Backyard Aquaponics
A place to discuss aquaponics

Hi, i'm struggling with this plant, believe or not but i'm not getting good results.
I'm trying to replicate the ideal ambient conditions , after reading multiple papers this is the range of nutrients that lemna needs :

I grow outdoors in 10 cm deep plastic containers

n = from 20 to 50 ml per litre
p = 1 ml per litre
k = 0.5 ml per litre
ph = ideal is 7
temperature = from 25 to 30 c
light intensity = 250 to 300 umol


Lemna minor seems stuck, it is green, a bit violet under the leaves (maybe it s the cold night?)
Temperature of the water goes from 25c to 30c in full sun, and at night it can reach 6-8 c
Water is still , no movement involved.
Having a tap water of kh 12 and ph 8.5, i tried cutting it with 70 r-o water, bringing the kh to 4, added some acid until ph 7.
Unfortunately the ph keep rising to 8, 8.5 after 24-36 hours.

What am i m doing wrong? Why my ph keeps raising?

Thank you

Those are the main nutrients, are you providing any micro nutrients?

The high pH could be locking out some nutrients that the lemna (duckweed) needs. Phosphorus deficiency causes purpling in leaves of some crops although I'm not sure about lemna. Lack of phosphorus or unavailable phosphorus can cause stunting or slow growth so this might be what's happening. Your system has phosphorus but it might not be available to the plants because of the pH.

I don't know anything about your system but assuming you have a biofilter that has the capacity to process the waste then the pH will come down as the biofilter matures. If you don't have the patience to wait you can slowly and carefully adjust the pH down using hydrochloric acid (also known as muriatic acid. The adjustment process in the next paragraph works well). If you decide to use the acid be careful and observe all the precautions and warnings.

You can remove a known amount of the water from the system into a bucket and adjust with acid to the desired pH then let it sit and see if you need to adjust again to make it stable (don't use a galvanized bucket, zinc can cause problems for fish). Keep track of how much acid it takes to get the pH of the water in the bucket stable at the reading you want. Once this is done you'll know how much acid it takes to adjust a known volume of water - this ratio should be the same for the entire volume of your system so you can figure the amount of acid you need to make the adjustment. Gradually adjust the system pH over a number of days - no more than 0.4 pH units per day. Most fish can handle larger changes than this but it causes them stress. If you overshoot the fish will probably wind up OK so don't get too concerned - just try to avoid overshooting.

Once you have the system pH down you'll need to adjust the top up water before adding so that it doesn't raise the pH again.

I'm not an expert on lemna so I can't help you much on the details of that but I hope this gives you a place to start.

You re probably right about phosphorous. I m going to solve the ph issue.
thanks