Loan rates for Gentry County were used in place of harvest price starting in 1998. Averaged over years and locations corn and soybeans have not responded to additional phosphorous. See Table 4 below. Phosphorus and potassium is removed with the grain and not applying fertilizer for many years will deplete the soil and cause deficiencies. Corn, Soybean Yield and Return to fertilizer application after 4 years of corn and 3 years of soybean at different phosphorus levels.
Numbers are averages over years and locations.
With the higher potassium subsoils, it may not be profitable to build soils to that level yet further research is needed to confirm this. On average, over years corn seems to respond more favorably to the potassium than does soybeans. That during difficult times soil sampling is even more important to find and apply fertilizer to those areas that are lowest first priority. Keep soils at least up to the medium level for phosphorus and potassium. Since 1997 on location 1, which is two years of corn and two years of beans, we have already mined 173 lbs of phosphorus and 192 lbs. of potassium from the control plots.