Simple Calculation Reveals Soil Phosphorus and Potassium Removal

Progressive growers monitor soil fertility using soil tests, but also closely manage fertility programs by predicting future crop nutrient needs.

Published on: Apr 14, 2011

Soil nutrient programs will soon be put to the test as the growing season approaches quickly. These programs should be carefully developed with input from an agronomist or fertilizer dealer because fertility is responsible for as much as 40% of crop yield. As growers see their investments pay off and yields increase, it's important to remember that high-yielding crops remove proportionally more nutrients such as phosphorus and potassium from the soil. Nutrient application must keep pace with crop removal.

Progressive growers monitor soil fertility using soil tests, but also closely manage fertility programs by predicting future crop nutrient needs. This is especially important in years when ideal growing conditions result in yields which go beyond yield goals and the corresponding nutrition program designed to reach that yield goal.

"When crop yields increase and fertilizer rates are not increased to meet the crop's increasing nutrient needs, the plants turn to the soil's nutrient bank, pulling nutrients from it and mining the soils," explains Dan Froehlich, agronomist with The Mosaic Company. "When removal rate exceeds application rate, a negative soil nutrient balance can occur that can be yield-limiting for future crops."

One way to get an idea of future fertility needs is to calculate the difference in crop removal rate between the yield goal and actual yield harvested. Simple equations found HERE may be used to calculate P and K removal rates from the soil for both corn and soybeans.

For example, if a soil fertility program is designed to produce a 180-bushel-per-acre corn yield, the crop P removal rate will be 63. (180 yield bu. x .35 [P2O5/bu. removal constant] = 63 removal rate). If growing conditions are positive and crop yield increases to 220 bushels per acre, the crop P removal rate increases to 77 (220 yield bu. x .35 = 77). This difference of 14 is taken from the soil by the crop, reducing soil test phosphorus and potentially creating a negative balance for soil phosphorus levels.