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Understanding Properties of
Growing Media
P
LANT GROWTH is largely dependent
on the medium in which the plant
grows. Not all growing media are the
same. What makes one better than
another? Extensive research has been
done on the properties of growing
media. Some of those properties are
discussed in this unit.
Objective:
þ
Describe characteristics of growing media.
Key Terms:
Ñ
aeration
available water
bulk density
cation exchange
capacity
growing medium
hydroponics
leach
organic matter
pH
pore space
porosity
soil
soilless media
unavailable water
water-holding ability
Growing Media
The health and quality of horticultural crops rest largely with the growing medium. The
growing medium is the material in which plants are grown. In field production and in the
home garden, soil is the growing medium. Soil is the outer portion of the earth’s crust that
supports plant growth. In greenhouse production, a variety of materials are used as growing
media. Often, soilless media, or media that contain no soil, are used.
Some greenhouse operations grow plants in a nutrient solution without any soil, a practice
known as hydroponics. The plants in a hydroponics unit are sometimes supported in sand or
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gravel substrate. However, these
materials do not provide any
nutrients. Some hydroponics systems are designed so the roots
float in the nutrient-rich solution,
while in others a nutrient solution
is sprayed directly onto the roots.
FUNCTIONS OF THE
GROWING MEDIUM
The medium used has a direct
impact on the quality of the crop
grown. This makes sense because
the growing medium provides
much of the plant’s needs. A
growing medium has four basic
functions in regard to plant growth.
FIGURE 1. Lettuce is being grown without soils in this hydroponics facility.
t
The growing medium serves as a reservoir of water for plant use.
t
The growing medium provides nutrients for plant growth and development.
t
The growing medium permits the exchange of gases to and from the plant roots.
t
The growing medium provides support for the plant by giving roots firm anchorage.
PROPERTIES OF THE GROWING MEDIUM
Properties of growing media
have been identified. These properties must be kept in mind when
selecting or preparing the growing
medium for plants. Among the
properties that must be considered are organic matter, bulk density, porosity, aeration,
water-holding ability, pH, cation
exchange capacity, and
uniformity.
Organic Matter
Organic matter is decayed
or partially decayed remains of
plants and animals. The organic
FIGURE 2. Crop residue adds to the organic-matter content of a field.
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matter in the medium must be stable and must function throughout the life of the crop. Peat
moss and bark are the most common sources of organic matter in growing media. In recent
years, a material from coconuts called coir has gained wider use. Organic matter gives a
medium a dark color. Organic matter contributes to the soil’s fertility as well as to improved
aeration and water-holding capacity.
Bulk Density
Bulk density is the ratio of the mass of dry solids in a medium to the volume of the
medium. Light bulk density eases handling and shipping of the potted plants. Heavy bulk density provides support for plants and stability of the pots. For a potted plant, the growing
medium’s bulk density should be light enough to ease handling and heavy enough to support
the plant.
Porosity
Root hairs
Ro
ot
The space between solid particles of a
growing medium is called pore space.
Collectively, pore spaces in a medium
determine soil porosity. A higher percentage of porosity in a medium results
in good water drainage and aeration.
Good mineral or garden soils have about
50 percent pore space. Organic media
used in greenhouses have between 75
and 85 percent pore space.
Aeration
Soil
particles
Available
water
Aeration is the exchange of gases in a
medium. Pore spaces that allow air pockets within the medium are vital for
healthy root growth. Cellular respiration
conducted in the root cells requires oxygen to convert sugars to energy. A
byproduct of this respiration is carbon
dioxide. A medium must have sufficient
porosity to allow an exchange of carbon
dioxide and oxygen.
Air
pocket
FIGURE 3. Good mineral or garden soils have about 50 percent pore
space. The pores are filled with either water or air.
Water-Holding Ability
Water-holding ability means the medium has the characteristics to retain water that can
be absorbed by plant roots. Good water-holding ability is necessary to promote optimal plant
growth.
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Water in a growing medium may be available or unavailable for plants to absorb. Available
water is found in the pore spaces of the medium and is readily absorbed by plant roots.
Unavailable water is a thin film of water that binds so tightly on particles of medium that it
cannot be absorbed by plants.
pH
pH is the measure of acidity or
alkalinity of the medium. A
14-point scale is used to measure
pH. A neutral pH is 7.0. Any
reading between 0 and 7.0 is acid,
and any reading between 7.0 and
14.0 is alkaline, or basic. The pH
is determined by the concentration of hydrogen ions (H+) and
hydroxyl ions (OH–) in the soil
solution. A sample of pure water
has an equal number of hydrogen
ions and hydroxyl ions and is
therefore neutral. When hydrogen
ions outnumber hydroxyl ions,
FIGURE 4. pH is important in the availability of nutrients in the soil.
(Courtesy, Natural Resources conservation Service, USDA)
the solution is acidic. When
hydroxyl ions outnumber hydrogen ions, the solution is alkaline.
pH plays a large role in the availability of nutrients in the medium. Most essential elements
for plant growth are available to most plants when the soil pH is between 5.5 and 7.0. If the pH
is above or below that range, the availability of nutrients may be restricted.
Cation Exchange Capacity
Cation exchange capacity is the measure of a medium’s capacity to hold nutrients.
Many nutrients are positively charged cations, such as potassium (K+), calcium (Ca+2), magnesium (Mg+), copper (Cu+), iron (Fe+2 or Fe+3), manganese (Mn+2), and zinc (Zn+2). Clay and
organic matter particles in a medium have negatively charged sites. The cations are attracted to
these negatively charged sites on particles of the medium. This is the same principle of attraction that applies to magnets.
A cation can leave the medium particle and be replaced by a cation held in the medium
solution. For instance, a potassium atom may leave the particle and be replaced by a copper
atom dissolved in the soil water. This replacement of one cation by another is called cation
exchange. The fertility of a medium is directly related to the number of cations the medium
can attract and hold. The greater the cation exchange capacity, the greater the fertility of the
medium.
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Mg
+
Negatively charged sites
H+
Zn
+2
Cu+
Ca+2
–
Mn+2
Cu
+
H+
–
– Particle –
–
H
–
Fe+2
Zn+2
+
Cation
exchange
K+
Mg+
Mn+2
Positively
charged
cations
Ca+2
Medium
+
K
Fe+2
+
H
FIGURE 5. Negatively charged clay and humus particles attract and hold positively charged cations.
Nutrients in solution are absorbed with water into plant roots. Nutrients also leach or
wash out of the medium during watering. The loss of nutrients from a growing medium
requires the grower to replace them with fertilizers.
Uniformity
In the greenhouse industry, the growing medium must provide conditions that encourage
uniform plant growth. This is because a crop in which all the plants have the same rate of
growth and development is highly valued by customers. A uniform growing medium has the
same texture and composition from one pot to the next. Uniform medium helps to ensure that
an entire crop grows and matures at the same rate.
Summary:
2
Growing medium is the material in which plants are grown. Soil is the outer portion of the earth’s crust that supports plant growth. Soilless media, or media that
contain no soil, are common in the horticulture industry.
A growing medium has four basic functions in regard to plant growth. The growing
medium serves as a reservoir of water for plant use. It provides nutrients for plant
growth and development. It permits the exchange of gases to and from the plant
roots. It provides support for the plant by giving roots firm anchorage.
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Properties of growing media that influence plant growth include organic matter,
bulk density, porosity, aeration, water-holding ability, pH, cation exchange capacity,
and uniformity.
Checking Your Knowledge:
´
1. What is growing medium?
2. What are the functions of a growing medium in regard to plant growth?
3. Why is soil porosity important?
4. Why is soil pH important?
5. What is cation exchange capacity?
Expanding Your Knowledge:
L
Try mixing your own growing media using a variety of components. Then, plant
plants of the same type and age in same-size pots using the different media. Give all
the plants the same growing conditions and observe the growth. After several
months, note which growing medium provided the best growing conditions for the
plants.
Web Links:
:
Quality Growing Media
http://www.suite101.com/article.cfm/propagation_seed_starting/105871
Growing Media for Greenhouse Production
http://www.utextension.utk.edu/publications/pbfiles/PB1618.pdf
Soils and Growing Media
http://www.hort.purdue.edu/hort/courses/hort101spring/Soils&GrowingMedia/
sld001.htm
Agricultural Career Profiles
http://www.mycaert.com/career-profiles
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