Are we really seeing K-deficient corn, soybean?
click image to zoomUniversity of Illinois Plant ClinicPotassium deficiency symptoms in corn develop on the edges of older leaves, typically starting at the tip, while the new growth appears normal. 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.
click image to zoomPotassium deficiency symptoms in soybean develop on the edges of older trifoliates, while the new growth appears normal. 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.
click image to zoomFigure 2. Distribution of soil potassium concentrations for nearly 550 randomly selected fields throughout Illinois during a survey of soil fertility conducted during the falls of 2007 and 2008. Results are divided among the state’s potassium-supplying power regions, based on cation exchange capacity (CEC); sandy soils are also low-CEC soils. Within a region, different colors represent fields testing in three categories: below the recommended critical level (CL), above the level at which no additional fertilization is recommended (no appl. point), and at maintenance levels (between CL and no appl. point). The number of fields and percentage of surveyed fields for each category within each region are also presented. 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.