Horticulture Science
Lesson 20
Understanding Properties of
Growing Media
Interest Approach
Collect samples of growing media. Some suggestions are
water, sand, peat moss, gravel, garden soil, potting mix,
etc. Have the students vote on which growing medium
would be most effective for growing plants. Then lead a
discussion on what each sample contains that allows it to
be used as a growing media.
Student Learning Objectives
•Define the types of growing
media.
•Describe the functions of the
growing media.
•Determine desirable properties
of growing media.
Terms
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aeration
available water
bulk density
cation exchange capacity
growing medium
hydroponics
leach
organic matter
Terms
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pH
pore spaces
porosity
soil
soilless media
unavailable water
water-holding ability
What are the types of growing media
used in floriculture production?
• 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 the
home garden, soil is the
growing medium.
What are the types of growing media
used in floriculture production?
• Soil is the outer portion of the Earth’s
crust, which supports plant growth.
• In many cases,
soilless media or
media that contain no
soil are used.
What are the types of growing media
used in floriculture production?
• Some greenhouse operations grow plants in a
nutrient solution without any soil, which is a
practice known as hydroponics.
• The plants in a hydroponics unit are sometimes
supported in sand or gravel substrate for
support.
• However, these materials do not provide any
nutrients.
• With some hydroponic systems,
the roots float in the nutrient-rich
solution or a nutrient solution is
sprayed directly onto the roots.
What are the functions of the
growing media?
• Growing medium serves four basic
functions for plants.
1. Growing medium holds water for plant
use.
2. Growing medium provides nutrients for
plant growth and development.
3. Growing medium permits the exchange
of gases to and from the plant roots.
4. Growing medium provides support for
the plant by giving roots firm anchorage.
What are desirable properties of
growing media?
• Through research, desirable properties of
growing medium have been identified.
• Some desirable properties for growing
medium include organic matter, bulk
density, porosity, aeration, pH, and cation
exchange capacity.
What are desirable properties of
growing media?
• Organic matter is decayed or partially
decayed remains of plants and animals
found in the medium.
• The organic matter in the medium must be
stable and must function throughout the
life of the crop.
• It should remain
stable and not break
down before the crop
is finished.
What are desirable properties of
growing media?
• Peat moss and bark are the most common
sources of organic matter in growing
medium.
• In recent years, a material from coconuts
called coir has gained wider use.
• Organic matter gives soil its dark color.
• Organic matter contributes to the soil’s
fertility as well as improved aeration and
water-holding capacity.
What are desirable properties of
growing media?
• 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.
• High or heavy bulk density is needed to
provide support for plants.
• For potted plants the growing
medium’s bulk density should be
light enough to ease handling and
heavy enough to support the
plant.
What are desirable properties of
growing media?
• The spaces between the solid particles of a
growing medium are pore spaces.
• A higher percentage of pores or porosity
in a media results in better water drainage
and aeration or the exchange of gases.
• Good mineral or garden soils have about
50 percent pore space.
• Organic media used in greenhouses have
between 75 and 85 percent pore spaces.
What are desirable properties of
growing media?
What are desirable properties of
growing media?
• Aeration is the exchange of gases in the
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.
What are desirable properties of
growing media?
• A byproduct of this respiration is carbon
dioxide.
• It is important that a medium has sufficient
pore spaces to allow an exchange of
carbon dioxide and oxygen.
What are desirable properties of
growing media?
• For optimal plant growth, a growing
medium should have good water-holding
ability.
• Available water is water
that can be absorbed by
the plant roots.
• Available water is
found in the pore spaces
of the medium.
What are desirable properties of
growing media?
• Some water in the growing medium is
considered unavailable for plant use.
• Unavailable water is a thin film of water
that binds so tightly on media particles that
it cannot be used by plants.
What are desirable properties of
growing media?
• Medium pH plays a large role in the
availability of nutrients in the medium and
is the measure of acidity or alkalinity of the
medium.
1. A 14-point scale is used to measure pH.
– A neutral pH is 7.0.
– Any reading between zero and 7.0 is acid, and a solution
between 7.0 and 14.0 is said to be alkaline or basic.
– The pH is determined by the concentration of hydrogen
(H+) ions and hydroxyl ions (OHB) in the soil solution.
– A sample of pure water has an equal number of H+ and
OHB ions and is therefore neutral.
What are desirable properties of
growing media?
2. 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.
What are desirable properties of
growing media?
• Cation exchange capacity is the
measure of a media’s capacity to hold
nutrients.
1. 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).
What are desirable properties of
growing media?
• Clay and organic matter particles in the
medium have negative charged sites.
• The cations are attracted to these
negatively charged sites on media
particles.
• This is the same principle of attraction that
applies to magnets.
What are desirable properties of
growing media?
2. The cations 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 for another
is called cation exchange.
– The fertility of a medium is directly related to
the number of cations a medium can attract
and hold.
– The greater the cation exchange capacity, the
greater the fertility of the medium.
What are desirable properties of
growing media?
What are desirable properties of
growing media?
3. Nutrients leach or wash out of the
medium during watering.
• The loss of nutrients requires the grower to
replace them with fertilizers.
What are desirable properties of
growing media?
• 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 helps to ensure
that the plants grow at the same rate.
Review/Summary
•What are the types of growing
media used in floriculture
production?
•What are the functions of the
growing media?
•What are desirable properties
of growing media?