Horticulture Principles and Practices

acteria that decompose sawdust first utilize some of the existing nitrogen. Thus, some
nitrogen fertilizer should be included in the mix when sawdust is used as an ingredient.
In areas of certain crop production, by-products of the industries may be good
sources of material for soil mixes. Usable plant by-products include peanut shells,
bagash (from crushing sugarcane), straw, and corncobs.
BARK Bark from hardwoods such as oak and maple or softwoods such as conifers may be
used in a growing mix. Bark consists primarily of lignin, which decomposes slowly. Hardwood
bark tends to be higher in nitrogen, phosphorus, and potassium and lower in calcium
content than softwood bark. Softwood bark, on the other hand, is higher in micronutrients,
especially manganese. Just like wood by-products, bark from certain species may contain
toxins. Growth inhibitors have been found in the barks of walnut, cherry, cedar, and white
pine. Loblolly pine (Pinus taeda) and slash pine (Pinus caribaea) barks do not have growth
inhibitors and thus are the most widely used in artificial soil mixes.
OTHER MINOR SOURCES Leaf mold (especially from maple, oak, and sycamore) is
composited material that can improve drainage, aeration, and water-holding capacity of
the medium. Bagasse (by-products of the sugar industry) has potential for use in media
formulation. However, its high sugar content triggers a high microbial activity leading to
reduced media quality over time. Rice hulls, corn cobs, peanuts shells, and others are also
organic materials that are less commonly used in media formulation.
Carbon to nitrogen ration (C:N) is an indication of the susceptibility to decomposition
of an organic matter, a lower ratio indicating that the material can decompose
rapidly, while a high ratio indicates the material is highly resistant to decomposition.
High cellulose (high carbon) is the reason for resistance to decay of organic matter.
Rapid decay of organic matter in a container medium can result in decreased volume,
increased compaction, and reduced aeration.
Materials of Mineral (Rock) Origin A variety of inorganic materials (containing
no carbon) are used in soil mixes.
SAND Sand is a heavy ingredient in growing mixes. Easy to pasteurize, its role in the
mix is to improve drainage and infiltration; it does not hold moisture. Sand does not supply
any nutrients to the mix. Silica sands with particle sizes between 0.5 and 2.0 millimeters
are desirable. If larger particles are used, the mix may settle and become compacted,
thus reducing infiltration. Sand may be obtained from the riverbed or mined from
white mountain sand deposits. The latter source produces particles that have flat sides
(called sharp sand). This angular property gives preference to this kind of sand because
it does not settle or pack down in a mix as does sand with rounded particles such as that
from riverbeds. Packing down reduces the pore spaces in the soil.
PERLITE Perlite is a light rock material of volcanic origin. It is essentially heatexpanded
aluminum silicate rock. The volcanic ore is heated to extreme temperatures of
about 982°C (1,800°F) to cause the rock particles to expand to produce the white product
used in mixes. Its role in a mix is to improve aeration and drainage. It can be pasteurized.
If this ingredient is required in a mix, the horticultural grade should be selected since it
has larger particle size and is thus more effective.
Perlite is neutral in reaction and provides almost no nutrients to the mix (except for
small amounts of sodium and aluminum). A disadvantage of the use of perlite is its low
weight, which makes it float when the medium is watered. Further, during mixing, it
produces dust, which can be eliminated by wetting the material lightly before use.
VERMICULITE Vermiculite is heat-expanded mica. This mineral is heated at temperatures
of about 760°C (1,400°F) to produce the folded structure associated with the material. It
is very lightweight and has minerals (magnesium and potassium) for enriching the mix,
as well as good water-holding capacity. Neutral in reaction (pH), it is available in grades
according to sizes. Grade 1 includes the largest particles, and grades 4 and 5 are fine in
144 Chapter 4 Plant Growth Environment