The section on Details is relatively complicated, therefore the main points to note are as follow.
- Cations (especially K+) can be released into the ‘available’ pool by weathering of clay.
- Too high concentration of monovalent cations (e.g. Na+, K+) can cause deflocculation (layers become separated so structure breaks down) of clays and slumping of soils.
- Divalent cations (e.g. Ca2+, Mg2+) promote flocculation of clay and thus hold the soil structure together.
- The relative (physical) sizes of the ions in solution mean that Mg2+ is more prone to leaching than is Ca2+; and also that Ca2+ is better for holding the soil structure together. Thus to improve the structure of a clay soil, gypsum (in conjunction with organic matter) is a useful material to apply.
- Magnesium that originates from within the clay structure holds the clay layers very tightly together; this is the cause of ‘tightness’ in soils naturally high in Mg2+. The same effect will not be promoted by application of too much magnesium e.g. in fertilisers, however; this may have the opposite effect and destabilise the soil structure.
Please refer to the Chemistry Glossary [link] for further explanation of unfamiliar terms.
Interactions with clays
- Cations are important for binding together the tiny layers within clay. Therefore:
- When clays weather (due to exposure to hot/cold/wet/dry), they release cations, particularly K+;
- The correct balance of cation types is important to ensure decent clay structure is maintained.
- Clay crystals have an overall negative charge, therefore if they are surrounded by a high concentration of monovalent cations, the cations can each only associate with one layer of clay. Thus a positively charged layer is formed around each crystal and they repel each other, creating deflocculated clay, leading to slumping soils.
- Divalent cations have the capacity to associate with two layers of clay (one for each charge). Therefore if there is a sufficiently high concentration of divalent cations around the clay crystals, they will effectively ‘sandwich’ the clay layers together and maintain flocculated clays, and therefore better-structured soils.
- See Figure 1 for an illustration of flocculated and deflocculated clay crystals.
Figure 1. Illustration of flocculated and deflocculated clay layers
Effects of hydration
- Due to their relative positions on the periodic table, non-hydrated Ca2+ has a larger ionic radius than non-hydrated Mg2+. Both have the same amount of positive charge (‘two plus’), but in calcium this is spread over a larger sphere, so the ‘charge density’ in non-hydrated Ca2+ is lower than the ‘charge density’ in non-hydrated Mg2+. One result of this is that Mg2+ can more strongly attract water molecules (the oxygen atom in water molecules holds a slight negative charge) and so in solution (which they will always be in soil unless the cations originate from within the clay structure), the ions become hydrated and hydrated Mg2+ has a larger radius than hydrated Ca2+. See Figure 2 for an illustration of the positions of hydrated and non-hydrated cations within clay structure.
- The fact that hydrated Mg2+ has a larger radius than hydrated Ca2+ gives rise to two main outcomes:
- Mg2+ is less well held by clays due to its lower charge density (overall hydrated Mg2+ is larger than hydrated Ca2+ so the 2+ charge is spread over a larger sphere), thus is more prone to leaching than is Ca2+. Therefore especially in light soils, magnesium leaching can become a problem (see also Cation Competition for more information).
- If the divalent cation holding the layers of clay together is predominantly (hydrated) Mg2+, the clay layers will be held further apart and thus will be less stable. Therefore to improve soil structure, it is better to increase the ratio of Ca2+ : Mg2+, e.g. by using gypsum (gypsum will not affect the pH of the soil), which is most beneficial when used alongside organic matter.
- It is worth noting, however, that the smaller non-hydrated radius of Mg2+ compared to Ca2+ is the reason why soils naturally high in magnesium (serpentine soils) tend to be ‘tight’: the clay layers are held more closely together than if they were being held by calcium. This is true because the Mg2+ has originated from within the clay structure and therefore has not yet been exposed to water and has not yet become hydrated.
Figure 2. Illustration of hydrated and non-hydrated ions
Effects of cationic fertilisers; ‘ideal’ ratios for structure
Using the theories and explanations above, the following points about use of cationic fertilisers become important, and the effects are likely to be more pronounced in clay-rich soils.
- If the ratio of Ca2+:Mg2+ in soil is decreased too far by applications of fertilisers (e.g. by consistent application of dolomite where magnesium concentration was not previously low), the clay fraction of that soil is likely to become deflocculated, the implications of which include:
- More dispersed clays à blocked pores à lower hydraulic conductivity/water infiltration.
- Greater tendency to capping à
- Greater tendency to erosion.
- Animal manures, in particular pig manure, often contain high concentrations of sodium. Since this is a monovalent cation it can result in deflocculation of clays and therefore slumping of soils.
- This is often observed where pig slurry has been applied over long periods of time.
- This situation can be improved by applying gypsum and organic matter (ensure by lab testing that Na concentration in the OM is low).
- High sodium concentrations can also occur after saline (e.g. seawater) flooding and in arid areas where evaporation is extremely large (not generally relevant to UK). Irrigation water can be a source of lots of sodium, but this should be rare with fresh water.
- Heavy rainfall events tend to be most destructive to poorly structured soils and/or those with low Ca2+:Mg2+ ratio, but the heavier the rain, the smaller the difference in effect (when different soils are compared) becomes.
- An ‘optimum’ ratio of Ca2+:Mg2+ cannot be calculated without knowing Na+ concentration and overall electrolyte concentration of soil solution; the concentrations of ions in soil solution change very rapidly and it is not practical to measure and react to this, partly because values depend on soil moisture content.
- High Na+ is likely to exacerbate low Ca:Mg ratio issues for soil structure.
- Maintaining high electrolyte concentration is good for soil structure, i.e. overall concentration of ions in solution should be ‘high’.
Cations and soil structure notes
Calcium and magnesium 2018 (Roles of calcium and magnesium in soil structure and availability in different fertilisers)