cycl
growers know
MICROWAVE
You can pasteurize moistened media like potting mix or
vermiculite by microwaving it for three to five minutes,
depending on your microwave’s power rating. This
only works for small batches of up to two pounds or so.
Microwave pasteurizing has the advantage of being faster
than oven treatment, and a microwave can be temporarily
relocated to reduce odor problems. Sterilizing in the
microwave can be problematic, however. Microwave ovens
heat quickly, but they sometimes leave cooler spots that are
hard to detect or eliminate.
Be aware that these methods are equal opportunity options.
They kill beneficial as well as destructive organisms.
SOLAR
Outdoor solarization is another way to pasteurize media that
uses heat—in this case, sunshine. In this method, media is
moistened, covered with clear polyethylene tarp material, and
left to steam in the sun on a hot day. A sustained temperature
of 150°F for 30 minutes is hot enough to kill many common
microorganisms. You can test the temperature using a candy
thermometer. Adding additional layers of tarp can increase
the temperature somewhat. Covering the soil for a longer
duration can be helpful, too. One day is effective, but leaving
the tarp in place for a week to a month can yield even more
impressive results. To sterilize media outdoors, solarization
should be coupled with a chemical agent.
NUTRIENT WASHING
The big advantage to most modern growing media is
that it is inert. It provides a blank or relatively blank
slate. That way it’s easier for the gardener to control
nutrient composition and concentration through
the plant cultivation process. The problem with
reusing media is that any lingering nutrient
residue can spoil the “recipe” designed
for a specific project or produce toxic
concentrations of some chemicals.
You can often see this fertilizer salt
buildup as a white powder crusted
on some types of used media. An
important step
in recovering
media for
reuse is to
eliminate
residual nutrient
where necessary.
This is usually
done through
repeated rinsing with
pH-neutral water;
however, in some
cases, the addition
of a chemical
chelating agent
may be necessary.
A FEW TIPS AND TRICKS
Just as each growing media is unique, each type is best
cleaned by a particular method:
CHEMICALLY STERILIZING CLAY PEBBLES
Let’s take a look at a small sterilizing job using clay
pebbles. Reusing this media is a good example because it
is often recommended as the environmentally friendly and
economically sensible choice. Note that you can’t reclaim clay
indefinitely, though; it degrades after a few reuses.
For this example, we’re sterilizing with hydrogen peroxide.
Bleach is another common chemical option.
Also, before you begin any media sterilization procedure,
remove loose debris and scrape or lift away any large
pieces of stubborn organic matter you see. For hydroponic
media, you can speed up this process by using an enzyme
product designed to aid in the breakdown of organic matter
in hydroponic systems. It softens dead roots, making them
easier to wash or scrub away. Just be sure to read the label,
as some enzyme products are not recommended for use
with hydrogen peroxide.
“
REUSED GROW MEDIA IS ONLY
A MONEY SAVER IF YOU CAN
CULTIVATE HEALTHY
PLANTS WITH IT.”
GENERAL DIRECTIONS:
• Fill a large sieve, colander, or strainer half full with clay
pebbles. Use the largest mesh that will still retain the
media itself.
• Rinse with tap water to remove organic matter. If using a
sink or tub, install a drain strainer to catch debris.
• Stir pebbles and keep rinsing/stirring until the water runs
clear and no surface debris is visible. This can take five
minutes or longer. Set aside.
• Fill a large bucket with enough water to cover the pebbles
generously (but hold back on adding the pebbles just yet).
Make a note of how much water you’ve added.
• Optional: Install an air stone to the water to promote
better circulation.
• Put on gloves and eye protection.
• Add concentrated H 2 O 2 to the water using the dilution direc-
tions for sterilization listed on the bottle. Stir.
• Add pebbles carefully. At this point, the slurry will begin to
foam. That’s the H 2 O 2 doing its work.
• If necessary, use the colander as a weight to keep the
pebbles submerged.
• Soak for 15 minutes or in accordance with the
H 2 O 2 instructions.
• Remove the air stone and pour off the water.
• Rinse thoroughly to remove H 2 O 2 residue.
• In case there are lingering traces of H 2 O 2 , it’s a good idea to
let the pebbles sit overnight before reusing them.
This procedure can be scaled up as needed. It can also be used
to sterilize perlite; just use a finer mesh sieve.
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grow cycle