There's not enough salt to sterilize the land where dumped. The salt in fish sauce preparation may be as high as 20% initially but by the time it's done, after having been drained and refilled multiple times, it's 2-4%. To sterilize land requires 5-10kg/m^2 of salt applied repeatedly.

I’m not talking about sterilizing land. Just that salt is not a desirable ingredient in fertilizer. Most plants will not thrive in salty soil, even if it doesn’t kill them.