KanjiJLPT N1
Insert

insert, put in, graft, wear (sword)

Mnemonic

With your hand, you insert a thousand seedlings into the field, pressing each one firmly into the soft earth. One by one, your hand pushes them deep into the mud—a thousand tiny lives tucked snugly into the field before sunset.
Additional thoughts
Picture kneeling in a muddy rice paddy, your hand aching after inserting seedling after seedling. The scale (a thousand) and the setting (field) lock in the radicals while the repetitive pushing motion anchors the meaning of insert.
Quick recall
Your hand Inserts a thousand seedlings into the field.

Details

The keyword for 挿 is insert. This kanji refers to the act of putting or sticking something into something else, such as sliding an object into a narrow space or slot. It conveys the physical motion of pushing one thing inside another, and is commonly associated with actions like plugging something in, slipping something between pages, or fitting a piece into place.
On'yomi
そう
Kun'yomi
さ.す、はさ.む