The parameter 236 (Soil temperature level 4) specifies that its top level is 100 cm and its bottom level is 289 cm so to encode this information in GRIB edition 1 (GRIB1) we would need to set the value of the key "bottomLevel" to 289 (octet 12 in section 1. See GRIB1 section 1 spec).
Note for this parameter, the key "indicatorOfTypeOfLevel" is 112 i.e., Layer between two depths below land surface (See https://apps.ecmwf.int/codes/grib/format/grib1/level/3/)
In this case, we allocate octet 11 for the "topLevel" key and octet 12 for the "bottomLevel" key.
However since the bottomLevel key is only one octet it cannot accommodate a value larger than 255! Therefore it is not possible to encode levels larger than that value in GRIB1 so we simply set all the bits to 1 (which means MISSING).
GRIB edition 2 addresses this shortcoming.