Inside View – July 2015
By Gary Jones
An Extra Dimension to Rootzone Management
By Gary Jones
When we consider “rootzone management,” our thoughts often turn to growing medium structure or pH/EC, or water content and “overdrain” in hydroponic grow bags (e.g., coir or rockwool).
One of the most popular new production trends that seems to pop up just everywhere these days is that of aquaponics. This is the combination of “aqua-”culture (fish farming) and hydro-“ponics.”
The obvious benefit is that the system generates two income streams: fish and vegetables (in the broadest sense). Equally important however is the “closed loop” system this produces. The waste from the fish provides a valuable source of nitrogen for the plants, which in turn clean up the water that is recycled to the fish. Pretty simple really, and let’s face it we have been rather slow to catch on to the natural system that has been happening all around in nature for eons.
Anyway, when we start to delve into these systems, we need to be aware of a number of new dimensions to our rootzone management.
“The most common aquaponic systems currently in use employ either a media-filled plant bed, nutrient-film technique (NFT), or a floating raft system for the plant growing area integrated with a recirculating aquaculture tank system (RAS) for the fish production area.”1
Within any of these systems, one of the most critical factors that has to be controlled (that we don’t normally deliberately address in regular systems) is that of the oxygen content of the water. We’re talking about dissolved oxygen (DO) levels.
Typically, the oxygen content of the water must be kept above 5 ppm to meet the needs of fish, plants and bacteria that we need to work for us in the system, and it is affected by a number of factors including salinity and temperature of the water and the size of the fish (smaller fish use more oxygen per kilogram of fish weight than do larger fish).
I wonder when was the last time any greenhouse grower checked the DO content of the growing medium?
I know there have been trials done in the past on DO levels in NFT systems, particularly in relation to row length of hydroponic greenhouse vegetable crops, but it’s not something that we typically pay much attention to.
A second critical rootzone factor that is carefully managed in aquaponic systems is the health of beneficial bacteria. As the fish in the system eat food provided, their waste contains nitrogen, about 10 per cent of it in ammonium nitrogen form. In water, some of the ammonium nitrogen, NH4+, becomes ammonia, NH3, and through nitrification this in turn becomes nitrate, NO3-. Going back to the other part of the aquaponics system, these now useful NO3- and NH4+ are taken up by the plants.
The point of this rather scary sidetrack into chemistry is that for this process to occur we need bacteria in the system. Nitrosomonas bacteria convert ammonia to nitrite, and nitrobacter convert nitrite to nitrate. And we need the right mixes of bacteria in the right relative numbers to make the process happen successfully.
So, the aquaponics practitioner needs to manage the rootzone with reference to another factor that most regular greenhouse managers do not even consider, let alone manage. Again, one wonders if managers should be paying more attention to the biological activity in our growing media as part of our nutrient management system, let alone as part of the biology.
A third factor to be managed in these systems is water temperature. (There are other factors, but we’ll stop at three!) We’ve already mentioned temperature’s effect on DO levels. But it also affects the well-being of the necessary bacteria and, of course, the fish and plants. Apart from making sure we don’t water plants with ice-cold water, how often do we take care to manage water temperature in our crop rootzone environment?
Next time you consider your rootzone management, think about those things we don’t think about… Until next time.
1 Dr. Richard Tyson, Sustainable Aquaponic Vegetable and Fish Co-production, Orange County, Fla.
Gary Jones is co-chair of Horticulture at Kwantlen Polytechnic University, Langley, B.C. He serves on several industry committees and welcomes comments at Gary.Jones@kpu.ca.