• Symptoms of potassium deficiency appear on the lower portion of the plant first with chlorosis (yellowing) at the leaf margins followed by necrosis (death).
• Potassium deficiency can be caused by soil pH, extreme liming or calcium rich areas of fields, lack of soil oxygen or true soil deficiency.
• Foliar application of potassium can help in cases where deficiencies are caused by reasons other than true soil deficiency.
• Grid soil sampling and application of lime to build the soil pH and maintaining it at the 6.0-6.5 range remains the best way to manage deficient soils.

By the time crop nutrient deficiencies become visible, yield damage has already occurred to some degree. This doesn’t warrant ignoring it, but should signal us to investigate further as to the cause of the deficiency. Is it a true soil deficiency or a root or soil moisture related? Tissue sampling accompanied by soil samples is our only tool to detect nutrient deficiencies prior to any yield damage. Recently, potassium deficiency symptoms have been reported in some soybean fields in Illinois.

Even though potassium is immobile in the soil, except at pH levels of 5.8, it is mobile in plants to move from old tissue to new growth. For this reason deficiency symptoms occur first on older plant tissue. While symptomology from crop to crop varies slightly, symptoms always begin at the leaf margins with chlorosis (yellowing) followed by necrosis (death) of plant tissue. Potassium deficiency symptoms can be confused in alfalfa with boron deficiency symptoms; however boron is plant immobile so symptoms appear first on new plant growth and not old growth as with potassium.

Potassium deficiencies can develop in soils testing in the sufficient to high range, soil pH < 5.8, soils overloaded with Ca⁺² ions or in cases where soils are oxygen deprived. At pH < 5.8 H⁺ ions begin to occupy cation exchange sites (CE-sites), making it difficult for potassium to find places to attach. Fertilizer applied potassium or potassium released from fixed state in the soil cannot displace H⁺ ions held tightly to these CE-sites and travels deeper in the soil profile until finding CE-sites where it can reside. While low pH can cause potassium deficiencies, symptomology may not appear until pH is well below 5.0. A similar process occurs with Ca⁺² ions when the soil profile is over loaded by extreme over liming or in fields located next to gravel roads where lime dust settles on a field. Calcium can displace potassium from the CEC leaving it free in soil solution to leach down thru the soil profile. Soil oxygen content also affects potassium uptake. When soil oxygen becomes low due to soil compaction or extreme soil moisture root respiration is decreased, hampering soil K uptake.

Correcting in season crop deficiencies when soils are truly low in potassium is difficult. Soil application of potassium in season is not usually an option unless liquid or dry potassium can be banded close to the plant roots. Broadcast potassium is not a viable option due to potassium only moving 1-2 cm. per season from the point of application. With broadcast potassium time is needed for the product to diffuse into the root zone. Foliar fertilizers containing potassium may be of help depending on the severity and underlying cause of the deficiency. Temporal deficiencies due to soil environmental conditions and moisture stress stand the best chance of success from foliar fertilization due to a low doseof potassium contained in foliar products.

Potassium in the plant serves many purposes. Among these purposes is internal water pressure regulation (turgor pressure). With regards to turgor pressure, potassium regulates guard cells and opening and closing of stomata which dictates plant transpiration. Potassium also acts to preserve crops standability in later stages of development, aiding in warding off plant diseases. While foliar products may help alleviate the plants temporal potassium deficiencies, under true soil deficient conditions, grid sampling needs to be applied.

All images- Visual symptomology of potassium deficiency varies from crop to crop as seen in the images in this article.  (Courtesy of the International Plant Nutrient Institutes Crop Nutrient Deficiency Image Collection)

Image 1: Claudinei Kappes IPNI Library
Image 2: C. R. Crozier IPNI Library
Image 3: T. L. Roberts IPNI Library