Components of growing media

a. Peat- Peat moss, sphagnum moss and reed-sedge peat

Peat moss

• The least decomposed form of the peat types, is typically light tan to brown in color, lightweight, high in moisture-holding capacity and very acid (pH 3.8 to 4.3).
• A significant problem with peat moss is “wetting up.”
• Peat moss is inherently hydro phobic (repels water).
• Most recipes call for peat moss on a volume basis (e.g., 50% peat moss: 50% perlite, vol:vol).

Sphagnum moss

• It differs from moss peat (peat moss) in that it is the young residue or live portion of the plant.
• Sphagnum, or “top moss,” is commonly used for plant shipment, propagation or to line hanging baskets.
• Substances have been extracted from sphagnum peat which inhibit the growth of fungi associated with damping off.

Reed-sedge peat

• It is formed principally from reeds, sedges, marsh grasses, cattails and other associated swamp plants.
• Peat humus is usually derived from reed-sedge

b. Coir (Coconut Fiber)

• Coir is a relatively new organic amendment primarily used in the greenhouse industry.
• Potential substitute for peat moss.
• Derived from the husk of the coconut fruit
• The typical pH range for coir is 5.5 to 6.8,
• It contains significant amounts of phosphorus (6 to 60ppm) and potassium (170 to 600 ppm)
• Can hold up to nine times its weight in water.
• Since coir contains more lignin and less cellulose than peat, it is more resistant to microbial breakdown and, therefore, may shrink less.
• Coir is easier to re-wet after drying than peat moss

c. Softwood Bark

• Bark is the primary component (80% to 100% by volume) in most outdoor container nursery mixes.
• Pine bark is preferred over hardwood bark since it resists decomposition and contains fewer leachable organic acids.
• A good potting medium usually consists of 70% to 80 %( by volume)
• Bark is described as either fresh, aged or composted.
• Composting bark typically takes 5 to 7 weeks.

d. Hardwood Bark

• The chemical properties of hardwood bark are significantly different from pine bark.
• The pH of fresh hardwood bark is usually less acid (pH5 to 5.5) than peat moss or pine bark.
• Composted bark may be rather alkaline (pH = 7 to 8.5).
• Hardwood bark typically contains toxic compounds and for this reason, should be composted before use.

e. Wood Substrates

• The use of ground whole-tree substrates (containing all shoot proportions of the tree and thus consisting of~80% wood fiber) from pine trees

f. Compost and Animal Manures: A large variety of compost or animal manure products is available

Manures:

• Disadvantages include possible high salts, fine particle size and weed seeds
• The advantages include the nutrient contribution and potential improvement in media physical properties.

Sludge:

• Plant-based composts:
• Compost products provide a low-cost media amendment.
• Critical issues to consider are the availability and consistency of the product and the particle size.

g. Rice Hulls

• Rice hulls can be found in a variety of forms including fresh, aged, composted and parboiled.
• Fresh rice hulls are typically avoided as container substrates because of residual rice and/or weed seed.
• Parboiled rice hulls (PRH) are produced by steaming and drying rice hulls after the milling process, which results in a product that is free of viable weed and/or rice seed.
• Rice hulls consist mainly of lignin, cutin and insoluble silica, providing a slow breakdown of particles and therefore making them an appropriate substrate for long-term crop production.
• The pH of composted rice and parboiled rice hulls ranges from 5.7 to 6.2, and 6.2 to 6.5, respectively.
• Fresh rice hulls are light in weight and are useful to increase drainage and aeration
• Fully composted rice hulls will hold more water than unprocessed hulls.

h. Cotton Gin Trash

• Composted gin trash can increase the water- and nutrient-holding properties of media
• Has a pH of 5.5 to 6.0.
• High soluble salts can be a concern, but this can be reduced quickly through leaching with water.
• Several studies have shown reduced plant growth when the media contains > 50% gin trash.

i. Perlite

• Perlite is most commonly used as a component in greenhouse growing media or nursery propagation applications.
• It is produced by heating igneous rock under high temperatures (1,100 to 1,600°F).
• Perlite differs from vermiculite in that the finished product is a “closed cell” that does not absorb or hold water.
• For this reason, it is usually included in a mix to improve the drainage or increase the percent of aeration.
• Perlite is lightweight, chemically inert, pH neutral, sterile and odorless.

j. Vermiculite

• Similar to perlite in that they both originate as mined minerals that are then heated to produce a finished product.
• Perlite is usually included in a mix to increase drainage but does not increase the retention of nutrients.
• In contrast, vermiculite with its plate-like structure holds large quantities of water and positive-charged nutrients like potassium, magnesium and calcium.
• Vermiculite is sterile and light in weight
• pH of vermiculite will vary depending on where it is mined.
• Extensively used in the greenhouse industry as a component of mixes or in propagation.

k. Sand

• Sand is a common amendment used in propagation applications and is occasionally used in a greenhouse or nursery mix.
• Sand is typically selected as a media component to improve the drainage or to act as a ballast to decrease container blow-over in outdoor container nurseries.
• While sand represents a wide range in particle sizes, growers generally use medium to coarse sands (0.25 to2 mm).
• Preference should be given to deep-mined sharp sands which are mostly silicon

l. Soil

• Soil is still occasionally used in a container mix primarily because it is locally available or to add weight to a predominantly organic-based mix.
• Some outdoor nurseries strip the topsoil in preparation for installing container beds, stockpile the soil and then use it over time as a minor component (e.g., 10%) in the mix.
• Major considerations when using would be the amount of weed seed and presence of residual chemicals.
• In general, soil should be a minor (<10%) component or not used at all in modern container mixes.

m. Rock wool

• Mineral or rock wool has been used extensively
• Like perlite and vermiculite, it originates from a natural mineral (alumino silicates with some calcium and magnesium) that is heated to 2,700°F to form fibers that are used to make blocks or cubes as a finished product.
• Blocks or slabs of rock wool are used extensively by hydroponic growers of greenhouse vegetables.
• Rock wool typically has an alkaline pH, is sterile and chemically inert.