Potassium chemical symbol (K) is one of 17 essential nutrients required for plant growth and reproduction. It is classified as a macronutrient, as are nitrogen (N) and phosphorus (P). Potash is defined as K2O and is used to express the content of various fertilizer materials containing potassium, such as muriate of potash (KCI), sulfate of potash (K2SO4), double sulfate of potash and magnesium (K2SO4 2MgSO4), and nitrate of potash (KNO3).

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Potassium in Plants

Potassium is essential in nearly all processes needed to sustain plant growth and reproduction. Plants deficient in potassium are less resistant to drought, excess water, and high and low temperatures. They are also less resistant to pests, diseases and nematode attacks. Because potassium improves the overall health of growing plants and helps them fight against disease, it is known as the "quality" nutrient. Potassium affects quality factors such as size, shape, color and vigor of the seed or grain, and improves the fiber quality of cotton.

Potassium increases crop yields because it:

  • Increases root growth and improves drought tolerance
  • Builds cellulose and reduces lodging
  • Activates at least 60 enzymes involved in growth
  • Aids in photosynthesis and food formation
  • Helps translocate sugars and starches
  • Produces grains rich in starch
  • Increases protein content of plants
  • Maintains turgor, reduces water loss and wilting
  • Helps retard the spread of crop diseases and nematodes.

Potassium Uptake by Crops

Time of potassium uptake varies with different plants. However, plants generally absorb the majority of their potassium at an earlier growth stage than they do nitrogen and phosphorus.

Experiments on potassium uptake by corn showed that 70 to 80 percent was absorbed by silking time, and 100 percent was absorbed three to four weeks after silking. Translocation of potassium from the leaves and stems to the grain was much less than for phosphorus and nitrogen. The period during grain formation is apparently not a critical one for supply of potassium.

Crop Yield Uptake (K2O)
Alfalfa 10 lb/ton 600 lb/acre
Corn 200 bu/acre 266 lb/acre
Cotton (Lint) 1,500 lb/acre 210 lb/acre
Grain Sorghum 800 lb/acre 240 lb/acre
Peanuts 4,000 lb/acre 185 lb/acre
Soybeans 60 bu/acre 205 lb/acre
Wheat 80 bu/acre 162 lb/acre
Source: IPNI
Note: Potassium content of fertilizers is expressed as K2O, although there is no such compound in fertilizers, nor is it absorbed by or found in the plant in that form. Soil and plant tissue analysis values are usually expressed in terms of percent potassium (K), but fertilizer recommendations are expressed as K2O. To convert from K to K2O, multiply K by 1.2. To convert from K2O to K, multiply K2O by a factor of 0.83.

Potassium Removal by Crops

Nutrient uptake or utilization is an important consideration, but crops take up far more potassium than they remove with the harvested portion. For example, a 200 bu/acre corn crop takes up or utilizes about 266 lb/acre of potash. But when the corn is harvested as grain, only 0.25 lb/bu is removed, or 50 lb/acre K2O is harvested and removed from the field. However, if the crop were harvested as silage, then 7.3 lb/ton K2O are vested and removed from the field. Therefore, a 32-ton/acre silage crop would remove 234 lb/acre K2O. Harvest management is the major consideration in developing a potash fertilization program. Crops harvested in which the whole plant is removed from the field, like alfalfa hay, must have more potash applied than crops where only grain, lint or fruit are removed. Often with hay and silage crops, removal is an excellent guide for planning the potash fertilization program. With other crops, such as grain, soil tests offer the best guide.

Crop Removal (K2O)
Alfalfa 49.0 lb/ton
Corn Grain 0.25 lb/bu
Corn Silage 7.30 lb/ton
Coflon (Lint) 19.0 lb/bale
Grain Sorghum 0.27 lb/bu
Peanuts (Nuts) 17.0 lb/ton
Rice Grain 0.16 lb/bu
Soybean Grain 1.18 lb/bu
Sugarcane 3.50 lb/ton
Fescue (DM) 54.0 lb/ton
Spring Wheat Grain 0.33 lb/bu
Winter Wheat Grain 0.29 lb/bu
Source: IPNI

Potassium Deficiency Symptoms

Potassium is a highly mobile element in the plant and is translocated from the older to younger tissue. Consequently, potassium deficiency symptoms usually occur first on the lower leaves of the plant, and progress toward the top as the severity of the deficiency increases. One of the most common signs of potassium deficiency is the yellow scorching, or firing (chlorosis), along the leaf margin. In severe cases, the fired margin of the leaf may fall out. However, with broadleaf crops, such as soybeans and cotton, the entire leaf may shed, resulting in premature defoliation of the crop.

Potassium-deficient crops grow slowly and have poorly developed root systems. Stalks are weak, and lodging of cereal crops such as corn and small grain is common. Legumes are not strong competitors for soil potassium and are often crowded out by grasses in a grass-legume pasture. When potassium is not sufficient, winter killing of perennial crops such as alfalfa and grasses can occur.

For more information on potassium deficiency, click here.

All photos are provided courtesy of the International Plant Nutrition Institute (IPNI) and its IPNI Crop Nutrient Deficiency Image Collection. The photos above are a sample of a greater collection, which provides a comprehensive sampling of hundreds of classic cases of crop deficiency from research plots and farm fields located around the world. For access to the full collection, you can visit IPNI's website.

Potassium in Soil

Relatively Unavailable Potassium

From 90 to 98 percent of the total potassium present in soils is found in insoluble primary minerals that are resistant to chemical breakdown. They release potassium slowly, but in small quantities compared to total needs of growing crops.

Slowly Available Potassium

This form makes up 1–10 percent of the total potassium supply, and may originate from dissolved primary minerals or from potassium fertilizers. This potassium is attracted to the surface of clay minerals, where it may be firmly bound or fixed between the clay layers in a form slowly available to plants. The actual amount available depends on the type and amount of clay present.

Readily Available Potassium

Readily available forms of potassium make up only 0.1 to 2 percent of the total potassium in the soil, and consist of potassium dissolved in the soil solution and held on the exchange positions of the clay and organic matter. This potassium is "exchangeable" because it can be replaced by other positively charged ions (cations) such as hydrogen, calcium and magnesium. This exchange happens rapidly and frequently. The potassium in the soil solution may be taken up by the plant or lost from the soil by leaching, especially on sandy, coarse-textured soils.

Potassium and Balanced Crop Nutrition

Adequate supplies of other plant nutrients are required to obtain maximum responses to potassium fertilizer; however, there are several unique relations between potassium and other nutrients.

High-potassium fertilization can decrease the availability of magnesium to the plant, and may result in magnesium deficiency of crops grown on soils that are already low in magnesium. This problem is often encountered with crops grown on sandy soils, particularly in the coastal plain soils of the southern United States. Conversely, crops grown on soils high in magnesium can suffer potassium deficiency, especially if the soils are high phosphorus and low in potassium. This problem is especially severe in the soils of the Mississippi River flood plain.

Leaching of potassium in acidic, sandy soils may be reduced by liming the soil to a pH of 6.2 to 6.5; however, applications of high rates of limestone to a soil low in potassium may induce potassium deficiency of crops growing on those soils. This problem occurs more on soils with predominantly 2:1 type clays (such as montmorillonite clays) rather than the 1:1 type (such as kaolinitic clays).

Percent of soil samples that tested below critical levels for K for major crops in 2010. Source: IPNI

Potassium Fertilizers

Elemental potassium (K) is not found in a pure state in nature because of its high reactivity. It can be purified, but must be kept in oil to retain its purity and prevent violent reactivity. Potash deposits occur as beds of solid salts beneath the earth's surface and brines in dying lakes and seas.

Various Potassium Fertilizer Materials and their Percent Nutrient Content
Material Chemical Formula N P2O5 K2O S MG
Potassium Chloride KCl     60-62    
Potassium Sulfate K2SO4     50-52 18  
Potassium Magnesium Sulfate K2SO42MgSO4     22 22 11
Potassium Nitrate KNO3 13   44    
Potassium Sodium Nitrate KNa(NO3)2 15   14    
Potassium Hydroxide KOH     83    
Potassium Carbonate K2CO3KHCO3     <68    
Potassium Orthophosphate KH2PO4K2HPO4   30-60 30-50    
Potassium Polyphosphate K4P2O7   40-60 22-48    
Potassium Metaphosphate KPO3   55-57 38    

Placement of Potassium Fertilizers

Placement

The common potassium fertilizers are completely water soluble and, in some cases, have a high salt index. Consequently, when placed too close to seed or transplants, they can decrease seed germination and plant survival. This fertilizer injury is most severe on sandy soils, under dry conditions and with high fertilizer rates — especially nitrogen and potassium. Some crops such as soybeans, cotton and peanuts are much more sensitive to fertilizer injury than corn. Placement of the fertilizer in a band approximately 3 inches to the side and 2 inches below the seed is an effective method of preventing fertilizer injury. Band placement of potassium fertilizer is generally more efficient than broadcast application when the rate of application is low or soil levels of potassium are low.

Broadcast

Broadcast application of potassium under minimum tillage results in much of the applied potassium remaining in the top 1 to 2 inches of the soil; whereas, with conventional tillage, it is distributed throughout the plow layer. Corn usually absorbs sufficient potassium under no-till due to its extensive root system in the surface layer of the soil. Leaf analysis of corn shows lower potassium content under minimum tillage than with conventional tillage due to either the location of the applied potassium or to poorer aeration. Sufficient potassium can be supplied by using a higher rate of potassium fertilization with no-till systems.

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