husk, bract, straw wrapper, souvenir gift, bribe
苴 centers on a plant-based covering or wrapping: a husk, bract, or straw wrapper. The extended senses of souvenir gift and bribe likely come from the idea of something wrapped or presented in plant material.
苴 combines the grass/plant radical 艹 with 且, which may have contributed a phonetic element. The exact historical development is uncertain, but the plant radical fits the husk/wrapper meanings.
The grass top 艹 covers the bottom 且 like a straw wrapper around a bundle. Picture dried grass husks tied around a gift.
For ソ, imagine sewing a straw wrapper: sew -> ソ, and the grass husk is stitched together.
bundle of straw used as mulch