There is a formula but I just shot from the hip on that one.  What are your yields goals?  300 -400 pounds of potash per year for a few years.  4% K base on a soil with 30.8 CEC is 960 pounds.  

If you could get to 2.5% K base would really help.  Thats 600 pounds - 400 pounds what is in the soil (400ppm) =200 pounds or  nearly 333 pounds of potash product + the K used in crop removal.  I know thats expensive.  $100/ A  


The K strips Ed is talking could be broadcast.  They would show the crop response to K.  If your not ready to do the whole field, atleast try some trial strips this spring.  It will be a good experiment to learn from.

Many farmers in ND and MN are learning this same thing.   For decades they were told your K level is high based on the PPM.