Is the Yellow Corn from Nutrient Deficiency?

Many corn fields are showing uneven growth and yellowing on the bottom leaves of the plant, and growers are wondering if a nutrient deficiency is the cause. Fields that were planted into a red clover or alfalfa plowdown do not seem to show these symptoms as often.

Both nitrogen and potassium deficiencies will cause yellowing and firing of the lower leaves of corn. A little detective work should sort out if either of these is the culprit, and what is causing the deficiency in the plant. It may not necessarily be a lack of fertilizer.

N Deficiency

Corn plants that are short of nitrogen will be pale coloured and stunted, with yellowing starting on the lower leaves and proceeding up the plant. As the leaves yellow, there will also be firing (necrosis) that extends from the tip of the leaf along the midrib.

Dry weather can reduce the uptake of nitrogen, particularly if the N was surface applied. Most of the N flows into the crop with the water taken up for transpiration, but if the fertilizer is in the top few inches of soil, and the water is coming from deeper in the subsoil, the plants will run short of N. Fields where there was a legume plowdown will tend to have better moisture holding capacity, plus the N mineralized from the legume roots will be deeper in the soil so less affected by dry conditions at the surface.

K Deficiency

Corn plants that are short of potassium will be pale coloured and stunted, with yellowing starting on the lower leaves and proceeding up the plant. As the leaves yellow, there will also be firing (necrosis) that extends from the tip of the leaf along the leaf margins. If you have trouble remembering which is which, remember that nitrogen is inside, while potassium is outside.

A large part of potassium uptake is by diffusion, so dry soil conditions reduce availability my making the water films thinner and more tortuous, so the K has to travel farther to reach the root. Add to this the restriction on root growth due to compaction or cloddy seedbeds, and the amount of K available to the plant is significantly decreased even if the supply in the soil should be adequate. In some fields, the improved soil structure following a forage or cover crop will increase root density and K uptake. There are cases, however, where a long-term forage without adequate fertilizer has depleted the soil and increased the K deficiency.

Figure 1. Nutrient Deficiency Symptoms and other problems that could be
confused for nutrient deficiency problems.