Why are potassium deficiency symptoms showing up?
The post-planting soil application of fluid or granulated K fertilizer broadcast or banded between the rows as a rescue treatment may, or may not, be more effective than foliar fertilization. This application will be inefficient when rainfall continues to be deficient or when a K deficiency is induced by the soil and root problems mentioned above. On the other hand, higher rates than with a foliar spray can be applied, and if the deficiency is due to low soil-test K the applied K will begin to increase soil-test K levels for the next crop. For the current crop, however, an application to the soil will not be effective if sufficient rainfall is not received soon.
How can you prevent this type of deficiency for future crops?
If you see deficiency symptoms this spring, you need to think about how to prevent deficiency in future crops.
If the reason for deficiency symptoms is low soil-test K, the deficient areas can be targeted for post-harvest soil sampling, testing, and appropriate fertilization. Deficient areas can be easily marked with hand-held global positioning devices, and this information can be provided to a dealer having variable-rate fertilization capability. Deep-band K placement for the next crop, mainly with ridge-till, strip-till, or no-till, can go a long way toward alleviating K deficiencies. A planter-band (starter) fertilizer treatment at planting also will help, but in-furrow K starter is not recommended for soybean and low rates should be applied for corn. For further information see Use new potassium soil test and fertilizer recommendations (ICM 10/20/2003), Iowa State University Extension and Outreach publications Take a Good Soil Sample to Help Make Good Decisions (PM 287), and General Guide for Crop Nutrient and Limestone Recommendations in Iowa (PM 1688).
If the deficiency symptoms are induced by soil or root issues other than low soil-test K, your area extension Field Agronomist or local crop consultant can provide additional suggestions about soil or crop management practices to help alleviate future problems.