September 2019 - 36

RESEARCH
Figure 5. A bioassay with broccoli seedlings is an in-house test. Plants were irrigated with water
from the case study operation had either no GAC (left), 1, 2, or 3 GAC filters (middle) or a tap
water control (right). The shorter hypocotyl length with no or limited GAC filtration is the result of
paclobutrazol contaminant in the water.
Figure 6. Increasing amounts of GAC filtration (left to
right) will remove fertilizer dye (top) and iron-EDDHA
chelate fertilizer (bottom).
To check that the GAC is functioning correctly or if carbon needs
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EVERYTHING WATER
replacement, before and after water samples were taken weekly by the
case study operation and sent to Waters Agricultural Labs for testing of
paclobutrazol concentration. Over 17 months of testing, the post-GAC
level of paclobutrazol never reached above the 5-ppb target level even
though pre-treatment levels reached as high as 73 ppb.
To save costs, we recommend monthly (rather than weekly) testing of
the target chemical before and after GAC. When sending in a water sample
to test, let the lab know the specific target agrichemical to avoid a very
high analytical cost.
There are other ways to check whether the filter is still effective. Broccoli
seedlings can be sown and irrigated with your raw recirculated water, GACtreated
water, and clean (bottled or tap) water (Figure 5). Check growth
after two to three weeks as a non-specific chemical bioassay. Begonia
plugs are another good " canary in the coal mine " for paclobutrazol.
You can also check non-target chemicals before and after GAC filtration.
HAND
WATERING
CROP
PROTECTION
For example, we found that GAC is very effective (95% removal after 64
seconds) at removing the soracid blue dye used to color many watersoluble
fertilizers. In addition, 48% of iron-EDDHA at 2-ppm iron was
removed at the same GAC level. Both fertilizer dye and Iron-EDDHA
provide a visual indicator of GAC efficacy (Figure 6). In addition, if you
are repeatedly treating a recirculating nutrient solution containing ironchelates,
it is likely that you will need to add iron back into your water
after filtration.
In this article, we covered several aspects to consider on the design
and operation of GAC filters. If you are interested in using GAC to treat
irrigation water, please contact the authors for more information.
AIR
MOVEMENT
IRRIGATION
SYSTEMS
WATER
TREATMENT
Paul Fisher is professor and Extension specialist at the University of Florida
(pfisher@ufl.edu). George Grant is a GGSPro technical specialist for Griffin
(ggrant@griffinmail.com). Rosa Raudales is assistant professor at the University
of Connecticut (rosa.raudales@uconn.edu).
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Integrated Plant Health
information@dramm.com
www.dramm.com
920.684.0227
36 SEPTEMBER 2019 GPNMAG.COM
This research was funded by the Clean WateR3 USDA-ARS Specialty Crops Research
Initiative grant (cleanwater3.org), and industry partners in the Floriculture Research Alliance
(floriculturealliance.org).

http://www.cleanwater3.org http://www.floriculturealliance.org http://www.dramm.com http://www.GPNMAG.COM

September 2019

Table of Contents for the Digital Edition of September 2019