- Bhat, N.R., Thomas L. Prince, Harry K. Tayama and Stephen A. Carver. 1992. Rooted cutting establishment in media containing wetting agent. HortScience, Vol. 27, No. 1. January. p. 78.
- Williams, Greg and Pat Williams. 1989. Mixing your own growing media for potted plants. HortIdeas. October. p. 118ñ119.
- GMPro
- Greenhouse Product News
Organic Potting Mixes
| Horticulture Technical Notes |
Appropriate
Technology Transfer for Rural Areas (ATTRA)
Index
Introduction
Since the 1960s, most commercial greenhouse businesses have been using
potting mixes that contain no real soil at all. These types of mixes are
referred to as soilless mixes and are made from ingredients such as sphagnum
peat moss, vermiculite or perlite. Producers began using soilless potting
mixes because of the problems associated with using topsoil: quality topsoil
was hard to find, it was too heavy for good air and water circulation,
and it often contained disease organisms.
All good potting mixes should:
1) Provide water and air.
4) Be of consistent quality.
Organic Potting Mixes
The production of certified organic vegetable transplants
is dependent on an organic potting mix. Potting soils containing prohibited
materials will not be allowed under the proposed rules of the National
Organic Program. Synthetic chemical fertilizers and wetting agents--amendments
commonly used in commercial mixes--are restricted in most programs and
therefore alternative mixes are needed.
Purchasing a commercially prepared organic mix is the
easiest way to get started. Organic mixes are available from a few dealers
around the country. Unless a mix specifically states that it is organic,
it probably contains prohibited ingredients. A partial listing of suppliers
is included in the table below. A second option is to arrange a special
order from one of the large commercial potting mix suppliers, whereby they
agree to exclude starter fertilizers and wetting agents. A third option
is to mix your own. As freight is often prohibitive in the first two cases,
many growers prefer the last method, but this presents a new set of challenges.
For recipes of potting mixes, see Appendix C.
There are no industry-wide standards concerning potting
media, so the grower should be wary of buying bagged materials with which
he or she is not familiar. Bags labeled as compost, for example, may contain
inorganic materials, depending on where the compost came from and what
composes it.
Types of Potting Mixes
The types and amounts of potting mix ingredients should
change depending on the intended use. Mixes used to start seeds must be
very light and full of air because seedlings die easily in heavy, wet soil.
Plants grown in fairly small plug trays need a soil mix that is lighter
than one used in a larger plug trays. In general, the younger and smaller
the plant, the lighter the soil mix needs to be. Soil mixes that are used
after the seeds are started usually contain some perlite or sand, as well
as peat.
Sphagnum peat moss. Also called peat moss or simply
peat. Peat moss is a very stable source of organic material that holds
a great deal of water and air and does not decompose quickly. Peat moss
is quite acidic (pH 3.5-4.0); limestone is added to the mix to balance
the pH. Younger, lighter colored peat moss does a better job of providing
air space than does older, darker peats that have few large pores for air
space. A wide range of quality exists. Peat is the most widely used soilless
medium, because of its wide availability and relatively low cost.
Composted pine bark. Its high lignin content makes
it slow to degrade, so it nourishes beneficial organisms for months. Can
be substituted in part for peat moss. Lightens the mix.
Sawdust. Similar to peat moss in most ways. Quality
depends on type of tree: cedar, walnut, and redwood can be toxic to plants,
and oak, hickory and maple deplete soil nitrogen more so than sawdust from
evergreen needle trees.
Vermiculite: Handled gently, vermiculite provides
plenty of air spaces in a mix. Handled roughly, vermiculite compacts and
loses its ability to hold air. Vermiculite holds water and fertilizer in
the potting mix. It also contains calcium and magnesium, and has a near-neutral
pH. Vermiculite comes in different grades. Medium grade is usually used
for starting seeds. A coarse grade can be used in soil mix for older plants.
Perlite: Perlite is a volcanic rock that has been
heated and expanded to become a lightweight, white material. Perlite is
sterile and pH-neutral. When added to a soil mix, perlite can improve air
space and water drainage. It is a hard material that does not break apart
easily. Perlite pieces create tiny air tunnels, that allow water and air
to flow freely to the roots. Perlite will hold from 3 to 4 times its weight
in water, yet will not become soggy. Perlite can be used instead of sand
to reduce the soil mix weight. It holds very little water and costs are
relatively high. It is much lighter than sand and can be used instead of
it.
Limestone: Calcium carbonate (CaC03)
or calcium magnesium carbonate (called dolomitic limestone) is used to
adjust pH. The range can be adjusted for specific crops, but a pH range
of 5.5 to 6 is ideal for most crops. Lime should be well-ground.
Sand: Coarse sand (sometimes called builderís sand)
will add air space to the potting mix. Fine sand settles into the spaces
between other ingredients and makes a dense mix that excludes air. Clean,
washed sand has a near-neutral pH and little if any food value for plants.
Sand is much heavier than any other ingredient used in potting mixes. The
added weight is good for tall, top-heavy plants that might blow or tip
over, but it is not the best choice for plants that will be shipped or
moved a lot. Sand is the least expensive and most readily available source
of larger-particle material.
Commercial Starter Fertilizer: Most commercial
mixes contain starter fertilizer blends. Whether the blends are made of
micro- or macro-nutrients, they are usually formed from synthetic fertilizer.
These fertilizers are prohibited from use in potting media used for certified
organic production.
Wetting Agents: Wetting agents are a common ingredient
in commercial potting mixes. They are included to help regulate moisture,
improve aeration, and increase nutrient availability. Most wetting agents
do not qualify for approval by organic certifying agencies. Check with
a local certifying agent to see if there are any approved materials with
wetting properties. Safers Soap® products have surfactant qualities
and might be suitable. Some certifying agencies also accept Shakleeís Basic-H®
and similar materials considered to be biodegradable. Wetting agents should
be used conservatively. Some commercial materials have been found to be
deleterious at too high a concentration (1). A University
of California Peat-Lite mix employs 3 oz. of wetting agent for a 17 cubic
foot media mix consisting of equal portions of peat moss and vermiculite(2).
The wetting qualities of compost-based mixes may be superior to peat-based
mixtures and may not need wetting agents.
Compost: Made from a variety of locally available
materials, usually a combination of ingredients that are high in nitrogen
(such as manure or alfalfa) and those high in carbon (such as straw or
corn stocks). Compost holds water well and provides nutrients; the amount
of nutrients provided depends on what it is made from. It is important
to select the highest quality compost available. It can be used with or
in place of peat moss. Compost is cheaper than peat moss and contains nutrients.
Compost can also provide natural protection against diseases of the seedlings
and roots of plants due to beneficial organisms that live in well-made
compost. For more information on disease-suppressive composts, contact
ATTRA.
Alfalfa: Alfalfa may be a good, locally available,
alternative to peat moss. Alfalfa provides nutrients, especially nitrogen,
that releases slowly in the mix, making the fertilizer
less likely to flow out of the pot when irrigated. Raw alfalfa must be
processed before use in a potting mix. To process: Grind raw alfalfa through
a 2 cm screen. Add water and decompose for 20 days. Air dry for another
20 days after decomposition.
Newspaper: Ground-up newspapers might be a good
ingredient in a potting mix, and could be used instead of peat moss. Newsprint
should not be more than 25% by volume of the mix.
Coir: Coir comes from coconut husks and is a waste
product of the coco fiber industry. Coirís structure is similar to that
of peat, but its pH is higher (5.5-6.0). It holds up to nine times its
weight in water. Coir can have a high salt content. See below for coir
suppliers.
Mineral wool: This is a coarse material resembling
fiberglass, spun from blast furnace slag. Very little research has been
done with mineral wool. It might be a sustainable option for growers near
factories that produce blast furnace slag.
Kenaf: Kenaf is a fibrous plant grown in the Deep
South. At the end of the growing season, kenaf plants are cut down and
parts of them used to make paper. The waste products can be used as growing
media. Growers who have used kenaf have seen excellent results.
For information on sources of organic fertilizers, request
ATTRAís Sources
or Organic Fertilizers and Amendments. For information on sources
of organic seeds and transplants, request ATTRAís
Alternative Seed Suppliers ó Untreated, Heirloom, and Organic.
Consult the Buyerís Guide issues of GMPro (3)
and Greenhouse Product News (4) for more
extensive coverage of other supplies.
References:
Suppliers of Coir
Crystal Company
Farm Wholesale, Inc.
Peaceful Valley Farm Supply
Suppliers of Soil Mixing Equipment
Amadas Industries
Bouldin & Lawson, Inc.
Gleason Equipment
H.C. Davis Sons Manufacturing
Co.
Javo USA Inc.
Ohio Central Steel Co.
Appendix A
P.O.
Box 3657
Fayetteville,
AR 72702
Phone:
1-800-346-9140 --- FAX: (501) 442-9842
Introduction
Organic Potting Mixes
Types of Portting Mixes
Suppliers
Appendix A - Potting Mixes
Appendix B - Suppliers of Commercial Potting Mixes
Appendix C - Potting Soil Recipes
Potting mixes are made of both solid particles and the open spaces
(pores) between them. Large pores are filled with air and small pores hold
water. A good potting mix should have a balance of both solid particles
and large and small pore space. Potting mixes with too many large pores
require frequent watering, but those with too many small pores are too
heavy and can promote disease. It is important to handle a potting mix
carefully once it is made to avoid compacting it and losing the air spaces.
2) Retain fertilizer.
Potting mixes made mostly from sand will have little ability to
hold plant food, while mixes formed from lots of peat moss or compost will
hold plant food well. The pH of the mix also affects nutrient availability.
Peat moss, which is the most commonly used ingredient in potting mixes,
is very acidic (low pH). Dolomitic limestone is often added to adjust the
pH and bring it closer to neutral. Compost and sand are usually neutral
(depending on what they are made from) and may require less limestone when
they are used in potting mixes.
3) Support the plant.
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