Aggregate media such as grow rocks, Geolite, Hydroton, etc. should be cleaned between crops to remove debris and roots that accumulated in the media. As well, any system parts that come in contact with nutrient solution, such as growing trays, feed/drain lines, water pump, etc., should be sterilized before putting the system back into use. Cleaned media can be sterilized at the same time simply by placing it in the system beforehand, or it can be sterilized separately.
It's important to note that, in spite of sterilization, any remaining root fragments will begin to disintegrate and turn brown after approximately 2-3 weeks into the next growing cycle. This will leave a brown sediment on the reservoir floor and may tint the nutrient solution with a brown color as well. Thus the cleaning procedure outlined below is intended to minimize the number of root fragments remaining in the cleaned media.
Hydrogen peroxide (H2O2) is by far the most practical product to use for sterilizing aggregate media. Unlike system parts with smooth surfaces, media such as hydroton are designed with irregular surfaces and small pores meant to capture & store nutrient solution. As a result, bleach can be flushed from smooth system parts with one fresh-water rinse, but media requires several flushes before chlorine levels in the remaining solution become safe enough for plants. This is not a concern with H2O2 as one flush is enough to return media to a safe condition. And unlike bleach, because the weak H2O2 solution will naturally breakdown to a harmless condition when exposed to air for several hours, no rinsing is required if you don't need to replant right away.
Because it is the least expensive, bleach may be more practical to use for the rest of your system. However, many find that the small savings doesn't outweigh the convenience of sterilizing both the system & media together at the same time and flushing only once, or not at all depending on the urgency to replant.
To sterilize with H2O2, you can use the same 5 gallon bucket that was used to clean the media. Fill it with 4 gallons of water, then add 2 cups (16 oz) of the common drug store variety hydrogen peroxide, this is typically a 3% solution (check the label). For other strengths use a scaled down quantity. For example, if using a 30% strength use only 1/10 or 1.6 oz. See the H2O2 page for more.
Place the container (with the media inside) into the H2O2 solution. After standing in the solution for 1 hour, remove it and let it drain back into the bucket. Rinse if needed, then repeat with the next container of media.
Copyright © 1997-2004 pH