With the wet weather we have had this year, it is likely that some cornfields will start to show "firing" or yellowing on the bottom of corn plants.

Agronomists indicate two most common nutritional causes of lower leaf firing are nitrogen and potassium deficiencies. While both of these show up on the lower leaves, it is very easy to tell them apart.

Nitrogen deficiency will start as yellowing at the tip of the leaf and then go back the middle of the leaf following the midrib, forming a V-shape. Potassium will also start at the tip of the leaf but the yellowing will go down the outside edge of the leaf. The symptoms will start at the bottom of the plant and go up as the deficiency becomes worse. These nutrients are mobile and the plant will strip them out of the older leaves and move them to the new growth. Thus, the higher up on the plant symptoms are observed, the worse the deficiency. Eventually the yellowing may cover the entire leaf and the leaf my die.

There are several possible causes for these deficiencies. Most obvious is the lack of nitrogen or potassium in the soil. For nitrogen, either not enough was applied or a significant amount may have been lost after application, by leaching and denitrification because of the wet weather pattern.

For potassium, look at the soil test. If the soil test is low, then this is likely the cause of the deficiency. If the soil test is not very recent and potassium levels were marginal, they could be critically low now, especially following a good crop year like we had last year and/or if high potassium consuming crops like a hay crop or soybeans preceded the corn. Incomplete starter fertilizers may be a factor. There is not much you can do at this point. Just be sure to apply sufficient potassium as fertilizer or manure before the next crop.

However, potassium deficiency can show up even with optimum or high soil test levels. This is usually because the plant is not being able to take up the potassium that is in the soil. Anything that limits root growth can cause this; compaction, low pH, root injury by herbicides, and root feeding insects.

Compaction is a very common cause of this problem, especially sidewall compaction resulting from wet conditions at planting. Again, there is not much that can be done about this at this time, but you need to plan on minimizing the problem in the future.

For less obvious symptoms, plant analysis is an excellent tool for diagnosing these problems. Be sure to follow the sampling recommendations from the lab. A good way to determine the problem using plant analysis is to take a sample from the problem area and a similar sample from a nearby normal area and compare the results.