There have been numerous reports of yellowing of the older (lower) corn leaves in central and northern Illinois, especially in the western areas. The symptoms described are characteristic of potassium (K) deficiency: yellowing begins at the leaf tip and proceeds along the leaf margin of the older leaves, with some chlorosis of the in-terior of the leaves, and newer leaves remain green. The fact that the newer leaves are green does not necessarily mean that the plant is growing out of the problem but rather that the plant is translocating the potassium from the older leaves to the newer ones.
Greg Jones, with Pioneer Hi-Bred near Galesburg, indicates that these symptoms are appearing in some fields where soil conditions have been relatively good. Eric Adee, at Monmouth, also reported that these symptoms were associated with grape colaspis injury, which caused uneven growth and K deficiency symptoms on the smaller plants. It's thus possible that there may be multiple causes of this root-related problem and that K deficiency symptoms are just the "indicator."
This problem was widespread in 2000, in about the same area of western-northwestern Illinois, roughly west of Peoria and south of the Quad Cities, and at about the same time and crop-growth stage. Fortunately, a good rain in early July appeared to alleviate the problem in 2000, and the affected fields produced good corn yields. That is not to say that the same thing will happen this year, but we can hope.
Causes of the Problem
Poor root system. The root system seems to have not developed rapidly enough to allow for adequate K uptake to meet the needs of the vegetation. There are several possible reasons for the slow root development, including cool and wet or cool and dry soils, compaction, nematodes, insects, herbicide injury, and fertilizer burn. Based on reports that we have received, it appears that the most likely cause this year is related to the cool soils experienced in May and much of early June. If that is the case, the root system should start to expand rapidly and overcome the problem.
Low soil K. Over the past several years, yields have been high, and thus crop removal of K has been higher than normal. If producers did not adjust their fertilizer application accordingly, it is possible that the soil supply has been depleted to the point where the soil is not capable of supplying an adequate quantity. Soil testing will confirm whether this is the problem. If you are seeing streaks through the field of good and deficient plants, check to see whether the good plants line up with the center of last year's harvest area. If they do, it is possible that the residue left behind the combine, particularly if it did not have a straw spreader, could have provided enough K to meet the needs of the crop. Remember that K is soluble in plant tissue and will be leached out into the soil with the first rains on the dead tissue.
Tillage. In 2000, this problem seemed to be worse on fields that had a history of long-term reduced or no-tillage. Under those tillage systems, K and other immobile nutrients will remain near the surface of the soil, and if it is dry near the surface but moist underneath, the roots will be most active in the moist zone, which is also where K may be low.
Oxygen availability. Potassium uptake may be inhibited by low soil oxygen levels, a problem that can be caused by compaction and/or excessively wet soils. Most of the soils in the affected area received a lot of rainfall in May but have not been excessively wet for long periods after the crops emerged.
Solution to the Problem
It is too late to correct the problem this year. Attempts to use foliar and or sidedress potassium have not been successful.
If the field in question has not been soil tested for several years, make sure that you have it tested this fall to ascertain the potassium status and to correct it if the level is low. Other practices that will provide better conditions for root growth may also be helpful, but as previously indicated, this problem may not have the same cause in all fields.--Robert G. Hoeft and Emerson D. Nafziger