Why is Potassium Fixed in the Soil?

Ever wondered why potassium is fixed in soils? It's all about atomic-level processes, where its small ionic radius and low hydration energy allow it to perfectly fit and become "fixed" within the interlayer cavities of specific 2:1 silicate minerals like illite, smectite, and vermiculite.

Ionic radius and hydrated energy

Ionic radius, expressed in picometers (pm) or angstroms (Å), measures the size of an ion after electron loss or gain. As you move down a Group in the Periodic Table, ionic radii generally decrease due to the addition of new electron shells. Hydration energy is the energy released when an ion attracts and binds to water molecules, represented as a negative value because it's an exothermic process. This energy is significantly affected by the ion's charge density and ionic radius. Cations with higher charge densities (meaning a high charge and small size) have stronger electrostatic interactions with water molecules, leading to higher hydration energies. For instance, Group 2A alkaline earth metals have a higher charge (+2) than Group 1A alkali metals (+1).

Figure 1 below clearly shows an inverse relationship between these ionic properties. As ionic radius increases, hydration energy generally decreases. Furthermore, alkaline earth metals (e.g., Mg2+, Ca2+) exhibit significantly higher hydration energies than alkali metals of similar size due to their higher charge. This higher hydration energy means these ions are more tightly bound to water molecules.

Figure 1. Chart by: KALI-SPAR. Data source: R.D. Shannon (1976). Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crys. A32 751-767.

The role of “dehydrated K+” and hexagonal cavities

The "fit" of the dehydrated K+ ion within the hexagonal cavities formed by the oxygen atoms on the surfaces of 2:1 clay mineral layers warrants further explanation.

Dehydrated K+ occurs because K+ ion mostly perturbs the water molecules in its surrounding and does not attract and bind the water molecules strongly. The ionic radius of the “dehydrated K+ ion” remains approximately 1.38 Å and is remarkably close to the size of the hexagonal cavities formed by the oxygen atoms in the tetrahedral sheets of 2:1 clay minerals. This specific size allows K+ to shed its hydration shell (water molecules) and fit snugly into these cavities.

Compared to smaller, more highly charged cations like Mg2+ or Ca2+, K+ has a relatively low hydration energy. This means that K+ requires less energy to shed its surrounding water molecules to form dehydrated K+. Ions with higher hydration energies (e.g., Ca2+, Mg2+) retain their hydration shells more strongly, making it energetically unfavorable for them to enter the narrow interlayer spaces and be fixed. Their hydrated ionic radii are too large to fit into the hexagonal cavities.

Mechanism of K+ fixation

The overall mechanism of fixation involves a competition between the electrostatic attraction of the clay mineral layers for the K+ ion and the expansive forces of the hydrated cation:

When 2:1 clay minerals, such as illite or vermiculite, are in a moist environment, their interlayer spaces are often expanded, allowing various hydrated cations to reside on exchange sites.
Upon drying, or when K+ is abundant in the soil solution, the dehydrated K+ ions can enter the hexagonal cavities on the surfaces of the clay layers.
Once seated in these cavities, the K+ ions act as strong electrostatic bridges, drawing the negatively charged clay layers closer together. This "locks" the K+ ions in place, making them non-exchangeable and physically inaccessible to plant roots and standard soil extractants.
In essence, the forces of attraction between the negatively charged clay layers and the positive K+ ions are stronger than the weak hydration forces. This allows the layers to “re-collapse”and trap the K+.

Comparison with other cations

Na+ (Ionic Radius ≈ 1.02 Å, Hydration Energy ≈ -406 kJ/mol): Although smaller than K+, Na+ has a slightly higher hydration energy and thus a larger hydrated radius. This makes it less prone to fixation, as it prefers to remain hydrated and mobile.
Ca2+ (Ionic Radius ≈ 1.00 Å, Hydration Energy ≈ -1577 kJ/mol): Despite a similar ionic radius to Na+, Ca2+ has a much higher charge (+2) and thus a significantly higher hydration energy. It strongly retains its hydration shell, preventing it from entering the interlayer spaces and leading to fixation.

Conclusion

The interplay between the ionic radius of a “dehydrated K+” and its relatively low hydration energy, combined with the specific dimensions of hexagonal cavities in 2:1 clay minerals, is the fundamental mechanism of potassium fixation. This understanding highlights why K+ is uniquely susceptible to becoming trapped in clay interlayers, impacting its availability as a crucial plant nutrient. Recognizing these atomic-level processes is vital for developing effective soil tests for K fertilizer recommendations that account for soil mineralogy and ensure sustainable crop production.

Report abuse