The way I understood it is that the Sugya here in Chulin refers to a scenario such as where the knife is stuck in the wall and is being used for Shechitah. This is called "Talush ul'v'Sof Chibro" because the knife was originally detached but afterwards one affixed it on the wall. In contrast, in the case in Bava Metzia 56b, when one plants wheat in the soil one is not fixing the wheat in the soil, but rather in order for the wheat to grow it has to be sowed in the ground. This is why the Gemara there uses the phrase "Batlinhu" -- because it may be that we consider that the wheat seeds have no independent existence but merely became part of the field. This is very different from a knife in a wall, where we would not say that the knife became Batel to the wall.

Kol Tuv,

Dovid Bloom