Some farmers have reported corn and soybean showing symptoms of potassium (K) deficiency this year. During a prolonged drought, it is not uncommon for corn and soybean to show symptoms of K deficiency (Photos 1 and 2). This was a common occurrence during the 2012 drought. K is highly mobile in the plant. Consequently, K deficiency symptoms occur first on the lower leaves and progress toward the top as the severity of the deficiency increases. One of the most common signs of K deficiency is yellowing along leaf margins followed by scorching and dieback. In most severe cases the necrotic leaves may be shed. The K deficiencies will most likely appear on low soil test K soils or under prolonged dry top soil conditions. K moves to the roots by diffusion in the soil solution. Therefore, prolonged lack of soil moisture can drastically reduce K uptake.
Although K, unlike nitrogen and phosphorus, does not become a part of the chemical structure of plants, many plant physiologists consider K as second only to nitrogen in importance for plant growth. This is because K “activates” as many as 60 enzymatic and plant hormonal reactions, regulating many physiological and biochemical processes. Plants depend on K to regulate the opening and closing of leaf stomates, the pores that allow gases and water vapor to pass through the leaf.
When there is adequate K in the plant, K moves into the guard cells surrounding the stomate, which takes up water (Photo 3). These guard cells become turgid and swell. As the inner walls of the guard cells are thicker than the outer walls, the pores begin to open. This allows free exchange of gases and water. When the water supply becomes short, K is pumped out of guard cells and the pores are closed tightly, conserving water. This minimizes drought stress.
However, if K supplies in the plant are inadequate, the stomate becomes slow to respond and may take hours, rather than minutes, to close. As a result, plants become much more susceptible to drought.
For crops showing deficiency symptoms this year, there are no certain economically effective rescue treatments. Crops take up most of their K requirements during early vegetative stages. The best preventive treatment is to soil test and apply K as recommended before or at planting of the next crop. Also, try and avoid induced deficiency situations where root growth may be restricted, such as soil compaction and pest damage. If only certain areas of the field are affected, those areas should be recorded, post-harvest soil tested and treated. Fertilizer applicators with GPS and variable rate technologies would find it feasible to spot treat these areas for the benefit of the next crop.
Source: George Silva, Michigan State University
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