During December and January of 2017-2018 Plant Chicago experimented with a different method of feeding our plants called the Kratky Method. It is a means of passively fertilizing plants hydroponically, first developed in the 1990’s by Bernard A. Kratky at the University of Hawaii. This method is great for small scale growing (such as mason jars), because no special equipment is needed, but how does it compare to Plant Chicago’s large aquaponic system? Thanks to a donation of lights from Happy Leaf LED, Inc., and a flat of oak lettuce from Backyard Fresh Farms, we were able to find out just that.
What is the Kratky Method?
The Kratky Method makes use of the same bed you would find in a deep water culture type system (where water pumps provide a flow of nutrients and air pumps provide oxygen to the plants’ roots), but eliminates the need of the water and air pumps. The bed is filled with a hydroponic nutrient solution, but instead of letting foam rafts float on the surface of the water, rigid boards are suspended over the water. A gap between the boards and the water allows the plants’ roots to access oxygen (as the water in the solution evaporates, more root area is exposed, allowing more access to oxygen). At the end of the plants’ growth cycle (i.e. harvest time), the nutrient level can then be adjusted for the next incoming crop, and the process starts all over again.
On December 28, 2017 we isolated one of our grow beds from our main aquaponic system. We left the adjoining bed connected, and both were installed with 6 – 33” Procyon Ultra LED grow lights from Happy Leaf. The isolated bed was filled with fresh water and dosed with General Hydroponics MaxiGrow, while the aquaponic bed remained the same. Both beds were planted with 90 x 17-day old oak lettuce plants from Backyard Fresh Farms. Plant size was monitored and water testing performed weekly up to the harvest date on January 25, 2018. On that date, one board (18 plants) from each bed was harvested, dried and weighed. The charts below show the water test results for Ammonia (TAN), Nitrite (NO2), Nitrate (NO3), pH, and Temperature (T) from the 4 weeks of the experiment:
Water parameters in the aquaponic system remained fairly steady for the duration of the experiment, with ammonia and nitrite levels at 0 ppm (parts per million), and nitrate at 80 ppm. The pH dropped briefly during the second week due to fluctuations in alkalinity. Parameters in the Kratky system shows much more volatility. The total ammonia started out at 8 ppm, a level that would be quite toxic if fish were present. This began to decline after the second week, ending at 0 ppm. Nitrite concentration remained a steady 5 ppm for the entire experiment. (This result may be flawed as the test kit we were using cannot measure higher than 5 ppm.)
Nitrate concentration began at 80 ppm (same as the aquaponic system), was reduced to 0 ppm, and then rebounded to over 100 ppm by the end of the experiment. This initial decline was due to absorption by the lettuce, while there was no mechanism to quickly replace it. By the third week, bacteria populations grew to an appropriate size, accounting for both the decrease in ammonia and increase in nitrate. Just like in an aquaponic system, these bacteria were converting the ammonia to nitrite and then to nitrate. After the third week, enough nitrate was being produced that it once again was in surplus. Note: for the week that the nitrate registered as 0 ppm, it is unlikely that there was no nitrate to be found in the system, rather the lettuce was absorbing it as fast as it was being produced.
We harvested 18 plants in total, in groups of 9 from each of the two systems. Each group was dried in an electric oven (the most the oven would allow). The first group of 9 from the Kratky system, after drying, was found to actually have 10 plants (two originating from the same pot), and so was removed from comparison. The entire bed was not used as 180 plants would have taken over a week to dry in our small oven, and we wanted to be able to save some plants for eating. The 9 plants from the Aquaponic bed weighed in at 22.72 oz (0.8 oz after being dried), and the 9 Kratky system plants weighed 38.96 oz (1.2 oz dry). Comparing the straight-from-harvest weights of the two groups (see chart 3) shows a 71% increase in growth with the Kratky method. If we compare the dry weights, that increase is lower – only 50%. While it’s possible that the differences in nutrient availability between the two growing methods resulted in different amounts of water retention, it’s more likely that differences in oven temperature and drying time resulted in groups of ‘dry’ plants that actually had different amounts of moisture content left, changing their weights.