alkaline, alkali, lye, salt
碱 centers on alkaline substances: chemical bases, lye, and salts that are basic rather than acidic. The salt sense is specifically alkaline salt, not table salt in general.
碱 combines 石 (stone) with 咸, which likely contributes the sound and the idea of saltiness or alkalinity. The exact historical development is uncertain, but the character is used in Chinese for alkaline substances.
The left side 石 (stone) suggests a mineral, and the right side 咸 looks like a container with a salty taste. Picture a stone-like lump of alkaline salt or lye.
For ケン, imagine a chemist named Ken testing an alkaline stone: Ken -> ケン, and the stone fizzes with basic reaction.