soil, loam, earth, rich
壤 centers on fertile, rich soil or loam—the kind of earth that supports growth. The meaning extends naturally to soil in general and the idea of richness.
壤 combines 土 (earth, soil) with 襄, which likely contributes the sound and may carry a sense of abundance or richness. The exact historical development of 襄 is uncertain, but the phono-semantic structure is clear.
The left side is 土 (earth), and the right side 襄 looks like a full, rich container. Picture a handful of dark, crumbly loam—soil so rich it almost overflows.
For ジョウ, imagine a rich field of soil where a farmer named Joe works: Joe -> ジョウ, and his land is always fertile.