Crop Nutrition

Potassium Deficiency in Soybeans

eKonomics News Team

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Prolonged dry weather conditions bring the potential for potassium deficiency, especially in soybeans that require a lot of potassium and remove up to 1.4 lb. per bushel.

Potassium is vital to plant health as it regulates the opening and closing of pores (leaf stomata) on the plant’s surfaces that allow gases and water vapor to pass through. Without enough potassium, these pores are slower to respond to environmental conditions and may take longer to close meaning poorer regulation of water potential and greater susceptibility to drought.

Potassium depends on adequate soil water to move from high concentration on soil surfaces to areas of low concentration near plant roots. When the soil is dry, the diffusion rate of potassium decreases to a point that deficiency symptoms may appear. Dry conditions also limit root growth so there is less capacity for the plant to take up potassium.

Recognizing the symptoms

One of the key nutrients for plant health, potassium moves very easily throughout the plant and signs of deficiency show up first on lower leaves, and progress towards the top of the plant as deficiency increases. Deficiency can be seen from the V3 stage up to the more advanced vegetative stages in older leaves. The most common sign of potassium deficiency in soybeans appears as yellowing along the leaf margin. In more severe cases, deficiency symptoms can progress to the upper leaves of the plant, where leaves become scorched and die back, and ultimately drop off the plant.

Visual symptoms indicative of potassium deficiency may be associated with low soil test potassium and prolonged dry soil conditions. Learn more about how potassium moves through the soil and plant from the Potassium Cycle.

The problem with low potassium

Across parts of the US we are seeing lower potassium levels in soybeans for a few different reasons. As yields increase, crops are requiring a higher level of nutrient replacement, including potassium. Growers who have relied on a traditional fertilizer strategy may also be missing out on potassium – especially in areas that have had natively high potassium levels in the past, leading growers to pay less attention to this part of their nutrient management plan. Managing potential potassium deficiencies is important to ensure growers aren’t missing out on soybean yield potential.

Complicating factors

Potassium deficiency can be confused with soybean cyst nematode (SCN) damage. Fields with high SCN populations and low potassium conditions can exhibit intensified SCN symptoms. And potassium deficiency is difficult to control in a growing crop, so prevention is the best management practice.

Soybeans growing with less-than-optimum potassium levels have also shown an increase in soybean aphid pressure. Testing in Wisconsin and Michigan fields found low soil potassium was associated with increased aphid populations at the low end of the range of soil potassium in soybean fields. While these results don’t imply that adequate potassium is a reliable control strategy for aphids, it’s another reason soybean growers should manage soil potassium levels as part of an integrated pest management strategy, and be extra vigilant with scouting for aphids in fields that have low potassium levels.

Sources

  1. Drought and potassium deficiency in corn and soybeans, George Silva, Michigan State University Extension, July 26, 2018
  2. Soybean potassium deficiency symptoms during early and late growing stages, ICM News, August 21, 2018
  3. How Potassium Nutrition Can Suppress Soybean Aphids, Tom Bruulsema, Christina DiFonzo, and Claudio Gratton, Better Crops, Vol 94, 2010, No. 2
  4. Are we really seeing potassium deficient corn and soybean? Illinois Acre, June 20, 2012