Page 22 - Griffin Gazette Fall 2015
P. 22
growing media componentsAreviewofthemajor
By Dan Jacques and Ron Walden, Sun Gro Horticulture
The major role of a growing mix is to support Bark mix and is stable in maintaining its
the plant, while holding water and nutrients The most common bark used is southern aeration properties.
for the plant to use during growth. The pine bark. In the west, fir bark is more
components selected to make the growing common, while northern barks could Vermiculite
mix will have an impact on its physical and include a combination of pine, spruce and Vermiculite is also an expanded ore. Once
chemical properties (with emphasis on fir. Bark used in greenhouse media is expanded, it mainly adds air space to a mix.
physical properties). We will look at the most generally either aged or composted, and However, it does have some water- and
common components added to growing often referred to as processed bark. It is nutrient-holding properties, especially in
media and their impact on properties of the less acidic than peat moss, with a common comparison to perlite. Unlike perlite, it will
medium produced. pH range of 4.5 – 6.5. Bark has high air tend to break down somewhat over time,
space, but lower water- and nutrient- thereby reducing its aeration properties.
There are five main components commonly holding capacities than peat moss. Bark
used in making growing media: peat will draw nitrogen from the mix. This is Putting it All Together
moss, bark, coir, perlite and vermiculite. often compensated for by addition of slow- Growing mixes, whether made by the
They are combined in various amounts release nitrogen. grower or a commercial mix company,
as a percentage of the mix. These five will rely mainly on the five components
components are compared in Table 1 and Coir/Coconut Coir Pith described above. These components are
will be the components discussed in Coir, or coconut coir pith, is a byproduct combined in varying percentages with other
this article. There are a number of other of processing coconut fiber. It is the finer additives such as wetting agent, starter
components that can be used, but these “dust” particles that have historically been nutrient charge and limestone. Coarseness
tend to be less common or regional considered waste material. Coir is used properties will determine the best use of
components. These include peanut mainly as a partial substitute for peat the components in greenhouse production.
hulls, rice hulls, pumice, wood chips and moss. Coir provides very good air space Table 3 includes desired properties of mixes
composts. Table 2 outlines some of the as well as good water holding. It tends to for various areas of greenhouse production.
uses of the components. be higher in soluble salts (mainly potassium
and/or sodium, depending on how it was A combination of, at most, three of the
Peat Moss processed). It will have lower nutrient above components at any one time
Peat moss is the partially decomposed plant holding capacity than peat and may, at will provide the necessary physical and
material derived from the sphagnum moss times, draw some nitrogen as bark does. chemical properties needed to successfully
plant. This material is the most common The pH range is 5.5 – 7.5. grow a crop. Mix blending does not have to
component used in most greenhouse mixes be very complicated, as long as the starting
due to its physical and chemical properties. Perlite components are of good quality. The
Peat moss will provide good aeration Perlite is derived from volcanic rock that components will have their greatest impact
and moderate to high water and nutrient is expanded at high temperatures. The on physical properties, while the grower’s
holding capacity to a mix. The pH of peat expanded product is used to add air space water quality and fertilizer selection will have
moss is generally in the range of 3.0 – 5.0. to mixes. It does not impact pH of the the greatest impact on pH and nutrition.
Table 1 Properties of the 5 major growing media components Table 3 Desired properties of mixes for various uses in greenhouse production
Raw Total Water Holding Air pH CEC Attribute Propagation Mix Peat-Lite Bark-Based
Materials Porosity Capacity Capacity (me/cc) Growing on Mix Growing on Mix
Peat 89-94 74-77 12-20 3.0-5.0 7.0-13.0 Particle Size Fine Medium to Coarse Medium to Coarse
Bark 75-80 56 19-24 4.5-6.5 9 - 12 Air Space Low to Moderate Moderate to High Moderate to High
Coir 92-94 9-12 6.5-7.5 6.1 Water Holding Moderate to Low
Perlite 82-83 28-32 7.0-7.5 0.15 Longevity High Moderate
Vermiculite 68 36-40 6-10 5.0 - 7.5 1.9-2.7 Nutrient Needs Short Short to Medium Short to Long
78-80 70-72 Low Moderate to High1
Moderate
1Nutrient needs in a bark based mix may be offset by the use of slow release nitrogen.
Table 2 Uses of the regional growing media components
Component Common Use Concerns Comments
Peanut Hulls Substitute for bark Customer fear related to peanut allergies • The composting process removes the possibility of allergic reactions.
Wood Chips Substitute for bark or perlite Nitrogen drawdown More of a regional component
Rice Hulls Substitute for perlite Rodents are attracted to rice hulls and In large quantities, they can provide some silicon for plants
rice hull mixes
Pumice Substitute for perlite None Common in the west
Yard Waste Small percentage addition to mix Source of waste may sometimes have • Can increase nutrient and water holding in a mix
Compost undesirable contaminants • Need to make sure there are no herbicide contaminants
Vermicompost Small percentage addition to mix None • Excellent results when added at 5 – 10% of the mix.
