Nutrient concentration is the other piece of information needed to determine nutrient removal amounts. In a normal growing season, phosphorus (P) and potassium (K) levels in biomass can vary considerably. Typical values for stover after grain harvest range from 5 to 8 lbs. of P2O5/ton and from 5 to 40 lbs. of K2O/ton, with 20-30 pounds K2O/ton being the normal range. Low K levels are usually due to rain leaching it out of the plant after the crop starts dying – obviously not a problem this season.
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 says.
By the R1-R2 development stage in a normal growing season, plants have taken up nearly all of the K (around 170 lbs. K2O/acre) and approximately half of the P (around 40 lbs. of P2O5/acre). All of the nutrients are present in the vegetative tissues at these stages. As seeds start to form, some of the P and K in the tissues translocates into the seeds. By physiological maturity (R6), approximately 20 lbs. of P2O5/acre and 120 lbs. 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 advises. “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 P and K analysis at the same time does not add much to the cost.”