Dramatic Potassium Deficiencies are Appearing in Soybean Fields

Richard Taylor, Extension Agronomist;rtaylor@udel.edu

Potassium deficiency symptoms occur when plants cannot extract adequate K from the surface soil. Probably the majority of the soybean root system is located in the top four inches of soil where the plant obtains much of its needed water. Although inadequate K uptake can occur due to root restricting problems such as compaction, the most likely cause of an inadequate uptake rate for K is when the soil test K level is below optimum. When growers have used intensive grid sampling for soil nutrient evaluation, they have almost always found that there is a huge variability in soil test K levels. This variation plus inadequate K fertilization in the past few years when the price of K fertilizer was very high has resulted in some of the problems we’ve been observing this year.

Legumes such as soybean use some potassium (K) as young seedlings but dramatically increase uptake during rapid vegetative growth just prior to flowering and during reproductive growth. Uptake peaks just before flowering begins. Potassium taken up by the vegetative mass is used to supply the large amount of K transferred to the seed during seed development. As much as 60% of total plant K found in the vegetative tissue is transferred to the developing seed. On a weight basis, this results in the soybean seed containing more than twice the K of corn seed. For this reason, deficiency symptoms often are observed during the period from late flowering (late R2) to early seed fill (R5—beginning seed to R6—full seed). A deficit of K during the late vegetative stage or during the reproductive stages will be reflected in the yield of the soybean crop.

In the past two weeks, a half dozen or more fields have come to my attention in which severe K deficiency symptoms have appeared (Photos 1 and 2). Most of the severely affected fields have been located in the northern portion of the Delmarva Peninsula. The size of the affected area within a field has been increasing the last couple of years according to the individuals scouting the fields.

Potassium and phosphorus (P) are two nutrients that are taken up by plants primarily through the process of diffusion rather than the process of mass flow [nitrogen (N), sulfur (S), the other macronutrients and almost all the micronutrients]. The slow rate of diffusion and limited distance that K and P can move in soils means that these nutrients aren’t as easily available to crop plants as N. Availability is more influenced by nutrient placement than for soil mobile nutrients such as N and S. Typically for K, deficiency symptoms appear first on the oldest leaves since K is mobile in plants and can be quickly moved by the plant from old leaves to the growing point (younger leaves).

Since K is taken up by diffusion, symptoms often occur after a dry spell and usually first in no-till fields where all fertilizer is surface applied. If rainfall resumes or returns to normal, many of the affected plants will recover and the severity of the symptoms will lessen or go away entirely (Photo 2). Where just areas of a field are affected, the size of the area will often decrease following the resumption of rainfall.

Photo 1. Soybean field at the R3 growth stage showing severe K deficiency.

Photo 2. Soybean field at the R3 growth stage showing K deficiency lessened slightly following recent rainfall.

Potassium deficiency shows up initially as general yellowing (chlorosis) around the margins of individual leaflets on the older leaves (Photo 3). As the deficiency worsens, the marginal yellowing can change into marginal burning or necrosis on the leaf edges and the entire set of symptoms gradually moves up the plant to younger and younger leaves.

Photo 3. Soybean K deficiency begins as marginal yellowing on the leaflets but as the deficiency worsens a marginal burn or necrosis develops.

Photo 4. Marginal burning of the leaflet edges on soybean due to K deficiency.

Potassium is critical to the soybean plant since it is used to regulate water use in the plant and control transpiration by the plant. Transpiration is the cooling mechanism in plants but also has a major influence on nutrient uptake for all nutrients other than phosphorus and potassium. In addition, potassium is important since it is needed for the movement of sugars and this is critical during the seed fill period. Potassium also is essential in helping plants withstand all kinds of stress problems ranging from disease pressure to insect feeding to heat and drought tolerance. For a following corn crop, inadequate K can lead to stalk strength and quality concerns especially if high rates of nitrogen are applied to the corn crop. Late season wind from thunderstorms or a hurricane can cause severe lodging in K deficient fields.

In one field, a green stripe was observed that showed regular changes in the width of the stripe (Photo 5). Broadcast fertilizer had been applied to this field on a previous crop and the pattern possibly indicated that some unevenness occurred during spreading. In the past, we’ve observed even very small changes in soil test K level when it was near the critical level could dramatically affect the growth and appearance of the soybean crop.

Photo 5. Potassium deficiency in a soybean production field possibly showing the impact of a fertilizer spread pattern [note the greener stripe on the right side of the photo that expands and contracts across the field in a regular (man-made) pattern].

If the problem areas are the result of low soil test K levels, growers or consultants should make note of the areas affected and mark them and a border around them for extra K fertilizer in the future. Another option is to begin intensive soil mapping in the field to allow the grower or fertilizer dealer to use variable rate applications of K fertilizer. If the problem is the result of compaction, root pruning, seed furrow sidewall compaction, or soil test K stratification, the grower will need to consider management techniques to eliminate the particular problem that is limiting root growth or K uptake by the crop.