Propagation Soil
A good propagation mix must provide proper amounts of oxygen and moisture if seeds
and other plant material to maintain active growth and health. It must also provide
physical support as well as nutrients to support growth and development. Finally, it must
provide an environment that is pest, disease, and weed free.
Propagation soil may consist of one or more components mixed together. Organic
components include peat moss and Sphagnum moss. Peat consists of decomposed
aquatic vegetation, which has been preserved under water in a partially decomposed state.
There are three types of peat classified by horticulturalists: moss peat, reed sedge, and
peat humus. Peat is not a uniform material and should be pasteurized as a propagation
soil.
Sphagnum moss is decomposed acid-bog plants. Common species include S. papillasum,
S. capillaceum, and S. palustre. It is pest free, light weight, holds high volumes of water,
and has a pH of about 3.5 to 4.0. It contains a fungistatic chemicals, which supports its
ability to inhibit damping-off of seed germination.
Inorganic components include sand, vermiculite, perlite, and pumice. Sand consists of
decomposed rock particles, 0.05 to 2.00 mm in size. Quartz sand, consisting of silica
complex, is often used for propagation purposes. Sand contains few minerals and has no
buffering capacity, or cation exchange capacity.
Vermiculite is a micaceous mineral that is mined in South Carolina and Montana. By
introducing extreme heat, vermiculite is manufactured by placing ore in a furnace where
it is heated to 2,000 degrees Fahrenheit. At this temperature, water inside the ore changes
to steam and expands the particles into an accordionlike structure. Vermiculite has a high
cation exchange capacity and thus can hold nutrients in reserve for absorption by roots.
Horticulture vermiculite is graded to four sizes: No. 1 has particles from 5 to 8 mm in
diameter; No. 2, a common propagation grade, from 2 to 3 mm; No. 3, from 1 to 2 mm;
and No 4, which is most useful as a seed germination medium, from 0.75 to 1 mm.
Perlite reacts the same way as vermiculite to high temperatures, but instead of swelling,
the particles expand like popcorn at 1,400 degrees Fahrenheit. A particle size of 1.6 to
3.00 mm is produced and is most useful in horticultural applications. Unlike verticulite, it
has no cation exchange capacity and contains no mineral nutrients. Perlite presents some
problems with fluoride-sensitive plants. In either case, both perlite and vermiculite are
common propagation components. They are pest and disease free, light weight, and
possess high water holding capacities.
Pumice is volcanic rock containing silicon dioxide and aluminum oxide, with trace
amounts of iron, calcium, magnesium and sodium oxides. It is added to propagation soil
to increase aeration and drainage.
Propagation Soil Preparation
Plant pathogens are often killed in propagation soils by applying steam or chemicals.
The treatment given to soil to free it of pathogens and pests is called sterilization or
pasteurization. Soil sterilization kills all organisms in the soil, while pasteurization kills
harmful microorganisms and weed seeds. The most common used methods are steam
pasteurization, electrical pasteurization, or chemical fumigation.
The recommended method of steaming soil is to bring the temperature in the coldest part
of the soil to 180 degree Fahrenheit. This temperature is maintained for at least one-half
hour. Oversteaming may cause a buildup of harmful substances in the soil. For example,
manganese is a nutrient commonly used by plants in small amounts. Oversteaming
causes manganese to be released by the soil in quantities that are toxic to the plant. Also,
soils with high amounts of organic matter produce toxic levels of salt when exposed to
oversteaming.
In the horticulture industry, soil technicians prefer using aerated steam at a temperature of
140 degree Fahrenheit for 30 minutes to treat propagation soil. Because higher
temperatures (180 degrees Fahrenheit) kill all microorganisms, diseases may spread very
rapidly with the absence of their natural enemies. On the other hand, when soil is treated
with aerated steam at 140 degrees Fahrenheit, most of the beneficial microorgamism
survive. Their presence stops or slows the growth of plant diseases.
Steam and air are mixed and injected into a soil bed or trailer. The aerated steam flows
through the vents and into the soil, elevating the temperature to a recommended level to
eliminate diseases.
Some plant propagators treat a soil with chemicals to eliminate harmful pathogens.
Because fumigant gases diffuse slowly at low temperatures, soil fumigants are most
effective if the soil is at the proper temperature.
Vapam is a liquid carbamate soil fumigant that kills nematodes, fungi, and pest weeds. It
is effective at temperatures between 30 and 90 degrees Fahrenheit. At a rate of one quart
per one hundred square feet, Vapam is often applied from a watering can or proportioner
attached to a water hose. After application, Vapam must be watered to spread it
thoroughly into the soil. Because Vapam treated soil is toxic to plants, it must be turned
2 or 3 times before use in propagation or growing soils.
Other soil fumigants used in the horticulture industry include chloropicrin (tear gas),
methyl bromide, ethylene bromide, and formalin (40% formaldehyde). These chemicals
are poisonous to microorganisms as well as animals, humans and plants. Because
inappropriate use of fumigants may cause negative side effects to plant growth, the
manufacturer’s directions must be followed exactly when applying these chemicals.
Moreover, caution and common sense also are applied when using fumigants.