The Natural Cycling of Potassium in Agricultural Soils

The natural cycling of potassium (K) in agricultural soils is a continuous biogeochemical process that involves mineral weathering, K release/fixation, plant uptake, and return of K through the soil, plants, and organic matter, without a gaseous phase.

Weathering of K-Bearing Minerals: Potassium is initially released into the soil through the slow chemical weathering of silicate minerals like feldspar and mica, influenced by soil pH, moisture, and chemical conditions. Mica can weather to illite, smectite, or kaolinite while feldspars can form smectite and kaolinite. Smectite can weather irreversibly to kaolinite, but under certain conditions, smectite can transform to illite and back again. In K-depleted soil, K+ moves out of illite interlayers, and hydrated cations move in, prying open the mineral edges. This makes illite more "smectite-like" as the interlayers expand. If soil is enriched with K+ (e.g., through fertilization or crop residue), K+ can move back into smectite-like layers, transforming them back to illite. This dynamic conversion demonstrates the close relationship between soil K levels and clay mineral type.

Simplified depiction of mica and feldspar weathering. Adapted from Franzen and Bu (2018). NDSU Extension SF1881.

Structurally, these minerals are classified as 1:1 phyllosilicate (one silicon oxide tetrahedral sheet to one aluminum hydroxide sheet) or 2:1 phyllosilicate (an aluminum hydroxide sheet sandwiched between two silicon oxide tetrahedral sheets).

• Illite: A non-expanding 2:1 phyllosilicate (similar to mica) where the 2:1 mineral sheets are held tightly together by K+ cations, with a narrower interlayer space. It has lower cation exchange capacity (CEC) compared to smectite.

• Smectite: An expanding 2:1 phyllosilicate (e.g., montmorillonite, beidellite) with wider interlayer spacing than illite, allowing movement of cations and water. It has a high CEC, with almost all of the CEC coming from interlayers.

• Kaolinite: A 1:1 layered phyllosilicate, one of the end products of mica weathering, found in highly weathered soils. It has much less isomorphic substitution and no interlayer spacing, resulting in lower CEC.

Isomorphic substitution is the process where cations with less positive charge replace those with more positive charge within the mineral structure, creating the negative charge that is balanced with cations such as K+. For example, Al3+ can substitute for Si4+ in the tetrahedral sheet, or Fe2+/Mg2+ for Al3+ in the octahedral sheet, increasing the mineral's negative charge.

• In beidellite (a smectite variant), substitution primarily occurs in the tetrahedral sheet, leading to stronger K+ binding due to closer proximity of charges.

• In montmorillonite (another smectite variant), substitution is mainly in the octahedral sheet, resulting in weaker K+ binding due to greater distance from the K+ cation.

Plant Uptake and Utilization: Plants absorb K as the K+ ion from the soil solution. This absorbed K moves to growing tissues such as young leaves, flowers, and fruits, supporting their development.

Return to Soil: Unlike other nutrients, K is inorganic in plant tissue and readily leached by rainfall or snowmelt without microbial mineralization.

Soil K Pools: Soil K exists in various pools: exchangeable K (adsorbed on soil particles), fixed K (trapped within clay minerals), mineral K (in primary minerals), and soil solution K (immediately available for plant uptake). These pools maintain a dynamic equilibrium.

Losses: K can be lost from the soil through leaching, crop removal during harvest, runoff, and erosion.

Agricultural Management: Practices such as K fertilization, crop residue management, and organic amendments (e.g., manure, compost) are crucial K inputs for maintaining soil K levels.

Simplified potassium cycle. Adapted from "Updated K Cycle" presented at the Frontiers of Potassium Workshop in 2015, based on the work of Barros et al. (2004), Ecol. Model. 178:441-46.