Nutrient removal with drought-stressed corn
How much nutrient is present in the biomass produced? Nutrient concentration is the other important piece of information needed to determine nutrient removal amounts. In a growing season with normal conditions, there can be large variability in the amounts of phosphorus (P) and especially potassium (K) present in biomass. Typical values for stover after grain harvest range from 5 to 8 lb of P2O5/ton and from 5 to 40 lb of K20/ton (with 20 to 30 lb K2O/ton being most common). The lower K contents are caused by the leaching of K from the plant by rain after the crop starts dying, which obviously would not apply at this time.
So nutrient removal values can vary even under normal circumstances; the droughty conditions make it even harder to estimate a value that would work across all locations. The amount of P and K present in the plant material is a function of how much is taken up by the crop. Under severe drought conditions the capacity of the crop to take up nutrients is diminished (see "Water Stress and Nutrient Deficiency" in issue 15 on July 13), so amounts lower than normal would be expected in the corn crop's vegetative tissues this year.
Under normal growing-season conditions, by R1-R2 development stage, nearly all of the K (around 170 lb K2O/acre) and about 50% of the P (around 40 lb P2O5/acre) is taken up. Of course, all of the nutrients are present in the vegetative tissues at these stages, as there are no large seeds present yet. As the seed starts to form, part of the P and K in vegetative tissues translocates into the seed, and by physiological maturity (R6), about 20 lb of P2O5/acre and 120 lb K2O/acre are present in vegetative tissues and cobs. Again, I suspect that the amounts present at R1 for the current crop would be lower because of the droughty conditions.
The best approach to accurately determining nutrient content is to send a representative sample of the harvested material to a commercial lab for analysis. If for no other reason, nutrient analysis is important with drought-stressed corn to determine nitrate concentrations for managing livestock feeding. It would not cost much more to have P and K analysis done at the same time. For more information regarding how to manage livestock feeding of drought-stressed corn with high nitrate concentrations, see the University of Illinois Extension website on drought resources.
What's the bottom line? While there are many factors that make it difficult to generalize about how much nutrient will be removed if biomass is harvested out of drought-damaged corn, the following notes offer general guidelines or at least illustrate important points to consider when making P and K fertilizer decisions for this fall.