Potassium Efficiency

OVERVIEW

Potassium is a primary plant nutrient that plays a major role in achieving the maximum economic yields from our production acres.  Management of Potassium, and other essential nutrients, is part of a balanced fertility program.

POTASSIUM REQUIREMENTS

Plants absorb Potassium in greater amounts than any other essential nutrient except Nitrogen in most crops.  In order for a crop to achieve its maximum yield potential, Potassium is needed in large quantities.  Please reference the table below for specified crop Potassium requirements.


Potassium Requirements of Several Crops

Corn Soybeans Wheat Alfalfa
   
    bu/A     
    Yield  

     K2O
  Required  
    bu/A     
    Yield 
     K2O
  Required  
    bu/A     
    Yield 
     K2O
  Required  
   tons/A   
    Yield
     K2O
  Required  
150 199 40 97 50 90 5 250
175 233 55 134 75 135 7.5 375
200 266 70 170 100 180 10 500
*Approximations  (Corn, soybeans, and wheat requirements are based on grain and stover potassium requirements.)
 


AVAILABILITY

Potassium is relatively immobile in the soil so availability depends on several factors.  No-till and minimum tillage methods, as well as with compaction can limit soil aeration limiting root growth.  Soil temperature and the pH of the soil can also limit availability.  Optimum availability of Potassium is at a soil pH of 6.0 – 7.0.  Soil moisture is essential for Potassium to move in the soil, so in dry years Potassium can be limiting.

Soils that have high clay content can retain high levels of Potassium reserves.  Availability of the Potassium depends on the type of clay and several other factors.  


FORMS OF POTASSIUM TAKEN UP BY THE PLANT

The plants absorb the ionic form of Potassium (K+) from the soil.  Plant roots will only come in contact with approximately 2% of the soil area.  This is very critical in Potassium management.  Once the plant is growing, the Potassium ion will only move a limited distance through the soil solution by diffusion during the growing season.


FUNCTIONS IN THE PLANT

Potassium is one of the greatest investments in protecting a crop against disease.  It has the ability to strengthen stalks and stems against disease, protecting the plant from lodging.  It has the ability to make plant cells thicker, making it more difficult for certain diseases to invade the plant after a heavy rain or other stressful conditions.  In alfalfa and other forage crops, it is essential in providing optimum nutrition for winter hardiness.  In soybeans Potassium has proved to increase seed and grain quality by reducing the number of infected and shriveled seeds.

Potassium has this impact because it is involved in over 60 enzyme systems that regulate plant growth reactions.  One of these major roles is in regulating water use efficiency in the plant.  The opening and closing of the stomates in the leaves is directly related to the concentration of Potassium in the cells that surround the stomates.  If the plant has a shortage of Potassium, the stomates will only open partially and be slower in closing. 

Potassium is also vital in photosynthesis.  If a plant is deficient in Potassium, photosynthesis will decline.  In turn, the plants respiration rate increases.  This causes the plant’s carbohydrate supply to decrease.  Carbohydrates provide energy for plant growth when they are broken down, which Potassium plays a key role in.

Potassium deficiency can show up in several ways and at different stages of plant growth.  The most common is the firing of leaf margins in grasses.  Typically, this will appear on older leaves because of the mobility of Potassium in the plant.  In mid to late season, high yielding cotton can show deficiency on younger leaves if there is not enough Potassium to meet the high growth demands. 

In alfalfa a Potassium deficiency will start as small white or yellow dots on the leaf margins.  Then they will continue to develop until the outer leaf margin is entirely white or yellow.

If a crop is deficient in Potassium, it will grow slower with poorly developed root systems and weaker stalks or stems.  Seed and fruit will be smaller in size and shriveled.


FERTILIZER PLACEMENT

With the high demand for Potassium throughout the growing season and especially in the early growing season, placement in the root zone is extremely important.  Placement on the seed is recommended for optimum results.  This provides the Potassium right where the growing seedling can utilize it to promote early season plant growth.   

Foliar feeding of Potassium is practical and efficient as the crop continues to grow.  Potassium is absorbed into the plant in 10-24 hours and will translocate throughout the plant.  This makes foliar feeding of Potassium a logical decision when you consider the impact it has on the growing crop.  With a true solution liquid formulation of Potassium, it may be combined with post-emergent herbicides and insecticides.  (Consult your fertilizer representative for specific recommendations.)


Potassium is an essential nutrient that cannot be overlooked.  It has a greater impact on yield than most growers realize, especially when you consider the effect it has on stalk and stem quality, regulating water use in the plant, disease management, and ultimately yield.