insert, stick into, plant
插 centers on putting something into a space or medium: inserting, sticking into, or planting something firmly.
插 combines 扌 (hand) with 臿, which likely contributes both sound and the idea of inserting or piercing. The exact historical development of 臿 is uncertain.
The left hand 扌 holds something and pushes it into the right component 臿, which looks like a tool or container. Picture a hand inserting a plant into soil.
For ソウ, imagine sowing seeds by inserting them into the ground: sow -> ソウ, and your hand plants them firmly.