Alfalfa would be a good alternative.  Every ton of alfalfa uses about 80 lbs. of potash.  An eight ton yield would use over 600 lbs. of potash.

Once established it can tolerate the uninterrupted irrigation better than row crops too.   I know this, because we use to receive waste water from a green bean processor.  There were times we might get a two-inch-plus rain, and the next day they were out there irrigating their waste water.  The alfalfa was much better at handling this amount of water as compared to how a row crop could have handled it.

Turning it into an alfalfa field probably wouldn't work too well for you unless you had a dairy farm near by that would like to buy the hay.   I wouldn't be surprised you might net more income selling the standing hay to a dairy farm versus planting a row crop.  

gordon