Grower 101: Soil Amendments --Sorting Through the Crowd

May 8, 2003 - 09:41

While having a myriad of soil amendment choices is great, the varied list can create some confusion about which product does what; here are the basics.

A soil amendment is any material added to a soil (or
soilless mix) for the purpose of improving its physical and/or biological
characteristics, including water retention, water permeability, air movement,
root penetration, drainage, aggregation, increasing organic content, improving
microbial viability and so on. In short, the reason for using amendments is to
provide a better environment to support plant life and development.

Aside from holding the plant upright, the growing medium
also stores minerals for plant nutrition and water for growth, and it serves as
a home for important living organisms. The ideal growing media is one in which
there are enough large particles to ensure proper movement of oxygen to the
roots, as well as enough small particles to retain the nutrients and water needed
for proper plant growth and development.

The benefits provided by soilless growing media are
numerous. There are, however, challenges to deal with. By starting with the
same type of growing media (growing mixes or media) each and every time, any
existing variables, such as lack of nutrients or an inability to retain
nutrients for a long period of time, can be easily adjusted/manipulated if
necessary. And that's where soil amendments come in.

Organic amendments

Organic soil amendments are derived from things that are or
were alive at some point (mainly plants and animals). Generally, organics help
separate soil particles and increase nutrient and water-holding capacities of
the growing media. They are also a food source for important microbial

Sphagnum peat moss.
The most commonly used component in soilless growing media, sphagnum peat moss
is known to have excellent water-holding characteristics, and it works well
with other components to provide the physical properties necessary for optimum
plant growth. There are different grades of peat moss, and they are priced
accordingly. Peat moss does not add any nutrients to the mix.

Beneficial microbes.
These include forms of bacteria and fungi that, under natural growing
conditions, perform a number of tasks that are essential for proper plant
growth development. They are found in soils around the world. Unfortunately,
because greenhouse conditions are, for the most part, kept as sterile as
possible (to avoid disease), there is usually a marked absence of microbes
(beneficial or otherwise) in growing media. Adding a microbial amendment to
your media will accomplish a number of important things. First, with beneficial
microbes in the media, there will be less room for any would-be disease
pathogens that might find their way into your greenhouse. That means less
chance for plant disease. Microbial activity can also lead to increased root
development and is essential for the uptake of nutrients by plants.

Coir. Coir, a
byproduct of the coconut industry, is produced mainly in places such as the
Philippines, Indonesia, Mexico, the Caribbean and Costa Rica. It has a high
capacity for retaining air, nutrients and water. In fact, coir can hold 1,100
percent of its weight in water. Because of these properties, it can be used in
a similar fashion to sphagnum peat moss. It is important to note that coir
typically contains some mineral elements (especially phosphorus and potassium).

Wood residues. Bark
and bark ash provide large pores or openings in your growing media to help
promote aeration and drainage. These materials hold a substantial amount of
water, and they are heavier, which is an advantage for larger plants that might
otherwise become top heavy. Adding bark to media may offer some increased
resistance to root rot diseases. One drawback of using bark is that it has a
tendency to tie up nutrients ? especially nitrogen ? if the bark
has not been properly composted. It is also known to change the pH of the
growing media.

Kelp extract. Kelp
extract comes from seaweed that has been harvested, dried and ground. It adds
vitamins, plant growth hormones and trace minerals that help boost plant vigor
and vitality.

Inorganic amendments

Inorganic soil amendments are either mined or man-made. They
are most often used to physically separate soil particles, allowing for
improvements in things such as aeration.

Vermiculite. This
sterile, lightweight product comes in a variety of sizes. It is created when
mica is heated to approximately 1,800° F, at which the mica's plate-like
structure expands, allowing it to retain large quantities of air, water and
nutrients. Vermiculite is most often used to improve the water retention of
growing media. It can also add cation exchange capacity and buffering capacity.
Finer grades of these lightweight granules are most often used for seed
germination, while coarse grades are good for adding to potting media.

Perlite. Perlite is
produced when volcanic rock is crushed then heated at very high temperatures.
This process results in a sterile, lightweight, porous, bead-like material.
White in color, it is excellent for increasing aeration and drainage and
avoiding compaction. It will not change the nutrient quality or pH of the media
to which it is added. It is available in fine, medium and coarse grades. One
drawback is that perlite holds little, if any, water. And since perlite is
dusty, it can have a tendency to float out of the growing media during

Calcined clay. This
is created by heating clay particles to form large and irregularly shaped
particles that have excellent aeration and drainage properties. They also have
many small pores and a large surface area, resulting in a moderate capacity to
hold water.

Rock wool. Similar
in appearance to fiberglass building insulation, rock wool is created by
melting a mixture of industrial basalt, coke and limestone at extremely high
temperatures. The resulting liquid is then spun to form fibers (similar to how
cotton candy is created). Rock wool comes in a ground form for use as an
amendment. It has excellent water-holding capacity and also increases aeration
and drainage. It is sterile and chemically inert.

Sand. Sand is
usually added to greenhouse media to increase its weight, drainage and
aeration. When used as an amendment, sand should have a coarse texture. As
would be expected, the relatively large size of sand particles means it does
not hold water and nutrients well.

Wetting agents.
Water-absorbing polymers are available in various forms, including liquid,
powder or gel. These are used primarily during seed germination to improve the
ability of your soilless media to absorb water from below. While wetting agents
are generally derived from inorganic sources, there are a few created from
organic sources (such as Yucca plant extract).

With so many amendments available to today's grower, it is
critical to do your homework. Coupled with an understanding of your plants'
needs, knowing which amendments to utilize to achieve your objectives can have
a significant impact on your production and profits.

Note: Please note that the preceding is not meant to be an
exhaustive list of all soil amendments. Rather, it is intended to be a rundown
of some of the more common types of amendments available for use by growers.

About The Author

Kevin Hattori, formally of Growth Products, Ltd., has worked in the industry for five years. He may be reached at

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