Potassium Sulfate - A potash fertilizer with many benefits

Soluble potassium sulfate

Potassium fertilizer is commonly added to improve the yield and quality of plants growing in soils that are lacking an adequate supply of this essential nutrient. Most fertilizer K comes from ancient salt deposits located throughout the world. The word “potash” is a general term that most frequently refers to potassium chloride (KCl), but it also applies to all other K-containing fertilizers, such as potassium sulfate (K₂SO₄, commonly referred to as sulfate of potash or SOP).

Chemical formula; K2SO4

 Production

Potassium sulfate, Great Salt Lake, Utah

Potassium is a relatively abundant element in the Earth’s crust and production of potash fertilizer occurs in every inhabited continent. However, K₂SO₄ is rarely found in a pure form in nature. Instead it is naturally mixed with salts containing Mg, Na, and Cl. These minerals require additional processing to separate their components. Historically, K₂SO₄ was made by react­ing KCl with sulfuric acid. However, it was later discovered that a number of earth minerals could be manipulated to produce K₂SO₄ and this is now the most common method of production. For example, natural K-containing minerals (such as kainite and schoenite) are mined and carefully rinsed with water and salt solutions to remove byproducts and produce  K₂SO_4.  A simi­lar process is used to harvest  K₂SO₄ from the Great Salt Lake in Utah, and from underground mineral deposits.

In New Mexico (USA), K₂SO₄ is separated from langbeinite minerals by reacting it with a solution of KCl, which removes the byproducts (such as Mg) and leaves   K₂SO_4.   Similar processing techniques are used in many parts of the world, depending on the raw materials accessible.

Potassium sulfate

Chemical Properties

Chemical Formula:  K₂SO₄

K content: 40 to 44% (48 to 53% K2O)

S content: 17 to 18%

Solubility (25 ºC) 120 g/L

Solution pH approx. 7

 Agricultural Use

Concentrations of K in soil are often too low to support healthy plant growth. Potassium is needed to complete many es­sential functions in plants, such as activating enzyme reactions, synthesizing proteins, forming starch and sugars, and regulating water flow in cells and leaves.

Potassium-deficient cotton

Potassium sulfate is an excellent source of nutrition for plants. The K portion of the  K₂SO₄ is no different than other com­mon potash fertilizers. However, it also supplies a valuable source of S, which is sometimes deficient for plant growth. Sulfur is required for protein synthesis and enzyme function. There are certain soils and crops where the addition of Cl^- should be avoided. In these cases, K₂SO₄ makes a very suitable K source. Potassium sulfate is only one-third as soluble as KCl, so it is not as commonly dissolved for addition through irrigation water unless there is a need for additional S.

Potassium-deficient almonds

Several particle sizes are commonly available. Fine particles (<0.015 mm) are used for making solutions for irrigation or foliar sprays since it is more rapid to dissolve. Foliar sprays of  K₂SO₄ are a convenient way to apply additional K and S to plants, supplementing the nutrients taken up from the soil. Leaf damage can occur if the concentration is too high.

Management Practices

K₂SO₄ is frequently used for crops where additional Cl^- from more common KCl fertilizer is undesirable. The partial salt index of  K₂SO₄ is lower than some other common K fertilizers, so less total salinity is added per unit of K. The salt measurement (EC) from a  K₂SO₄  solution is less than a third of a similar concentration of a KCl solution (10 mmol/L). Where high rates of K₂SO₄ are needed, it is generally recommended to divide the application into multiple doses. This helps avoid surplus K accumulation by the plant and also minimizes any potential salt damage.

Potassium-deficient soybeans

A pdf version of this information is available
at the IPNI website here