ground that has been under cultivation for one year, evil, calamity
甾 centers on land that has been worked for a season, but the given meanings also include evil and calamity, which do not share an obvious conceptual link with cultivation. The core idea is unclear; the agricultural sense and the disaster sense may belong to separate uses of this rare character.
The origin of 甾 is uncertain. It visually combines 巛 (river) and 田 (field), but the historical relationship between these components and the meanings 'cultivated ground' and 'evil/calamity' is not clear.
The top 巛 looks like flowing water, and the bottom 田 is a field. Imagine a field that has been flooded for a year, making it both cultivated and a source of calamity.
For シ, picture a field that has been under cultivation for one year, and you see a single sheaf of wheat: sheaf -> シ.