Richard Taylor, Extension Agronomist; rtaylor@udel.edu
This past week while driving up and down the state, I saw a number of corn fields with yellow, sometimes slightly stunted plants. I also visited several growers concerned about the same set of symptoms (see photos 1 and 2 below).
Photo 1. Leaf interveinal chlorosis in field corn
(photo compliments of Flip Wright, Southern States, Inc.)
Photo 2. Leaf interveinal chlorosis on newest leaves in field corn
(photo compliments of Flip Wright, Southern States, Inc.)
Symptoms such as those in the photos above seem to be appearing more frequently in recent years. Symptoms including interveinal chlorosis on the newer leaves have traditionally been associated with micronutrient (manganese or iron) deficiency. The metal micronutrients are immobile in plants suggesting that the symptoms should be most visible on the newest leaves. Another essential element, magnesium, also produces interveinal chlorosis, although magnesium is mobile in the plant so that symptoms appear on the older leaves rather than the newer leaves.
A couple of years ago when this complex of symptoms began to appear frequently, I tried applying the metal micronutrients either alone or in combination to discover if the symptoms were indeed caused by one of the metal micronutrients. I applied manganese sulfate since this is the micronutrient most frequently seen as deficient in our area, iron sulfate, copper sulfate, zinc sulfate, and a combination product containing all these micronutrients. In no case did any of the micronutrients alleviate the symptoms that were seen on the corn.
Dr. Greg Binford, who at that time was our Nutrient Management Specialist, also tried a couple of sulfur products including ammonium sulfate at a couple of sites in Delaware. Dr. Binford did find some response to ammonium sulfate and actually got a yield increase with sulfur addition.
The symptoms that we have been seeing also fit with sulfur deficiency and the combination of a lack of response to micronutrients and the response the Dr. Binford found with sulfur indicate that this may be the essential element responsible for the symptoms we are observing. The small quantities of sulfur applied as manganese sulfate, iron sulfate, etc. apparently were inadequate to alleviate the symptoms we observed. However, even when sulfur is applied at rates adequate to overcome the symptoms, yield response can be lacking, as noted in previous studies several area researchers have reported. Much of the sulfur in Delaware soils is held in deeper soil layers and will be available to the plants as the soils warm up and rooting depth increases, giving plants access to this soil held sulfur.
A way to confirm that the symptoms are related to sulfur availability is to conduct a tissue test. However, tissue testing will not tell you if there will be an economic yield response to an application of sulfur. Our traditional 0-8 inch deep soil sample also does not help to assess the chance of an economic yield response to sulfur fertilization since sulfur is held deeper than that in our soils.
If you try tissue sampling, you should take a 15 to 20 plant sample from both an affected area and from a non-affected area if the plants have less than four leaf collars visible. Cut the plants off about a half inch above the soil surface to avoid soil contamination and lightly wash possible soil off the leaves with distilled water (usually available from a grocery store). Plants at four or more leaf collars should be sampled by pulling the youngest mature leaf—the upper most leaf with the leaf collar visible). Sample 15 to 20 plants from both the affected area and a nearby unaffected area. Be careful rising leaves or plants since this can affect nutrient concentration in the leaves, especially potassium. Dry the leaves or plants and ship to testing laboratories according to their suggested procedures.
When you get the laboratory results, you should not only look at the tissue sulfur concentration (less than 0.12% is considered deficient) but also look at the ratio of nitrogen to sulfur. When this ratio is greater than 20:1, sulfur deficiency is likely. Adequate sulfur is present in the plant when the tissue sulfur level is greater than 0.20% and the N:S ratio is less than 12:1. In between deficient and adequate, other factors such as cold soil, soil type (sandy spots in a field), soil compaction layers, inadequate sunlight restricting root growth can cause the plants in an area to show deficiency symptoms.
As mentioned earlier, the decision to spend the money to apply sulfur is a difficult call to make. In many cases, the response to fertilizer sulfur will not be enough to pay for the fertilizer and trip across the field. Perhaps one way to evaluate the choice is to look at the tissue test results. If plants are very deficient (<0.12% S and a N:S ratio >20:1), an application of a sulfur containing fertilizer such as ammonium sulfate at 100 to 150 lbs of 21-0-0-24S per acre can help overcome the symptoms and may increase yields enough to be economical. If the tissue test is in the in-between category, fertilization with sulfur can help you, the grower, feel better but in my experience will be less likely to pay for itself with a yield increase. The most likely fields to respond to sulfur will be coarse textured (sandy) soils that are heavily irrigated and heavily fertilized with nitrogen.