Potassium deficiency in field crops

Mike Rankin
Crops and Soils Agent
University of Wisconsin - Extension

This growing season there has been an increase in the number of fields (primarily corn) with plants exhibiting potassium (K) deficiency.  Discussions on the topic have been both local and statewide.  Potassium deficiency in corn is easily diagnosed.  Being mobile in the plant, K deficiency becomes prominent on lower (older) leaves and is characterized by a necrosis or yellowing of outside leaf margins.  Generally speaking, K deficiency has not been a problem for corn in the past.  That leads us to coming-up with some logical explanations, all of which won’t apply to every situation.

The most obvious explanation is simply that soil K fertility levels are low and not enough additional K was applied to meet crop needs.  Several changes in crop production practices might explain why this situation is more commonplace.  First, is the trend for more farms without livestock and consequently less manure applied. Manure is rich in K and annual applications generally result in more than adequate amounts available for corn and soybeans.  However, it should be pointed out that alfalfa is a heavy user of K and will tend to draw soil levels down where included in a crop rotation.  Another explanation is that more corn acres are being harvested for silage.  When this occurs, a significantly greater amount of K is removed from the field.  In fact, UW recommendations are to apply 80 additional pounds of K when corn is harvested as silage versus grain.

More corn growers are using liquid starter fertilizers. These fertilizers tend to be lower in K analysis than traditional dry starters. This is not a reason to quit using liquid starters, but it does point to the importance of broadcasting additional dry K as soil tests dictate.

There have also been situations where soil K levels are optimum or above and adequate amounts of K were applied but plant deficiency symptoms are present.  Again, there may be several explanations.  For proper uptake of K, oxygen must be present in the soil.  For this reason, K deficiency is common where severe soil compaction is a problem and may explain some of the increase in K-deficient plants.  Conversely, very dry and fluffy soil conditions may also limit soil K uptake.  As has been well documented, fields with a history of no-till crop production may develop stratified layers of crop nutrients (usually toward the top of the soil profile).  This may result in K being “positionally unavailable” to the crop even though adequate amounts were applied.

The good news in this story is that all of the previously mentioned conditions can be corrected.  Nevertheless, it once again points to the importance of a good soil testing program and knowing both crop nutrient needs as well as those factors that impact nutrient availability.