WHAT IS POTASSIUM?

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.

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).

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.

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.