Post by suska on Apr 24, 2007 19:37:55 GMT 7
Dulu pernah saya kirim di milis.
ma_suska
Selecting Growing Media
By Paul Westervelt, Department of Horticulture, Virginia Tech, Blacksburg,
Virginia
The role of a growing medium is to provide water, nutrients, and support to the plant while also allowing for gas exchange. It is commonly thought that a certain growing medium has specific,
unalterable characteristics when it comes to each of the previously mentioned functions. In fact,
only providing support is inherently a part of the medium. The other 75% of the equation is under the control of the grower. Why is this important? It’s important because a balance between
air and water in the growing medium is necessary for healthy plants. Before talking about the various potential growing medium components, we must look at some of the physical and chemical properties of media.
Physical Properties
Bulk Density: Bulk density is defined as the dry weight of the media per unit volume. Media with a high bulk density will be heavier than that with a low bulk density. Media with a low bulk
density has many benefits: it is less strain on workers, it’s cheaper to ship, and when used in hanging baskets it puts less strain on the greenhouse frame. Crops prone to tipping over like Easter lilies and poinsettias may require a media with a slightly higher bulk density in order to
stabilize the plant.
Texture: Texture is defined as the size and distribution of particles in the mix. It affects porosity
and the water holding capacity of a given medium.
Porosity: Total porosity is the percentage of the media (or component) made up of pore space.
Pore space is divided into two categories, macropores and micropores. During irrigation, most
pores are full of water. After irrigation, water will drain from the macropores due to gravity and
they will refill with air. Water that drains from the pot after irrigation is called gravitational
water. When the container of a freshly irrigated media finishes dripping, it is said to be at
container capacity. The adhesive property of water allows the micropores to retain water. Good
pore space is essential for both air and water to be available to roots. Total media porosity is
determined by the relative percentages of the chosen media components, handling of the media,
irrigation practices, and pot size.
Most growing media are composed of several components. Different components will fit
together differently to create pores of various sizes. It stands to reason that a component as
coarse as composted pine bark will have more macropores than one as fine as peat. Once
combined, individual components may not retain their original porosity and the media must be
reexamined.
Overhead watering, especially during the first irrigation, can compact the media so take care to
water gently.
The way a medium is handled may impact porosity. Compacting media by packing it in a pot or
stacking filled pots will increase bulk density and decrease the percentage of macropores thereby
decreasing plant available water and decreasing air space. For growers who choose to mix their
own media, over-mixing may also decrease total porosity of components and the media. In short,
it is up to the grower to not only select the proper growing medium for a crop, but also correctly
manage that media.
Unavailable plant water: As the media dries down, eventually a point will be reached where the
media can hold onto the water so tightly that it is unavailable to plant roots. This point is the
permanent wilting point (PWP). All the water between field capacity and the PWP is plant
available water.
Decomposition: Organic components may break down over time changing the physical
properties of the growing medium. Some components break down more quickly than others. For
example, sawdust decomposes quickly while composted pine bark and peat decompose much
more slowly. As a result, a medium containing pine bark may hold its original physical
properties longer than one in which sawdust is substituted for bark. Degradation of the organic
components in a medium may lead to decreased total porosity resulting in decreased plant
available water and decreased air space.
Chemical Properties
pH: pH is a measure of the acidity or alkalinity of a growing medium by measuring the relative
amount of hydrogen ions in the media solution. On a scale of 1.0 – 14.0, a pH of 7 is neutral,
below 7 is acidic, and above 7 is basic (alkaline). The pH of a given medium regulates the
availability of plant nutrients and pH requirements can vary within plant species. The pH of a
soilless medium should range from 5.4 to 6.0. For media with more than 20% soil, it should
range from 6.2 to 6.8 (Fonteno, 1996) Media pH can change in response to fertilizers and water
alkalinity. Regular monitoring is recommended to avoid plant health problems.
CEC: Cation Exchange Capacity (CEC) is a measure of the nutrient holding capacity of a given
medium. The higher CEC a growing medium has the more nutrients it can hold so a high CEC is
desirable in soilless media. Components with a high CEC are peat, bark, and vermiculite.
Components with a low CEC are perlite, Styrofoam, and sand.
EC: All the nutrients in the soil solution are considered soluble salts. Electrical Conductivity
(EC) is a measure of the soluble salts in a medium. These salts often come from fertilizer, so EC
is often used to track fertilizer levels in growing media. Soluble salts may also be a result of the
break down of organic components in the media or impurities in the irrigation water.
Impact of Containers
Another thing to consider when examining media/water relations is pot size. The size and shape
of a container can greatly impact the amount of water and air available to plant roots. A taller
container will have a greater gravitational pull and more air space. (Figure 1A) Shorter
containers have less gravitational pull and less air space. Plug flats are the extreme case and may
have no air space after watering. (Figure 1B). For this reason, special consideration should be
given to media selected for plug production.
Figure 1. A (top). Effect of gravity on
media drainage in a freely draining
container. As height increases, less
water is retained.
B (below). Effect of container size on
air-water relations of a peat :
vermiculite (1:1) medium. (From
Fonteno 1996)
Media Components
Soil: Occasionally soil is used in growing media but not very often. It has a very high water
holding capacity, but often very poor aeration and can be extremely variable depending on
source. In addition, it must be sterilized as there is a bunch of soil born pathogens present in even
a small amount of soil. Media containing field soil are referred to as soil-based media while
those without soil are called soilless media.
Peat: Sphagnum peat moss is the one of the most common growing medium components. The
peat we use in the United States comes mainly from Canada, but also Florida. Peat is very
lightweight when dry, has a high CEC, decent aeration, and excellent water holding capacity.
The similarly named Sphagnum moss is often used to line wire baskets, but is not used in soilless
mixes and is not the same thing as Sphagnum peat moss. To avoid confusion, most people call
the component simply “peat”. Dry peat can be very difficult to rewet so most commercially
available mixes have a wetting agent included in the media to aid with the initial irrigation. Also
be aware that peat can have a very low pH often below 4.
Bark: While many types of composted bark are available, pine bark is what we primarily use in
the southeast. Composted pine bark has a high CEC, is relatively lightweight, and provides
excellent aeration and drainage. pH normally falls between 5 and 6. There are many producers of
composted pine bark so quality can vary. Bark is used extensively in nursery production.
Perlite: Perlite is volcanic rock heated to a very high temperature (1,800F) until it expands into a
very lightweight aggregate. Because of its much lighter weight, perlite is often used as a
substitute for sand and can increase aeration. It has a pH slightly above 7, almost no CEC, and is
used to increase drainage and aeration in the growing medium. Several sources note potential
dangers of using perlite with Fluoride sensitive crops.
Vermiculite: Vermiculite is a mined silica that is heated much like perlite quickly turning water
trapped between the layers to steam and expanding the ore 15 to 20 times its original size. The
result is stacks of tiny layers. Vermiculite has a high CEC, is very lightweight, and has an
excellent water holding capacity. Note that it is easily compressed and therefore it is easy to
destroy the beneficial structure by excessively handling or mixing. Vermiculite is available in
several particle sizes, the smallest sizes are commonly used as a seed germinating medium.
Coir: Coir is a by-product of the coconut processing industry. The husk fibers are composted,
screened into several grades then dried and pressed into blocks. The finer grades are gaining
popularity as a renewable peat substitute. Coir has a high CEC, a pH of about 7, and excellent
water holding capacity. In addition, it decomposes more slowly than peat. As with other
components, there is some variation due to source. Some sources have produced coir with
excessively high soluble salt levels.
Crop by-products: Several crop by-products such as peanut shells, rice hulls, and cocoa hulls
are used in soilless mixes, but not commonly. Due to high Carbon:Nitrogen ratios, they may tie
up Nitrogen in the medium. pH is variable.
Sand: Sand is often used in combination with pine bark for nursery production, but is not a
common component in greenhouse media. Many lightweight options are favored since sand is
one of the heaviest components. Potential problems include salts, clay, and pathogens present in
untreated batches.
Rock wool: Rock wool is another of the lesser seen components. It is made by melting one of
several minerals and then spinning it resulting in a component similar in consistency to cotton
candy or fiberglass insulation. Large blocks are used in hydroponic production and smaller
blocks for rooting cuttings. Finer particles can be used in growing media where they provide
excellent aeration and water retention.
Mixing Media
Mixing your own medium may be tempting, but it can be costly. Mixing equipment is required
as well as skilled labor. Also consider the cost of components and the potential consequences of
improper mixing. If you have less 100,000 square feet of greenhouse space, it is probably
cheaper to buy commercially available media. Commercial media mixes are sterile, uniform, and
consistent from batch to batch. Also, proper mixing and handling are guaranteed. There are a
surprising number of commercial mixes available, one for every imaginable growing situation. If
you can’t manage to find one that suits you, some companies will produce customized blends.
Resources used for this article:
Dole, J.M. and H.F. Wilkins. 1999. Floriculture: Principles and Species. pp. 79-89. Prentice-
Hall. Upper Saddle River, NJ.
Evans, M.R., S. Konduru, and R.H. Stamps. 1996. Source variation in physical and chemical
properties of coconut coir dust. HortScience 31:965-967.
Fonteno, W.C. 1996. Growing media: types and physical/chemical properties, pp. 93-122. In:
Water, Media, and Nutrition for Greenhouse Crops, D.W. Reed, editor. Ball Publishing, Batavia,
IL.
Merrow, A.W. 1997. Coir dust, a viable alternative to peat moss. Greenhouse Product News
7:17-21.
ma_suska
Selecting Growing Media
By Paul Westervelt, Department of Horticulture, Virginia Tech, Blacksburg,
Virginia
The role of a growing medium is to provide water, nutrients, and support to the plant while also allowing for gas exchange. It is commonly thought that a certain growing medium has specific,
unalterable characteristics when it comes to each of the previously mentioned functions. In fact,
only providing support is inherently a part of the medium. The other 75% of the equation is under the control of the grower. Why is this important? It’s important because a balance between
air and water in the growing medium is necessary for healthy plants. Before talking about the various potential growing medium components, we must look at some of the physical and chemical properties of media.
Physical Properties
Bulk Density: Bulk density is defined as the dry weight of the media per unit volume. Media with a high bulk density will be heavier than that with a low bulk density. Media with a low bulk
density has many benefits: it is less strain on workers, it’s cheaper to ship, and when used in hanging baskets it puts less strain on the greenhouse frame. Crops prone to tipping over like Easter lilies and poinsettias may require a media with a slightly higher bulk density in order to
stabilize the plant.
Texture: Texture is defined as the size and distribution of particles in the mix. It affects porosity
and the water holding capacity of a given medium.
Porosity: Total porosity is the percentage of the media (or component) made up of pore space.
Pore space is divided into two categories, macropores and micropores. During irrigation, most
pores are full of water. After irrigation, water will drain from the macropores due to gravity and
they will refill with air. Water that drains from the pot after irrigation is called gravitational
water. When the container of a freshly irrigated media finishes dripping, it is said to be at
container capacity. The adhesive property of water allows the micropores to retain water. Good
pore space is essential for both air and water to be available to roots. Total media porosity is
determined by the relative percentages of the chosen media components, handling of the media,
irrigation practices, and pot size.
Most growing media are composed of several components. Different components will fit
together differently to create pores of various sizes. It stands to reason that a component as
coarse as composted pine bark will have more macropores than one as fine as peat. Once
combined, individual components may not retain their original porosity and the media must be
reexamined.
Overhead watering, especially during the first irrigation, can compact the media so take care to
water gently.
The way a medium is handled may impact porosity. Compacting media by packing it in a pot or
stacking filled pots will increase bulk density and decrease the percentage of macropores thereby
decreasing plant available water and decreasing air space. For growers who choose to mix their
own media, over-mixing may also decrease total porosity of components and the media. In short,
it is up to the grower to not only select the proper growing medium for a crop, but also correctly
manage that media.
Unavailable plant water: As the media dries down, eventually a point will be reached where the
media can hold onto the water so tightly that it is unavailable to plant roots. This point is the
permanent wilting point (PWP). All the water between field capacity and the PWP is plant
available water.
Decomposition: Organic components may break down over time changing the physical
properties of the growing medium. Some components break down more quickly than others. For
example, sawdust decomposes quickly while composted pine bark and peat decompose much
more slowly. As a result, a medium containing pine bark may hold its original physical
properties longer than one in which sawdust is substituted for bark. Degradation of the organic
components in a medium may lead to decreased total porosity resulting in decreased plant
available water and decreased air space.
Chemical Properties
pH: pH is a measure of the acidity or alkalinity of a growing medium by measuring the relative
amount of hydrogen ions in the media solution. On a scale of 1.0 – 14.0, a pH of 7 is neutral,
below 7 is acidic, and above 7 is basic (alkaline). The pH of a given medium regulates the
availability of plant nutrients and pH requirements can vary within plant species. The pH of a
soilless medium should range from 5.4 to 6.0. For media with more than 20% soil, it should
range from 6.2 to 6.8 (Fonteno, 1996) Media pH can change in response to fertilizers and water
alkalinity. Regular monitoring is recommended to avoid plant health problems.
CEC: Cation Exchange Capacity (CEC) is a measure of the nutrient holding capacity of a given
medium. The higher CEC a growing medium has the more nutrients it can hold so a high CEC is
desirable in soilless media. Components with a high CEC are peat, bark, and vermiculite.
Components with a low CEC are perlite, Styrofoam, and sand.
EC: All the nutrients in the soil solution are considered soluble salts. Electrical Conductivity
(EC) is a measure of the soluble salts in a medium. These salts often come from fertilizer, so EC
is often used to track fertilizer levels in growing media. Soluble salts may also be a result of the
break down of organic components in the media or impurities in the irrigation water.
Impact of Containers
Another thing to consider when examining media/water relations is pot size. The size and shape
of a container can greatly impact the amount of water and air available to plant roots. A taller
container will have a greater gravitational pull and more air space. (Figure 1A) Shorter
containers have less gravitational pull and less air space. Plug flats are the extreme case and may
have no air space after watering. (Figure 1B). For this reason, special consideration should be
given to media selected for plug production.
Figure 1. A (top). Effect of gravity on
media drainage in a freely draining
container. As height increases, less
water is retained.
B (below). Effect of container size on
air-water relations of a peat :
vermiculite (1:1) medium. (From
Fonteno 1996)
Media Components
Soil: Occasionally soil is used in growing media but not very often. It has a very high water
holding capacity, but often very poor aeration and can be extremely variable depending on
source. In addition, it must be sterilized as there is a bunch of soil born pathogens present in even
a small amount of soil. Media containing field soil are referred to as soil-based media while
those without soil are called soilless media.
Peat: Sphagnum peat moss is the one of the most common growing medium components. The
peat we use in the United States comes mainly from Canada, but also Florida. Peat is very
lightweight when dry, has a high CEC, decent aeration, and excellent water holding capacity.
The similarly named Sphagnum moss is often used to line wire baskets, but is not used in soilless
mixes and is not the same thing as Sphagnum peat moss. To avoid confusion, most people call
the component simply “peat”. Dry peat can be very difficult to rewet so most commercially
available mixes have a wetting agent included in the media to aid with the initial irrigation. Also
be aware that peat can have a very low pH often below 4.
Bark: While many types of composted bark are available, pine bark is what we primarily use in
the southeast. Composted pine bark has a high CEC, is relatively lightweight, and provides
excellent aeration and drainage. pH normally falls between 5 and 6. There are many producers of
composted pine bark so quality can vary. Bark is used extensively in nursery production.
Perlite: Perlite is volcanic rock heated to a very high temperature (1,800F) until it expands into a
very lightweight aggregate. Because of its much lighter weight, perlite is often used as a
substitute for sand and can increase aeration. It has a pH slightly above 7, almost no CEC, and is
used to increase drainage and aeration in the growing medium. Several sources note potential
dangers of using perlite with Fluoride sensitive crops.
Vermiculite: Vermiculite is a mined silica that is heated much like perlite quickly turning water
trapped between the layers to steam and expanding the ore 15 to 20 times its original size. The
result is stacks of tiny layers. Vermiculite has a high CEC, is very lightweight, and has an
excellent water holding capacity. Note that it is easily compressed and therefore it is easy to
destroy the beneficial structure by excessively handling or mixing. Vermiculite is available in
several particle sizes, the smallest sizes are commonly used as a seed germinating medium.
Coir: Coir is a by-product of the coconut processing industry. The husk fibers are composted,
screened into several grades then dried and pressed into blocks. The finer grades are gaining
popularity as a renewable peat substitute. Coir has a high CEC, a pH of about 7, and excellent
water holding capacity. In addition, it decomposes more slowly than peat. As with other
components, there is some variation due to source. Some sources have produced coir with
excessively high soluble salt levels.
Crop by-products: Several crop by-products such as peanut shells, rice hulls, and cocoa hulls
are used in soilless mixes, but not commonly. Due to high Carbon:Nitrogen ratios, they may tie
up Nitrogen in the medium. pH is variable.
Sand: Sand is often used in combination with pine bark for nursery production, but is not a
common component in greenhouse media. Many lightweight options are favored since sand is
one of the heaviest components. Potential problems include salts, clay, and pathogens present in
untreated batches.
Rock wool: Rock wool is another of the lesser seen components. It is made by melting one of
several minerals and then spinning it resulting in a component similar in consistency to cotton
candy or fiberglass insulation. Large blocks are used in hydroponic production and smaller
blocks for rooting cuttings. Finer particles can be used in growing media where they provide
excellent aeration and water retention.
Mixing Media
Mixing your own medium may be tempting, but it can be costly. Mixing equipment is required
as well as skilled labor. Also consider the cost of components and the potential consequences of
improper mixing. If you have less 100,000 square feet of greenhouse space, it is probably
cheaper to buy commercially available media. Commercial media mixes are sterile, uniform, and
consistent from batch to batch. Also, proper mixing and handling are guaranteed. There are a
surprising number of commercial mixes available, one for every imaginable growing situation. If
you can’t manage to find one that suits you, some companies will produce customized blends.
Resources used for this article:
Dole, J.M. and H.F. Wilkins. 1999. Floriculture: Principles and Species. pp. 79-89. Prentice-
Hall. Upper Saddle River, NJ.
Evans, M.R., S. Konduru, and R.H. Stamps. 1996. Source variation in physical and chemical
properties of coconut coir dust. HortScience 31:965-967.
Fonteno, W.C. 1996. Growing media: types and physical/chemical properties, pp. 93-122. In:
Water, Media, and Nutrition for Greenhouse Crops, D.W. Reed, editor. Ball Publishing, Batavia,
IL.
Merrow, A.W. 1997. Coir dust, a viable alternative to peat moss. Greenhouse Product News
7:17-21.