Fernández offers some general guidelines and indicated some important points to consider when making phosphorus and potassium fertilizer decisions for this fall.

The amount of P removed by harvesting biomass will probably be lower than what is removed when only grain is harvested. If a low yield of 4 tons of dry biomass/acre is harvested at 5 lbs. P2O5/ton, the removal would be 20 lbs. P2O5/acre. Even with 40 lbs. P2O5/acre (the amount accumulated in vegetative tissues by R1 under normal growing-season conditions), the removal values are lower than the 77 lbs. P2O5/acre removed when harvesting 180 bu. of grain/acre.

For K, however, removal rates can be much higher than normal removals with grain harvest. Using the same low yield of 4 tons of dry biomass/acre, if the material contains 20 lbs. K2O/ton, the crop would remove 80 lbs. K2O/acre; if the material contains 30 lbs. K2O/ton, then 120 lbs. K2O/acre would be removed. The estimated amounts are higher if 170 lbs. K2O/acre, which is the accumulation in vegetative tissues at R1-R2 under normal growing-season conditions, is used for the calculation.

Whichever estimate is used, they are all much higher than the normal removal rate of 50 lbs. K2O/acre for a grain yield of 180 bu./acre. However, if grain yields are low and the biomass is not harvested, the removal rates will be lower than normal.

“Whatever your particular situation is, it is very important to account correctly for removal rates to manage fertilizer applications this fall,” Fernández advises.

(For more information on how to feed drought-stressed corn with high nitrate concentrations to livestock, see: http://web.extension.illinois.edu/drought.)