Fact: While there are specific instances where thrusting a knife into hard soil ten times will kasher it, that does not work for other cutlery, and there is no halachic basis for leaving a knife in the soil for a long period of time.
Background: I was first exposed to this practice when visiting my grandparents in Jerusalem in the 1980s and seeing silverware planted in flower pots on their porch. This practice seems to have deep roots. Rabbi Hillel Herz (1615-1690), in a discussion on kashering knives, notes (Beit Hillel, YD 10:2) that one should not plunge a knife once in the ground and then leave it there, as this doesn’t accomplish anything.1
The practice stems from a legitimate method of kashering knives. Kashering utensils is necessary because non-kosher food can adhere to the utensil or the non-kosher taste (ta’am) can be absorbed into it. When utensils are used for kosher food, they must be free of any non-kosher residue or absorption of the taste of non-kosher food that could, in turn, get absorbed into the kosher food. Reasons for2 koshering utensils include: milchig utensils used for fleishig food, utensils used for non-kosher food that are subsequently to be used for kosher food or Passover utensils that were used with chametz. In general, all of the actual tangible prohibited substances that adhere to the keli must be removed, and the unwanted absorbed taste eliminated.
The basic principle regarding removing or destroying absorbed substances is k’bol’o kach polto—the manner in which the unwanted food was absorbed is the way to remove it. For example, the method of kashering metal utensils used directly on the fire with no liquid, such as baking pans or a barbeque grill, is libun—heating the utensil to a very high temperature (Shulchan Aruch, OC 451:4; Mishnah Berurah 451:27). This works by incinerating both the adhered and the absorbed unwanted substances (Shach, YD 121:17; cf. Mishnah Berurah 451:27). For utensils used to cook food including liquids, the method used is hagalah—the utensil is placed in boiling water that draws out the taste, which is then nullified in the water (Shulchan Aruch, OC 451:5).3 Sometimes it may be sufficient to pour boiling water on the utensil, iruy rotchim. In many cases, if the non-kosher food was cold when it contacted the utensil, the utensil merely needs to be washed with cold water (Shulchan Aruch, YD 121:1).4
The Shulchan Aruch states that kashering a knife is different from kashering other utensils and requires either libun (YD 121:7) or hagalah (OC 451:3).5 But there is also a method of kashering knives that involves sticking the knife into soil. When the Mishnah (Avodah Zarah 5:12 [75b]) enumerates kashering methods, it states that a non-kosher knife may be polished or buffed and thus rendered kosher. Commenting on this, the Gemara (ibid., 76b) mentions an alternative6 method, ne’itzah b’karka—thrusting it into hard earth ten times. The Gemara qualifies that this method only works if the knife will be used for cold food. In other words, a non-kosher knife which is thrust into the ground ten times may be used to cut cold kosher food or a fleishig knife so treated may be used to cut cold cheese. The Rema (YD 121:7) qualifies that this should be done only as a temporary measure, but for regular use, even for cold food, the custom is to fully kasher the knife.
This method is mentioned in a few different contexts in halachah. It would appear that it is not meant to expel the non-kosher taste absorbed into the knife but rather to remove surface fat (shamnunit). The hard dirt does the job of cleaning the knife, and removing all non-kosher remnants from the blade. Unlike the other kashering methods listed earlier, however, this method does not seem to remove the non-kosher taste absorbed in the utensil. However, Rabbi Yehudah Leibish Landau of Sadigura (d. 1900; Yad Yehudah 10:3), in a tour-de-force, expounds on the possibility of ne’itzah b’karka having an effect on some or all of the absorbed taste as well.
The Gemara mentions that one must thrust the blade into the earth ten times. Tosafot (Avodah Zarah 76b, s.v. ha’sakin) cites a Yerushalmi that says three thrusts but concludes that one should be careful to thrust it specifically ten times. Similarly, Rambam (Ma’achalot Assurot 17:7) and the Shulchan Aruch state ten times. The Knesset Hagedolah (Hagahot Tur, YD 121:99) quotes earlier authorities who say fewer thrusts are sufficient, but concludes with ten, and Yad Yehudah (YD 89, Perush Hakatzar:34) concludes that despite other opinions, the correct approach in his view is ten thrusts.
The soil is described in our version of the Gemara as being “unworked,” but other versions of the Gemara, as well as the Shulchan Aruch, state the earth must be hard (Gra, YD 121:20), and this is understood to mean that each thrust should be in a different spot in the soil.7 The blade must be smooth and not pitted. This method is only mentioned regarding a knife; the knife is unique that it involves both a stringency and a leniency. A chumrah associated with a knife is that it more easily acquires non-kosher matter and ta’am because of duchka d’sakina—the pressure of the [cutting] of the knife. On the other hand, because a knife can be thrust straight into the ground, the earth can serve as an abrasive cleaner for it but not for other utensils.
A knife used for shechitah must be kosher and if it wasn’t kosher when it was used to slaughter an animal, the areas surrounding the animal’s neck must be scraped clean or at least washed. If one has only a non-kosher knife available to use for shechitah, this method of thrusting into the ground may be used (Shulchan Aruch, YD 10:1) and then he can slaughter the animal. There is a difference of opinion regarding how effective the method is and whether it applies to a recently used knife (ben yomo), or if, like hagalah, the knife must remain unused for twenty-four hours (Taz, YD 10:4).
One may not use a knife that is usually used for meat to cut cheese or even to cut bread that will be eaten with cheese. This is true even if it was used only to cut cold food items (that are dairy) and it is now being used to cut cold food items (that are fleshig, or the reverse), because fatty residue adheres to the surface of a knife that is in frequent use (Nekudot HaKesef, YD 89:6). However, the Rema (YD 89:4) says it would be permitted to use the fleishig knife if it was jabbed into the hard ground ten times.8 It seems that at one point in history it might have been common, likely due to poverty, to regularly use a milchig knife for fleshing after thrusting it into the ground ten times (and the reverse as well). The Rema, however, notes that one should not use one knife for both milchig and fleshing; the custom is to have two knives, and to label them. Some people even had a third knife specifically to cut bread, which would be pareve (Aruch Hashulchan, YD 89:16).
Meat that had been salted but not rinsed is not considered kosher and thus, if it is cut with a kosher knife, the Shulchan Aruch (YD 69:20) rules that the knife requires hagalah. The Rema says it suffices to thrust such a knife into the hard ground ten times. Similarly if a dairy knife was used to cut warm meat, there is a concern that fats will linger on the surface of the knife; the Rema says it can be kashered by thrusting it into hard dirt (Darkei Moshe 105:5; Rema, YD 94:7).
This method is also mentioned as a means to kasher a non-kosher knife (bought from non-Jews) for cold use (Shulchan Aruch, YD 121:7).9 According to the Shulchan Aruch, a knife kashered in this manner can even be used to cut a davar charif—a “sharp” item, such as an etrog or radish (Gra, YD 121:22), although Darkei Teshuvah (YD 121:80) cites authorities who disagree with this.
It seems that this method works because the abrasive nature of the hard soil serves to clean and remove the fatty sheen. An obvious question is whether other methods of accomplishing this goal are acceptable. Rabbeinu Tam (Sefer Hayashar 790) writes that it is not specifically thrusting it into the earth but even rubbing it well on a stone suffices. The Meiri (Chullin 8b) says that merely washing or rubbing the knife with a cloth does not suffice because it does not remove old fat, but using sand or soap is equivalent to thrusting a knife into the ground. Rabbi Yosef Greenwald, the Puppa Rav (Vaya’an Yosef, YD 162), was asked if steel wool could serve the same function. He states that thrusting the knife into the soil is preferable, but otherwise he approves of using steel wood; however just as one is required to thrust the blade into a different spot in the dirt each time, the steel wool should be replaced for each of the ten rubs.
It is clear that in the sources there is an unusual method of kashering knives that involves thrusting in the ground ten times. This method, ne’itzah b’karka, has few practical uses and seems to have been widely misunderstood. The folk practice of burying silverware is clearly a misapplication of this technique, and as Beit Hillel (YD 10:2) stated there is no kashrut benefit to planting cutlery that has become non-kosher. This misconception obviously developed when thrusting a knife into hard soil morphed into burying, and knives were expanded to include other utensils. Today, as most people can afford to purchase a lot of utensils and they therefore kasher much less frequently than in the past, expertise in all methods of kashering, including ne’itzah b’karka, has decreased among the laity.
1. Well-known Israel novelist Yaakov Shabtai (1934–81) wrote in the short story “Adoshem” (Uncle Peretz Takes Off , p. 8): “…Sometimes he [my grandfather] would burst out of his chair, grab from his [someone else’s] hand a milchik utensil that had erroneously been used for meat or vice versa, bang it on the table and the door, then rush to stick it furiously in the flowerpot on the balcony . . . On days when he was particularly harried, the whole flowerpot was planted with spoons, forks, and knives glistening in the sun opposite the street.” This misconception also features prominently in a popular routine by comedian Buddy Hackett, in which he tries to explain to a policeman why he was burying a knife in his backyard. In an episode of Curb Your Enthusiasm (Season 5  Episode 48), a religious character explains why he needs to bury a plate, and Canadian folkloirst Shelley Posen, in his 2007 album Menorah: Songs from a Jewish Life, has a poignant song called “Fork Garden,” in which his daughter discovers a “garden” of forks in her grandmother’s flower pots.
2. Not all utensils are kasherable. For example, the Gemara (Pesachim 30b) derives from Vayikra 6:21 that it is impossible to purge absorbed taste from earthenware.
3. Because hagalah works by removing and nullifying the absorbed taste, the pot with the boiling water must be sufficiently large and the item being koshered must not have been used in twenty-four hours (eino ben yomo), and it must be thoroughly cleaned. If this is not possible, such as because of rust, hagalah will not be effective (OC 451:3; Mishnah Berurah 451:22).
4. Soaking for twenty-four hours (even in cold water) has the status of “pickling” (kavush) and thus if a prohibited food item soaked in a utensil for twenty-four hours, the utensil would need to be kashered (Kaf HaChaim, YD 105:1).
5. On the reason for two different methods, see Darkei Teshuvah, YD 121:81 and Mishnah Berurah 451:19.
6. Rashi says this method is in addition to another method used; most other commentators disagree and say it is in lieu of.
7. Rabbi Alexander Sender Schor (d. 1737) suggests (Tevu’ot Shor 10:16) that because Tosafot says the ground must be hard but not too hard, and we do not have clarity regarding how hard the soil should be, this method has fallen into disuse and he suggests alternatives.
8. See Taz 89:6, 7 and Shach 89:22 for when thrusting the knife is required and in what circumstances it is efficacious.
9. I.e., this method works even for a knife that had been used regularly with hot non-kosher foods. For a one-time non-kosher use, such as slaughtering a treifah, it is sufficient to merely wash the knife in cold water (Tosafot, Chullin 8b, s.v. v’hilchata).
Rabbi Dr. Ari Z. Zivotofsky is a professor of brain science at Bar-Ilan University in Israel.