Prepared by Maher Jabado
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 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.
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.
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.
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 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.
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.
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.
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.
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).
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.
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.
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.