Daniela:
After a bit of a leave of absence, I have now returned and made some adjustments to the duckponics system.
The new control bed with the repurposed white plastic bag.
The first new addition is added insulation to the control bed (see previous article for information on the different tanks) to try to reduce the temperature there. Before I left, I was seeing a temperature difference of about 10 degrees Fahrenheit, which can cause major differences in bacterial metabolic rate, and therefore not be an accurate reflection of the nitrate produced by the bacteria in the grow bed.
So, I have now added a white IBC plastic storage material around the black tub and a white bed sheet inside to help insulate the bed. I also added more gravel to increase gravel depth and reduce heat penetration to the water at the bottom. This also helps reflect more accurately the depth of the gravel in the grow bed. It seems to be working, too, which is very exciting. My last test showed that the water temperature in both the grow bed and control bed was 80 degrees Fahrenheit.
A surprise that I ran into on my return is the state of the duckweed. Before leaving, I had discovered that the duckweed pond was extremely basic; the pH was well over 8.6. I was planning to add some wood to try to lower the pH of the system (and idea I found online), but didn’t get the chance to do so before abruptly leaving.
Duckweed filled out its grow tank.
However, upon my return, I had found that the duckweed had finally grown to cover the entire water surface and the pH was much closer to the pH of the other ponds: 7.8. I would still like to see it a tad bit lower, but it was much improvement. Apparently, nature had taken care of the situation. I am thinking that perhaps that little mosquito larvae may have somehow helped reduce whatever chemical imbalance was causing the basic pH.
I have also now taken measurements of discharge rates for all the locations at the duck pond. I had already begun this process before, but I wanted to test again to check for accuracy of what I had previously measured, and finish measuring the other locations I had not gotten to. To measure the discharge into the grow beds and control beds, I plugged the outlet of container I was testing, dug a location deep enough to see the water surface, and then turned on the system. Once the water was coming out of the PVC pipes at a consistent level, I would take an initial measurement of depth to the water, and start the timer.
When the timer reached 120 seconds, I would take another measurement of water depth. From there, I can use the dimensions of the container and my previous measurement of how much volume the gravel takes up to calculate the volume of water that entered the bed in 120 seconds. When I convert that volume in square inches to mL and divide it by 120 seconds, I then have my discharge value.
The brassica's and mint thriving in the Northwest growbed.
For measuring discharge from the hoses, I was able to merely divert the water from the hose into containers of known volume, and time how long it took to fill up the containers. I can then divide the volume by the time and have my discharge. It was important, though, that I kept the highest point of the hose at the same level as it would have been flowing into the duck pond or duckweed bed, otherwise the discharge rate would be inaccurate. I also have to adjust the valves for water entering the grow beds and control bed to try to get the discharge between the three as equal as possible.