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TECHNICAL BACKGROUND ON THE
CHARACTERISTICS OF SOILLESS MEDIA AND HOW TO
JUDGE MEDIA QUALITY
Media is composed of solid, liquid,
and gaseous components
Solid materials usually constitute 33-50% of the
media volume. Spaces, or pores , between the
solid particles are filled with air or water. As
water moves through container media, it is
retained by smaller pores, but drains through
larger pores.
The second fraction of the media, the liquid
portion, consists of nutrients, organic
materials, dissolved gases, and water.
The third media phase consists of gaseous
materials including oxygen and carbon dioxide.
Although media oxygen levels vary from 0-21%, a
concentration of at least 12% oxygen is necessary
for root initiation to occur. Roots of most
plants fail to grow in a media atmosphere
containing less than 3% oxygen. The carbon
dioxide content of the media may range from 0.03%
to 21%; however, very high carbon dioxide
contents may be detrimental to plant health
(Bilderback, 1982).
Understanding the attributes of these media
components, as well as the interactions between
these components is essential.
Media pH
Media pH is a measure of the acidity or
alkalinity of a substrate, with a pH = 7
indicating a neutral pH. Measured on a
logarithmic scale ranging from 0 to 14, a pH
greater than 7 denotes alkaline media and a pH
less than 7 signifies acidic media. The acidity
of media is determined by the concentration of
hydrogen ions [H+] on media particles and in the
media solution. The chemical composition of media
particles, the ratio of media components in the
mix, and irrigation and fertilizer practices
affect the pH of growing media. Container media
can increase 0.5 - 1.0 pH units during the
growing season as a result of alkaline irrigation
water.
Microorganism activity
The pH of organic media influences the
activity of microorganisms; bacteria are more
prevalent at pH > 5.5, while fungi are most
active at pH < 5.5. Nitrification occurs most
readily at a neutral pH, contributing to the
transformation of the ammonium-nitrogen cation
(NH4+) to the nitrate-nitrogen anion (NO3-); this
increases the potential for nitrogen leaching
from the soilless media solution.
Nutrient availability
Micronutrient availability is optimal at media
5.0 < pH < 6.5. However, because these
nutrients are furnished through fertilization, pH
regulation is not as crucial with container-grown
nursery crops as it is with field-grown woody
ornamentals. It is usually unnecessary to modify
the container media to a pH greater than 6.5 for
most woody plant species if sufficient levels of
nutrients are available; for Ericaceous crops,
the media pH should not exceed the value of 5.5.
Soluble salts
Because media is restricted to a limited
container volume, ions from dissolved fertilizers
and irrigation water can accumulate and
contribute to high soluble salts levels in the
media water extract. Media, fertilizer materials,
and irrigation water sources should be selected
to minimize soluble salts buildup; in addition,
media solution soluble salts levels should be
monitored regularly.
Buffering Capacity
Buffering capacity is the ability of media to
withstand rapid pH fluctuations. Media with a
high buffering capacity requires incorporation of
a greater quantity of acid or base to alter the
pH than media with a low buffering capacity.
Media characterized by low buffering capacities
include sandy mixes containing little organic
matter, while media exhibiting high buffering
capacities are usually composed of greater
quantities of organic matter such as peat moss,
bark, punga fibre. Select a container media with
as high of a buffering capacity as possible to
alleviate unexpected pH fluctuations.
Low initial fertility
Nutrient levels can be more accurately
monitored in media characterized by minimal
inherent nutrient value than in purchased and
prepackaged media containing pre-incorporated
fertilizer materials. Low initial media fertility
affords the grower the opportunity to develop a
fertilization program targeted towards
fulfillment of the nutrient requirements
associated with the developmental stage of the
species in production.
Some types of media may render certain nutrients
unavailable for plant uptake. Use of media such
as sawdust or bark that is not adequately
composted can lead to the unavailability of
nitrogen for plant absorption as microorganisms
break down these materials and assimilate the
nitrogen for their own use. Vermiculite can
inhibit absorption of phosphorus and iron;
likewise, certain kinds of pine bark can
eliminate iron from the media solution.
Porosity
The amount of pore space in container media is
a critical physical characteristic which
influences water and nutrient absorption and gas
exchange by the root system. Pore space is
related to the shape, size, and arrangement of
media particles. Aeration porosity and
water-holding capacity are two critical physical
attributes of container media.
Total porosity reflects the total pore space
present in growing media; it represents the
percentage of the container media volume which is
not occupied by solid media particles. Porosity
is determined by media particle size and the
extent to which the particles can be compressed.
Total porosity is the sum of the aeration and
water-holding porosity of media and should
comprise over 50% of the container media volume.
Irrigating media to the point of saturation fills
the total pore space with water. As the media
drains by the force of gravity, smaller pores
remain filled with water while larger pores empty
and fill with air. When all water has drained
from the large pores, the amount of water
remaining in the medium's small pores is referred
to as container capacity. Aeration porosity is
comprised mainly of the large pore spaces,
macropores, which drain water freely as a result
of gravitational forces and remain filled with
air after media saturation and drainage.
For adequate gas exchange, aeration porosity
should constitute at least 15%, but ideally,
20-35% of the media volume. Water retaining
micropores should comprise 20-30% of the media
volume. Water held in even smaller pores is not
easily extracted by the plant. Conditions under
which these very small spaces are the only pores
retaining water often result in some stomatal
closure and wilting. As the media dries and water
is available only from the smallest pores,
significant wilting can occur.
For sufficient gas exchange, drainage, and
water-holding capacities, the proper proportion
of macropores to micropores is necessary. The
type of container media mix used determines the
amount of macropores and micropores in the media.
In addition, the size arrangement of pores is
important in the ultimate water-holding capacity
of the mix. A peat-sand mix contains a greater
number of large and medium sized pores than a
bark-sand mix. Media containing the greatest
amount of medium-sized pores has the potential to
hold more readily available water.
Media drainage
Sufficient media drainage is critical for
optimal plant growth. The rate at which water
drains from container media depends on the pore
size and cohesive and adhesive forces between the
water and container media. Media depth and pore
size affect the height of the perched water table
which is created when water saturates media pore
spaces. If coarse materials like gravel or sand
are placed in the bottom of the container, the
smaller pores in the media above this layer will
retain water until pressure forces the liquid
downward. Water accumulation above these coarse
materials elevate the perched water table.
If small pores are prevalent in the bottom layer
of container media, water will pass through the
larger pores above this layer fairly quickly and
saturate the base layer, potentially creating an
atmosphere too wet for vigorous root growth
(Swanson, 1989). Avoid media saturation in the
upper or lower layers of the container by
thoroughly mixing the media.
Drainage, or hydraulic conductivity, is the rate
at which water flows through the media.
Drainage is affected by the height of the
container. Containers that have identical heights
but different diameters have similar drainage
characteristics when the same media is used in
both. In general, water retention of container
media decreases as the height of the water column
increases. Media in a tall container
characterized by a greater depth drains more
readily than the same media in a short container
with a shallower media depth. Media in a short
container remains wetter than the same media in a
tall container because of a lack of drainage; use
a deeper container to improve media drainage.
When coarse material is placed at the bottom of a
container, the height of the column is shortened,
altering the drainage pattern. However, addition
of coarse materials to the container bottom aids
in drainage by constructing larger pore spaces.
Media capacity exists when large pore spaces do
not contain any free water after drainage. Water
is retained only in small pore spaces by adhesive
and cohesive forces. After drainage, such a
situation exists in the upper portion of
container media.
Water retention
As an indication of sufficient water
retention, media should absorb two inches of
water per hour without runoff. In addition, if
one quart of water can flow through media in a
one gallon container per minute, adequate
drainage exists.
Container media should also have the capability
to retain sufficient amounts of water for root
uptake. The ability of one cubic foot of media to
retain three gallons of water is indicative of
sufficient media moisture retention (Swanson,
1989).
Ions
Cation exchange capacity
Cation exchange capacity (CEC) quantifies the
ability of media to provide a nutrient reserve
for plant uptake. It is the sum of exchangeable
cations, or positively charged ions, media can
adsorb per unit weight or volume. It is usually
measured in milligram equivalents per 100 g or
100 cm3 (meq/100 g or meq/100 cm3, respectively).
A high CEC value characterizes media with a high
nutrient-holding capacity that can retain
nutrients for plant uptake between applications
of fertilizer. Media characterized by a high CEC
retains nutrients from leaching during
irrigation. In addition, a high CEC provides a
buffer from abrupt fluctuations in media salinity
and pH.
Important cations in the cation exchange complex
in order of adsorption strength include calcium
(Ca2+) > magnesium (Mg2+) > potassium (K+)
> ammonium (NH4+), and sodium (Na+).
Micronutrients which also are adsorbed to media
particles include iron (Fe2+ and Fe3+), manganese
(Mn2+), zinc (Zn2+), and copper (Cu2+).
The cations bind loosely to negatively charged
sites on media particles until they are released
into the liquid phase of the media. Once they are
released into the media solution, cations are
absorbed by plant roots or exchanged for other
cations held on the media particles.
Anion exchange capacity
Some media retains small quantities of anions,
negatively charged ions, in addition to cations.
However, anion exchange capacities are usually
negligible, allowing anions such as nitrate
(NO3-), chloride (Cl-), sulphate (SO4-), and
phosphate (H2PO4-) to leach from the media.
Percent base saturation
The concentration of potassium, magnesium, and
calcium expressed as a percentage of the cation
exchange capacity is referred to as the percent
base saturation. Values for percent base
saturation should be within the range of 1-5%,
10-15%, and 60-80% for potassium, magnesium, and
calcium, respectively. Media nutrient analysis
recommendations for the application of these
nutrients are established from the ratios of
potassium, magnesium, and calcium to each other
in addition to the quantity of these nutrients
present in the media.
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