Troubleshooting “Yellow” Corn
In my travel across South Dakota and southern Minnesota recently, the corn crop is generally in the V5 – V6 stage. The crop is growing very rapidly and looking good! Most corn has a healthy dark green color, but a few fields have areas of pale green or yellowing corn. In southern MN, 2013 and 2014 had widespread yellow-stripped, pale corn, but dryer years like 2015 didn’t have much yellow corn. This year some agronomists and producers have called with questions about these yellow areas, thinking they are related to soil fertility. Soil and tissue analysis are good tools to use together to “Trouble Shoot” or “Diagnose” if corn is yellow because of a nutrient deficiency or some other reason.
Here is an example from a SW Minnesota corn field that is mostly dark green except for a couple yellow areas. I’ve included soil tests and tissue tests for this example. The corn was in the V5 stage when I did the sampling. The yellow corn was in slightly lower areas of the field that had wetter soils compared to the adjacent green corn on slightly higher ground. Yellow corn is usually diagnostic for nitrogen and or sulfur deficiency. While sulfur deficiency is supposed to show up first on the upper new leaves and nitrogen deficiency should show up first on the lower older leaves, symptoms in the field don’t always look obvious like they do in text books.
The soil moisture in this field was good, with the soil where the yellow corn was located being more saturated than the green corn area. This field is highly productive and as you can see from the soil test levels (see table), it is well fertilized for maximize yield. The tissue tests for the yellow corn indicate the nitrogen is far below the sufficiency range and the potassium is slightly below, but looking at the soil test levels, all nutrients are in the adequate range for this time of the season.
So, in this situation, with adequate levels of nutrients in the soil, the yellow corn is struggling to take up enough nitrogen and even potassium because of the wet soil conditions. The root growth of the yellow corn is slower due to the water logged conditions. This slower root growth is causing lower nutrient concentrations in the plants at this time. As the soil conditions dry out, the nutrient levels in the plant should recover with better root growth. There is still a concern for N losses to denitrification if the soil stays wet too long. Similar situations happened in the 2013 and 2014 wet growing seasons. When you use both tissue and soil analysis to do trouble shooting, sometimes you will see that the nutrient levels in the soil are adequate and it is another stress that is preventing the plant from getting the nutrients it needs. Conditions like, water logged soils and compaction can cause yellow corn and low nutrient levels in plant tissue inspite of the soil having adequate nutrient levels.