The duckweed index has been invented and popularized by the user dw1305 at ukaps.org . He actually used Limnobium sp. which in the hobby is more commonly known as “amazon frogbit”. He describes the process as:
“… when the Limnobium looks like the plant in the bottom left hand corner of the image I feed with KNO3 (to give 2ppm K). If this doesn’t produce a rapid greening, I then add macro / micro elements. Subsequently I change the water to achieve my preferred conductivity and then only feed again when the “duckweed” index indicates I need to. ” You can read mor at the ukaps forum page. The index seems simple to use, when visible yellowing is observed first address the most likely limiting nutrient for floating plants : nitrogen. If this does not work add some other macro nutrients and end up with micros. He rightly argues that this system should mainly be used in low light systems, because the latency is pretty large and more sensitive plants will suffer more. While the idea is interesting, I was unable to find very good examples of different deficiencies or tests to see if the duckweed index works.
However, his idea made me curious. How would common duckweed (Lemna sp.) behave under different nutrient conditions. Lemna sp. is an excellent model organism for aquarists. It is an aquatic plant that takes all the nutrients from the water column and has access to atmospheric CO2. Therefore, there is little chance of CO2 limitation and no need for substrates. It also has a relative small size and a very fast growth rate. Perfect.
On this page I would like to provide better documentation and examples of how deficiencies in Lemna sp. look and behave. This is done in attempt to test if we can actually have a better informed and improved “duckweed index”.
More about the methods will be presented soon as the experiment is ongoing.