Phosphorus, Potassium, and Other Nutrients

For irrigated fields with yields close to normal, expect little impact from drought on soil test levels of phosphorus (P), potassium (K), and other nutrients. For dryland fields, crop uptake of these nutrients will be less than normal. However, this may have little impact on soil test levels for P, K and other nutrients. There may be some tendency toward higher soil test values, but soil tests for these nutrients are an estimate of plant availability over the growing season. There is evidence that soil test K can be influenced by soil drying, and some states have started analyzing soil K from field moist samples. However, most Nebraska soils are quite high in potassium, and thus slight differences in soil test K that can occur with the analytical process have no impact on the fertilizer recommendation.


Read: Phosphorus, Potassium, pH Management Issues Following Drought-Damaged Crops


Silage and Residue Harvest

In many cases, growers harvested fields intended for grain production as silage. In other cases, there will be opportunity to harvest crop residue as feed due to reduced pasture and hay production. Either way, growers should be aware of nutrient removal resulting from biomass harvest. Significant amounts of P, K and micronutrients, as well as carbon, can be removed from the field with biomass harvest. For example, a 150-bu./acre corn crop will uptake approximately 64 lbs. P2O5 and 42 lb K2O in grain, but will have approximately 36 lbs. P2O5 and 144 lbs. K2O/acre in stover. The value of these nutrients should be considered when pricing baled stalks for livestock feed. More information on the impacts of harvesting crop residues is available in the Extension publications Baling Corn Residue (pdf)  and Harvesting Crop Residues (pdf). If stalks are grazed rather than baled, expect much of the nutrient content in grazed residues to return to the field, minus the harvested weight of cattle once they are taken off the field.


The Bottom Line

Expect soil nutrient values – particularly residual nitrate – to be more variable this fall than in recent years from field to field, and within fields. The only way to know the availability of soil nutrients is to soil test. Fertilizer costs can easily exceed $100/acre for irrigated corn. Nitrogen prices currently range from about 50 to 70¢/lb. N, depending on the fertilizer source. Phosphorus prices range from 45 to 80¢/lb. P2O5, depending on P source and the price of N fertilizer. Thus, an irrigated cornfield requiring 180 lbs. N/acre at 60¢/lb., and 60 lbs. P2O5 at 60¢/lb., will have a fertilizer cost of $148/acre. Making accurate estimates of profitable fertilizer rates for next year will require accurate soil test information. Guidelines for soil testing are available in two Extension publications: Guidelines for Soil Sampling (pdf) and Soil Sampling for Precision Agriculture (pdf).

While there should be plenty of time for soil sampling this fall due to an early harvest, consider sampling next spring instead. Dry conditions this fall may make sampling difficult, and spring sampling may result in a more accurate prediction of nitrate-N availability, depending on weather conditions this winter. Nebraska is fortunate to have several good analytical labs where growers can have soil samples tested. Contact any of these Nebraska soil testing labs for information on current pricing and how to ship samples.


—Richard Ferguson, Gary Hergert, Charles Shapiro, Tim Shaver and Charles Wortmann,
Extension soils specialists