogen than what is used by the plant, and
this nitrogen is lost through the bottom of the container. Research is
being conducted to determine methods for reducing nitrogen leaching, mostly
for environmental purposes. The most promising research involves the
use of clay materials (similar to kitty litter) that absorb phosphorus and
ammonium. Zeolite is currently used for this purpose, although the
jury is still out on whether or not it’s effective. Suffice it to
that with current fertilizer technologies, bark-based medias provide sufficient
nutrient retention for producing high quality plants, despite the fact that
N is leached at higher than desirable rates.
Container medias must have sufficient pore spaces to allow free movement
of gases. Plant roots constantly undergo respiration. Respiration
is a cellular process that burns sugars to create energy (sugars are generated
by photosynthesis in leaves). Cellular respiration consumes oxygen
and releases carbon dioxide (CO2) as a byproduct. There must be sufficient
pore spaces in the media for plant roots to acquire oxygen and expel CO2.
This tradeoff between CO2 and O2, called gas exchange, is an often-overlooked
aspect of selecting container media. After containers are completely
saturated and allowed to drain, 10 to 30% of the container volume should
be air space for gas exchange.
Container media has to retain water for plant roots. All medias retain
water, some more than others. For example, peat retains a great deal
of water while sand retains very little. Bark can be purchased in a
wide spectrum of particle sizes, with smaller particles holding more water
than large particles. After containers are saturated and drained, 45
to 65% of the container should be filled with water.
Of water held by container media, some is available to plants and some is
not. Through physical processes called adhesion and cohesion, water
is bound to media to form a thin film over particle surfaces. This
thin film of water is generally unavailable to plant roots. Available
water content is the portion of the water in a container accessible to plant
roots. In most bark-based medias, 50% of water is available while the
other 50% is not.
Defining container physical properties
Providing sufficient gas exchange and water are the most important, yet the
least understood aspects of container
media. Fill a container with
any media and there will be solid particles, with small spaces called pores
between the particles. The percent of container volume composed of
pore space is referred to as total porosity (TP). Pores in a media
can be filled with either air or water. The fraction filled with air
is called air space (AS) and the fraction filled with water is called water
holding capacity (WHC).
Media particle size influences the size of pore spaces and TP. Consider
filling a basket with apples and a similar basket with peas. The size
of the pore spaces in the pea basket will be much smaller than that in the
apple basket, and the overall TP will also be much less. Similarly,
coarse bark (large particle size) will have larger pores between particles
than fine bark (small particle size), and will also have more TP.
|Bark particle size influences media physical properties. As particle
size increases from left to right, so does total porosity.
Pore size affects available water content, drainage, and the distribution
of water in containers. Small pores (<0.01 mm) hold water so tightly
that it is unavailable for plant uptake; pores between 0.01 and 0.8 mm in
diameter contain water that is readily available for plant uptake; and pores
between 0.8 to 6 mm are so large that they do not hold water and are mostly
filled with air.
When using coarse bark as the primary component, adding sand decreases pore
size and TP. Sand particles are small enough that they settle between
bark particles, thus decreasing pore size and TP. Coarse pumice added
to bark is generally too large to fit between bark particles; therefore it
increases pore size and TP. Increased pore size increases drainage.
So if your goal is to increase drainage, consider using pumice instead of
sand. If you goal is to improve root anchorage and prevent liners and/or
containers from blowing over, consider adding sand.
Air vs. water
Container media should contain 50 to 85% pore space (TP). Total porosity
of container media is important, but probably more crucial is the portion
that is AS versus WHC. Some plants prefer wet soils while others prefer
dry soils. On average, 10 to 30% of the container volume should be
composed of air space while 45 to 65% should be water. Consider two
different medias, both with 75% TP. A media with 10% AS and 65% WHC
would be ideal for plants that prefer wet soils, while a media with 30% AS
and just 45% WHC would be better for plants that prefer drier soils.
|The volume of water held at each level of
the container is listed. Water is not distributed evenly;
the contaienr bottom is saturated
while the top is drier.
Water is not distributed evenly throughout the container.
cohesion, and capillary action attract water to particles and resist gravity.
The ability of media to ‘hold’ water through adhesion and cohesion
is referred to as matric potential. Matric potential is the same throughout the
container. Gravity pulls water down through the container and out of
the drainage holes. While gravity is constant throughout the container, gravitational potential is
greater at the top of the container and lower at the bottom. Because of this gradual decrease in gravitational potential
towards the container bottom, matric potential is higher at the container
bottom and media particles are able to hold more water. This causes
water to form a perched water table at the container bottom. The perched
water table is a layer of saturation on the container bottom.
Container height affects the relative amount of water versus air. With
the same media, the perched water table occurs at the same height, regardless
of the container size. Short containers will have the same perched
water table as large containers, thus a greater percentage of container volume
is filled with water. This explains why a 5-gallon container hold less
water than a 5-gallon squat container.
|For a given media, the perched water table remains the same regardless
of plant height. It is therefore unwise to use the same media in large
containers as small.
Determining physical properties
Now that you’re armed with information on media physical properties, you
need to determine what the physical properties are for your media.
Determining physical properties is easy. You can send a sample to a
laboratory, or you can determine physical properties yourself with an easy step-by-step
guide. I am also available to visit your nursery and help you determine
these physical properties (Oregon growers only, of course).
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