On Friday, my grandmother made a comment about kashering a knife by sticking it in a potted plant (with very loose soil), something which will not kasher anything. What is the origin of this bubbe-meiße (old-wives' tale)?
You might be referring to the practice of thrusting a knife into dirt 10 times as part of kashering. I will cut and paste some posts I found on the subject from http://www.imamother.com/forum/viewtopic.php?p=248118
None of the posts on that website provides a source.
The Star-k website has the following statement:
"Sticking utensils in soil is generally not an effective way of kashering. (There is only one case where it is effective and so that is why some think that this method works.)"
[but I can't find anywhere on their website where they detail the one case]
This site details the practice at the beginning:
"However, when a knife or other utensil is used to cut, chop, grate, or grind a non-kosher food, a residue of that non-kosher food remains on the knife even after the knife is “cleaned”.1 This principle, known as duchka d’sakinah, is discussed in the Gemara and Shulchan Aruch,2 where it states that the knife may only be used for kosher food if it either undergoes a process known as נעיצה or is filed down (i.e. sharpened). נעיצה involves thrusting the knife into semi-solid earth 10 times, and it is generally understood that this process is effective, because the earth serves as an abrasive cleaner for the knife’s blade.
1 See, for example, Toras Chattas 23:7.
2 See Gemara, Avodah Zara 75b & 76b, and Shulchan Aruch 10:1, 96:1 & 121:7."
The Simlah Chadasha (10 "יתר דיני סכין"), discusses different ways to mess up a knife and how to fix them. If someone slaughters a טריפה1 (an animal that has certain physical deformities, such that it would not live 12 months), the knife must be cleaned (via rinsing הדחה or rubbing קינוח) from the fats of the טריפה.
But, if one killed three טריפות in a row, without a good שחיטה or הדחה\קינוח in the middle, then that knife needs נעיצה in hard ground to be kashered (10:13). הדחה\קינוח do not help for this case.
Sticking it into the ground ten times means sticking it into the ground ten times (there, 7). This does not mean sticking it into the ground nine times, nor does it mean sticking it into the ground once and leaving it there (leaving it in the ground does nothing -- we need abrasive force to scrape hardened [invisible to the naked eye] fat from the knife).
1 While it is not explicit in the text of the Simla Chadasha, I asked my rebbi and he told me that the same would also apply to neveila as well. [If you can source this to another ספר הלכה, please leave a comment below!]
2 That is a case where one used a knife that was used (for a long time) by a non-Jewish slaughterer -- one may assume that it was used for many טריפות (and, לכאורה, also נבילות as per the above footnote) and thus requires נעיצה.
It's called נעיצה, and all the sources I've seen have specified that the ground must be hard when you thrust the knife in. I don't know the exact origin, and this is not an exhaustive list, but it is mentioned by the Rambam (maachalot asurot 6:20 and 17:7), and followed by the Shulchan aruch (YD 121:7, YD 10:1, ), and the Rema (YD 89:4, YD 69:20) in certain circumstances. In other words, it's actually not totally a bubba-meisa, although in practice it is rare.
My guess on why it's so uncommon today (and this paragraph is purely speculation so please treat it as such), is that in times when running water was harder to obtain neitzah was relied upon more because the ground was more convenient than a well. Nowadays with the invention of indoor plumbing and faucets, water is easier to obtain so people use boiling water in cases where either neitzah or boiling water is an option.