I will soon be going to transplant lettuce seedlings into my Deep Water Culture tank—20 cubic metres (m3) for the first time. I would like to ask you if it will be OK to add some pool chlorine into the tank at this stage, before I put any plants in, just to sterilise the tank before I commence. If it is safe to do, what amount of chlorine would you suggest I add per 1000 litres?
Answer by RICK DONNAN
Good luck with your new system, you have prepared well.
Yes, it would be a good idea to chlorinate your entire system before starting (not just the tank—pipes are often more important). By far the most critical aspect is to ensure that the active chlorine has been almost totally deactivated before planting. Oxidising chemicals, such as the various forms of active chlorine, not only kill diseases, but also any organic matter present, including healthy roots.
What chemical to use?
This depends upon how you intend to deactivate your water. One method would be to totally dump your water and replace it, but this would be a huge waste of water, and sterilised water at that. In that case, you could use convenient liquid pool chlorine, which contains sodium hypochlorite. Take care that you don’t use a brand that also contains cyanuric acid.
If you plan to keep the water, then you should use calcium hypochlorite (solid/granulated pool chlorine). This will supply just over 50% active chlorine. I suggest a minimum treatment of no less than 10 ppm (mg/l) active chlorine, that is, 20 g/kl calcium hypochlorite. Go higher if you have sanitation concerns, but it will take longer to deactivate. Adding 20 ppm calcium hypochlorite will add 6 ppm each of calcium and chloride ions to your water. Calcium is a nutrient, of course, and chloride is readily taken up by lettuce without adverse impact.
Deactivation is by aeration and exposure to sunlight. You need to get down to no higher than 1 ppm active chlorine. My main experience with chlorine in water-based systems has been with NFT lettuce. Here there have been reports of problems with as low as 2 ppm. A classic case I met was a grower with a mature lettuce crop infected with Pythium. He dosed to 4 ppm in order to harvest, then planted his next crop. The young plants all died within hours.
With the huge volume of water your tank holds, it will probably take days to deactivate.
Pool chlorine test strips are good enough, provided you get one that has an obvious colour difference between 0, 1 and 2 ppm active chlorine. Otherwise, use a good pool chemical colour comparator kit. There is no need to buy a fancy meter. Ω
PH&G December 2015 / Issue 162