Over the last two weeks I have observed several fields and received numerous reports of potassium (K) deficiency symptoms developing in corn and soybean crops in many parts of the state. Many farmers have not seen this before in their fields and question if it really is K deficiency. Diagnosing crop nutrient deficiencies based solely on visual symptoms is not always easy, but K deficiency symptoms, shown in the photos here, are very distinct for corn and soybean. The symptoms start to develop in the older leaves with yellowing of the leaf margins. The yellowing normally starts in the leaf tip and extends to the base of the leaf along the edges. In more severe cases the leaf edges look dead (brown) while the new leaves remain green; this is because K present in older tissues is remobilized to supply K to the newer tissues.
Sometimes people confuse symptoms of K deficiency in corn with those of nitrogen deficiency. While nitrogen deficiency also occurs in older leaves, starting in the tip, yellowing develops along the midrib toward the leaf base, forming a V shape.
What is causing the deficiency? The symptoms showing up in so many fields reflect two general conditions: insufficient supply due to low soil-test K levels, and reduced K availability due to less-than-ideal soil conditions and slow root activity.
Some fields show K deficiency because the soil is K-deficient. A 2007-08 survey of nearly 550 randomly selected fields throughout Illinois indicated that approximately 45% tested low in K (below the critical level needed to maximize yield; details in Figure 2). When soils are below the critical level, there is a strong probability of yield reduction, even if the crop does not develop deficiency symptoms in the leaves. The fact that this year deficiency symptoms are developing in some fields due to low soil-test K levels exacerbated by dry conditions should be a warning signal to apply K this fall. The crops in most of these fields will likely not recover even after growing-season conditions improve.
In other fields showing K deficiency, the soil-test K levels are adequate for corn and soybean production, but environmental conditions are causing a temporary deficiency that should disappear soon after growing-season conditions improve. Potassium ions move in the soil solution by diffusion (from areas of high concentration near the soil particles to areas of low concentrations close to crop roots). The diffusion distance is very short, and where water is limited (soil pores have more air than water) the distance that K ions have to travel to reach the root becomes too far, as ions cannot diffuse through air. So under droughty conditions, as far as the crop is concerned it is as if the ions were not present.
Dry conditions also limit root growth and activity, which further reduces the capacity of the crop to take up K. Other factors that can reduce K availability by limiting roots include soils that are either too loose or compacted, root damage by disease or insect pruning, shallow seed-planting depth, and seed-furrow sidewall compaction that occurs when planting in wet soils (not a common problem this year).
What can be done about K deficiency? The best way to supply K is in the soil before planting is done. Similarly taking place before planting are management practices to avoid soil conditions that intensify the negative effect of dry weather on K availability, even in well-fertilized fields. There is thus little that can be done in an economically feasible way to correct a K deficiency for this year's crops.
Remember that if the deficiency is caused by factors other than low soil-test K, applying K fertilizer to the soil would not be profitable for this year's crop. Even in low-K soils, a liquid or dry application between crop rows is not a good option unless there is sufficient rain to move K into the root zone. A positive outcome, however, is that even if there is not sufficient rain to move K to the root zone, the application can begin to correct soil K levels for the next crop.
Another potential advantage to soil applications is that higher rates of fertilizer can be applied than with foliar applications. Both old and new research in midwestern states has shown that foliar K applications produce results too limited and inconsistent to be profitable in most years, especially given that deficiency tends to occur in isolated and small areas of the field. To my knowledge, none of the trials showing response to foliar applications occurred under droughty conditions. Still, if a foliar K application is deemed necessary, I recommend using fluid products that do not contain potassium chloride, potassium sulfate, or other ingredients with a high salt index so that higher K rates can be applied without concern for canopy injury. In general, for trials where there was a yield response to low-salt foliar K applications, the response was obtained without canopy damage at rates of 10 to 12 lb of K2O per acre or less.
The best action at this point is to look for clues that may help with determining how best to solve or minimize any future K deficiency. Take note of the general condition of the roots and the physical condition of the soil, and collect soil samples for K analysis from adjacent areas with seemingly normal growth and K-deficiency symptoms. If the deficiency is not extreme and is caused by dry conditions, the crop likely will recover with little or no yield reduction once there is sufficient rain. Most fields have areas that are more severely affected than others; it would be advisable to mark those areas with a handheld global positioning system (GPS) and go back after harvest to sample the soil and to target the area with variable rate applications if deemed necessary.