I think on hot days you are supposed to "flush" the rice fields with fresh (cooler) water anyway. The runoff then goes back to wherever (a river or in this case

I think on hot days you are supposed to "flush" the rice fields with fresh (cooler) water anyway. The runoff then goes back to wherever (a river or in this case maybe the ocean). Assuming that is true, I imagine that means any relevant nutrients are not flushed out by this process.