The parameter 236 (Soil temperature level 4) specifies that its top level is 100cm and its bottom level is 289cm so to encode this information in GRIB edition 1 we would need to set the value of the key "bottomLevel" to 289 (octet 12 in section 1. See GRIB edition 1 section 1 spec).
However since this key is only one octet it cannot accommodate any 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.