Submitted by John Heard, MAFRD Crop Nutrition Specialist
During seed fill, potassium deficiency symptoms is most commonly observed in soybeans. Potassium is a mobile nutrient, meaning that as deficiency occurs, usually lower leaves are scavenged of potassium in order to support new growth. This early season deficiency appears as yellowing and later browning of lower leaf margins (Figure 1).
Soybeans remove alot of potassium/acre in the seed – about 1.4 lbs K2O/bu. As seeds fill, some of the potassium that is stored in the plant leaves is mobilized for movement to seeds.. Then one may observe deficiency symptoms appearing on upper leaves (Figures 2 and 3).
The likely areas with potassium deficiency are lighter textured or sandy soils, peat soils or cropping systems where past potassium removals have been high without replenishment. These symptoms can be verified with tissue testing but should ultimately trigger soil sampling to determine the rate of potassium to be applied.