Where is Potassium Element Found in Earth?

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Potassium is the seventh most abundant element in the Earth's crust, accounting for about 2.6% by weight. Because potassium easily loses an electron to attract other anions to form compounds, its monomers do not exist in nature. It is abundant in ore, soil and dried salt lakes.

Potassium Element in Ores

Ores rich in potassium primarily include feldspar and mica. Feldspar, one of the most abundant rocks in the Earth's crust, is a type of aluminum silicate mineral that contains potassium (K₂O Al₂O₃ 6SiO₂). Mica is a silicate mineral with a layered structure, mainly divided into two categories: biotite (K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂) and muscovite (KAl₂(AlSi₃O₁₀)(F,OH)₂). During the cooling of magma, feldspar and mica are usually among the first components to crystallize. The ores formed in this manner are typically igneous rocks, such as granite. Metamorphism of rocks is another way in which feldspar and mica are formed. The compositions in the rocks rearrange and crystallize under high temperature and pressure, such as schist.

Potassium Element in Soil

Potassium is very abundant in soil, with a content of 0.3 to 3.6%, generally 1 to 2%. According to the ease that potassium can be utilized, it is divided into potassium in solution, exchangeable potassium, non-exchangeable potassium, and potassium in ores. Soluable and exchangeable potassium are easy to use by plants and microorganisms, but they only account for a minority. The rest of potassium, especially in ore crystals, serves as a reservoir that is difficult for organisms to utilize.

Potassium element in Ores

About 90% of the potassium in the soil are tightly fixed in the crystals of Feldspar and Mica, and are not easily released. Most of them come from the primitive rocks that formed the soil. Wind and rainwater also bring some small rock fragments into the soil. The size and composition of fragments, the level of potassium in the soil solution, and the acidity all affect the potassium releasing rate. However, they enter the soil so slowly that plants and microorganisms cannot utilize them during their growth.

Non-exchangeable Potassium element

It is also called fixed potassium and it accounts for about 2-8% of potassium in the soil. Not being part of the crystal structure is what sets them apart from potassium in rock. Since the attraction between the clay and potassium ions is greater than hydration of water molecules, they tend to be fixed on the clay surface. They also exist in wedge areas on the crystal surface. Non-exchangeable potassium cannot be replaced by cations in the soil, but it will slowly transform into exchangeable type, regulating the soil potassium ions content like a reservoir. If the level of potassium ions in the soil decreases, such as being removed by plants or washed away by rainwater, they will slowly be released. When the concentration of potassium ions increases, such as after applying potassium fertilizer, non-fixed potassium will transform into fixed potassium. Because the release speed is too slow, plants cannot utilize these potassium elements in the short term, even within their lifecycle.

Water-soluble Potassium and Exchangeable Potassium element

Water-soluble potassium is potassium ions in the soil solution. Since they can be directly absorbed by plants and washed away by rainwater, their content in the soil is low, usually 2-5mg/L. Exchangeable potassium is adsorbed on the negatively charged colloidal surfaces in the soil, accounting for about 1-2% of the potassium in the soil. They will be displaced into the solution by H₃O⁺ and NH₄⁺, except larger ions such as calcium and magnesium. Exchangeable potassium needs to be released from the colloidal surface to replenish the potassium taken up by plant. The potassium utilized by plants and microorganisms during their lifecycle essentially comes from these two forms.

Potassium in Oceans and Dried Salt Lakes

Although the average concentration of potassium in seawater is very low, only about 380 parts per million (ppm), the total amount of potassium is still very abundant due to the massive quantity of seawater. The geological changes of the Earth's crust turns bays and fjords into enclosed waters. Rainwater will also transport potassium from ores into lakes, and freshwater gradually become saltier and eventually evolve into salt lakes. If the climate near a salt lake is dry and evaporation exceeds precipitation, they will dry up and expose the salts inside. Thus, dried salt lakes and sedimentary basins contain a considerable potassium.

As the water decreases, the dissolved salts will exceed their saturation and begin to crystallize and precipitate. The first to precipitate are the hardly soluble carbonates and calcium salts. As evaporation continues, such as sodium and potassium salts will also begin to precipitate. They usually occur in the later stages, so thick layers of potassium salts (KCl, K₂CO₃, K₂SO₄) are often found in the upper layers of salt lake sediments. The very high potassium content makes such deposits ideal for fertilizers.

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