Potassium is the seventh most abundant element in the Earth's crust. It accounts for about 2.6% by crust weight. Because potassium easily loses an electron to attract other anions to form compounds, its monomers don’t exist in nature. It’s abundant in ore, soil and dried salt lakes as compound.
Potassium Element in Ores: Feldspar and Mica
Feldspar is the most abundant rock making up about 60% of the Earth's crust., so it’s easy to find in wild. Mountains that can be found everywhere contain these rocks, especially the one formed by magma eruptions. It’s a type of aluminum silicate mineral containing potassium (K₂O Al₂O₃ 6SiO₂). Another rock is mica, a silicate mineral with layered structure. There are two categories, biotite (K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂) and muscovite (KAl₂(AlSi₃O₁₀)(F,OH)₂). During the cooling of magma, these two rocks are usually the first components to crystallize. Metamorphism is another way for their formation. Their compositions rearrange under high temperature and pressure. One of the typical examples is schist.
Potassium Element in Soil
Its content in soi ranges from 0.3% to 3.6%, generally between 1% and 2%. According to the ease of becoming a cation in soil, it’s classified into soluble potassium, exchangeable and non-exchangeable potassium, and potassium in ores. Soluble and exchangeable ones are easy to use by plants and microorganisms, but they only account for a minority. The rest, especially in ore crystals, serves as a reservoir that is difficult for organisms to use.
Potassium element in Ores
About 90% of potassium in soil are tightly fixed in the crystals of Feldspar and Mica. They’re not easily released into soil. Most of them come from 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 ions in soil solution, and the acidity all affect its releasing rate. However, they enter the soil so slowly that plants and microorganisms can’t utilize them during their lives.
Non-exchangeable Potassium element
It’s also called fixed potassium and accounts for about 2-8%. It isn’t in the crystal structure. Since the attraction between clay and potassium ion is greater than hydration of water molecules, it tends to be fixed on the clay surface or wedge areas. The cations can’t replace it in the soil, but it will slowly transform into exchangeable type to regulate soil ions content like a reservoir. If the level of ions decreases, such as being removed by plants or washed away by rainwater, the fixed potassium will be released slowly for replenishment. When the ions increase, such as after applying fertilizer, non-exchangeable potassium will transform into fixed one again. Because the release speed is too slow, plants can’t utilize these elements in short term, even within their lifecycle.
Water-soluble and Exchangeable Potassium element
Water-soluble potassium is ion in the soil solution. Since they can be directly absorbed by plants and washed away by rainwater, their content is low, usually 2-5mg/L. Exchangeable potassium is adsorbed on negatively charged colloidal surfaces, and accounts for about 1-2%. They will be displaced into water by H₃O⁺ and NH₄⁺, except larger ions such as calcium and magnesium. These two forms are used by plants and microorganisms during their lifecycle.
Potassium in Oceans and Dried Salt Lakes
Although their average concentration in seawater is very low, only about 380ppm, the total amount is still very abundant due to the huge amount of seawater. The geological processes turn bays and fjords into enclosed waters. Rainwater will also transport potassium from ores into lakes, so the freshwater lakes gradually become saltier and eventually evolve into salt lakes. If the climate is dry and evaporation exceeds precipitation, lake will evaporate completely to expose the salt at bottom. Thus, dried salt lakes and sedimentary basins contain a considerable potassium.
As water decreases, the dissolved salts will exceed their saturation and begin to crystallize. The first to precipitate are 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 KCl, K₂CO₃, K₂SO₄ are often found in the top of salt lake sediments. The very high potassium content makes such deposits ideal for fertilizers.