to weed grass, land under cultivation
菑 centers on land that has been cleared of weeds and made ready for cultivation. The weeding action and the resulting cultivated field are two sides of the same agricultural idea.
菑 combines the grass radical 艹 with 甾, which likely contributes a phonetic element. The character is associated with clearing weeds from land, but the detailed historical development is uncertain.
The grass radical 艹 sits above a field-like shape 甾. Picture weeds being pulled from the top of a field, leaving the land clean and ready for planting.
For シ, imagine the sound of a scythe shearing through weeds: the 'sh' of the scythe cues シ, and the field is cleared for cultivation.