Removal values vary under normal circumstances. The droughty conditions make it even more difficult to estimate a value that applies to all locations.

“Severe drought conditions reduce the crop’s capacity to take up nutrients; thus, we expect to find lower-than-normal amounts in the vegetative tissues of the corn crop this year,” Fernández said.

By the R1 to R2 development stage in a normal growing season, plants have taken up nearly all of the potassium (around 170 pounds of K2O/acre) and approximately half of the phosphorus (around 40 pounds of P2O5/acre).

All of the nutrients are present in the vegetative tissues at these stages. As seed starts to form, some of the phosphorus and potassium in the tissues translocates into the seeds. By physiological maturity (R6), approximately 20 pounds of P2O5/acre and 120 pounds K2O/acre remain in vegetative tissues and cobs.

“The best way to determine nutrient content is to collect a representative sample of the harvested material and send to a commercial lab for nutrient analysis,” Fernández advised.

“It is important to perform nutrient analysis on drought-stressed corn to determine nitrate concentrations if it is going to be used for livestock feeding. Asking for phosphorus and potassium analysis at the same time does not add much to the cost.”

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

The amount of phosphorus 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 pounds P2O5/ton, the removal would be 20 pounds P2O5/acre.

Even with 40 pounds P2O5/acre (the amount accumulated in vegetative tissues by R1 under normal growing-season conditions), the removal values are lower than the 77 pounds P2O5/acre removed when harvesting 180 bushels of grain per 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 pounds K2O/ton, the crop would remove 80 pounds K2O/acre; if the material contains 30 pounds K2O/ton, then 120 pounds K2O/acre would be removed.

The estimated amounts are higher if 170 pounds 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 pounds K2O/acre for a grain yield of 180 bushels per 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 advised.

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