Source: Dr. John Sawyer, Iowa State Univ.
Yellowing of corn plants early in the season can be confusing to diagnose. And in some conditions there may be a period of time after corn emergence where small corn just does not look good. There can be a number of causes for plant yellowing. An example is the description in a recent ICM Blog by Alison Robertson (Anthracnose leaf blight prevalent in corn fields). Other reasons for yellow corn tissue are varied such as waterlogged soils, cold temperatures, herbicide issues, potassium deficiency (typically older leaf margins), etc. Two reasons that can be confusing due to similar plant symptoms are nitrogen (N) and sulfur (S) deficiency.
Nitrogen deficiency. Classic symptom description is yellowing of lower (older) leaves, from the leaf tip to the base down the midrib.
Sulfur deficiency. Classic symptom description is yellowing of new leaves (in the whorl, sometimes with interveinal striping), with lower (older) leaves remaining uniform green (Figure 1).
However, both N and S are tied together due to several common physiological process, therefore, early growth symptoms can be similar. Examples including overall leaf and plant yellowing, spindly plants, and interveinal striping. These similar symptoms most often occur when plants are small and there is severe deficiency (low soil supply and no fertilization). Also, plant response from fertilizer application can be quite similar for N and S, that is, good growth and green plants with uniform coloration (Figure 2). Sulfur does not move as readily in plants as N, so symptoms should differentiate on different plant parts. However, with young plants, early onset of symptoms, and with large and prolonged deficiency, such differentiation may not happen (Figures 2 and 3). One way to determine if an early season deficiency is N or S is to hand apply some S and N fertilizer to different areas and see if the plants green up.
