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By klasac
Posts:  18
Joined:  Sun Jan 03, 2010 11:00 am
Hi there!
After many experiments performed using Agar, I decided to switch to Gelrite. With my foodstore agar, I 'managed' to kill some precious drosera species, just because of its chemical variations depending on the batch. The agar I used raised the pH of media solution from desired 5.5 to 7.1 (without me noticing) which - of course- killed many germinating plants. So I got tired of checking the pH of solidified media and re-melting it to sterilize it, and bought 100g of gelrite.
I would like to ask someone who used it before, what is the best concentration and how does it alter pH after addition?
I read that unlike agar, it dissolves in warm water already so the heating stress is lower too.

Thanx for any comments:-)
By Frankco
Posts:  14
Joined:  Thu Jan 14, 2010 12:42 am
Hi klasac,

Gelrite will raise the pH just a bit more than agar. It gets a bit tricky because you will use different amounts of Gelrite depending on the starting pH of your media. I would check the pH after adding the Gelrite and then adjust your media.

Below are examples of how Gelrite and GP812 compare with agar when it comes to affecting the pH. Of course there are slight differences between brands of agar and some will change the pH more than others. The amount should be small unless you are not using micropropagation grade agar.

Starting out with media that has a pH of 4.89, gelrite will raise the pH to about 5.41.

Gelrite is pH dependant, meaning the amount of Gelrite you use will depend on the pH. The more acidic the media the more Gelrite you have to use.

Plants may raise the pH of the media and what looked like a nice gel to begin with may start to turn liquid after a while. You can adjust for this but if it's too much of a headache you may think about trying GP812 which isn't affected by pH, mixes cold and still has good clarity.

pH of De-ionised water is 6.63
Adding nutrients, PGRs etc. pH 4.89
(this is just an example. Your media will probably not have the same pH)

So starting with a pH of 4.89 the end results are:
with agar pH 5.09
with Gelrirte pH 5.41
with GP812 pH 6.01

Amount of Gelrite to use according to pH:
pH 4 use 4.92g/L
pH 5 use 2.93g/L
pH 6 use 1.36g/L
pH 7 use 0.64g/L
pH 8 use 0.50g/L
pH 9 use 0.40g/L

Hope this helps
Frankco, Frankco liked this
By klasac
Posts:  18
Joined:  Sun Jan 03, 2010 11:00 am
Hi, Frank!
Thank you so much for the detailed information!
I did some experiments and here are the results.
I used Gelrite (TM) from Duchefa company.
Your table of desired amounts of Gelrite depending on pH fully comply with my findings. I used 2.0g of GELRITE per liter of media and it gelled smoothly, with perfect consistency. To be exact, the hardness of the gel depends also on the amount of mono- and bi- valent metal cations present in the solution ( media concentration).I used 25%MS.
With the pH shift, it is another story from yours though. My GELRITE lowered the starting pH a bit. Because the gelling process means binding the gelrite structure units with metal ions, protons are released from the polymer during solidification of the media, which increases acidity. That is why, as expected, the pH dropped.
I started at 5.70 and after 25mins of autoclaving the pH dropped to 5.48 at room temperature (for abovementioned MS concentration).
This is perfect for dionaea and some drosera,,but somewhat low value for other drosera species. I guess I will start at pH 5.85 with concentration 2g of GELRITE per liter to get it to 5.65-5.70 when it is ready for use.
Thank you for your input once again:-)
I hope my experience helps too:-)
By Frankco
Posts:  14
Joined:  Thu Jan 14, 2010 12:42 am
Hi klasac,

Very interesting. The pH readings are pre-autoclave. I'll have to try it after autoclaving. Thank you for posting your experience with gelrite.

By Rocko Bonaparte
Posts:  74
Joined:  Mon Jul 05, 2010 8:23 am
How do you check the pH after the agar has been introduced? I would figure you would have to liquify the media, which means subjected it to high temperatures. Then you have to find a suitable probe that would work that hot.

I am also now a bit paranoid about what agar is doing to my media. My last media I got the pH to 5.4 after I had stirred in the agar, but I know it hadn't been completely incorporated. Does this mean my final media are probably at a pH too high to properly propagate carnivorous plants?
By klasac
Posts:  18
Joined:  Sun Jan 03, 2010 11:00 am
Hi, Rocko!
Most of widespread Ag/AgCl pH probes measure up to 70C. The meter has also temperature correction (since pH is temperature-dependant).
To measure the effect of agar on media solution, you dont have to melt it. Just measure the pH after you add solid agar powder. The reason for this is, that the pH is affected by salts present in agar and these salts are water-soluble and dissolve almost immediately even in cold solution. You will see the pH change almost instanneously after you add agar. The proton transfer is the fastest chemical reaction there is, so the speed of pH change is determined by the step of solvatation of the anorganic salts present in agar.
When agar is melted and medium autoclaved, part of agar is decomposed and the pH drops a bit, but this further change is much less than the initial change during salts solvatation.
So if you want to have the exact figures of how the agar affect the pH of your media, you can melt it/autoclave it and then measure the pH of cold solidified media.
I did both and it showed me that the agar can change the pH quite drastically (if you are not using expensive pure agar). That is why i switched to gelrite and so far i am very happy about the results:-)
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