We’ve gone over quite a few things in this mini-section on soil structure as we zoomed closer and closer in on the structure of soil! I do hope it’s becoming clearer as to why a good soil structure is so important, and how even poor soils can be improved given time and inputs. Soil microbes, organic matter, the clay and humus colloids, the rhizosphere — and time — all interact in a positive feedback loop to produce a rich soil high in granular peds (soil crumbs) best for healthy plant growth.

You’ll also have learnt how the large, negatively-charged surface areas of the clay and humus particles in those peds attract cations (positively-charged ions), and that these cations are exchangeable, swapping places on colloidal surfaces with other cations nearby and in the soil water.

Now you know what cations are, and where exchangeable cations get their name, it’s time to zoom in yet again, and take a closer look at those cations and the significance of their exchangability.

The cations most common to colloidal surfaces are H⁺ (hydrogen ions), Al³⁺ (aluminium ions), Ca²⁺ (calcium ions), Mg²⁺ (magnesium ions), K⁺ (potassium ions), Na⁺ (sodium) and NH₄⁺ (ammonium ions — not an element like the others, but a compound of one nitrogen (N) and four hydrogen (H) atoms with an overall charge of +1).

The number of cations able to bind to colloids is dependent on the number of negative charges available on the colloids’ surfaces. The type of cation that binds will be dependent on the concentration of that cation in the soil water. Higher concentrations of a particular cation in the soil water makes it easier to displace, and take the place of (exchange with), a different cation on a colloidal surface. For example, H⁺ ions are often released by plant roots — this increases their concentration in the soil water enough that they can swap places (exchange) with different cations already on colloids, such as Mg²⁺ (magnesium) or K⁺ (potassium). These dislodged ions then become available as nutrients that can be uptaken by the plant’s roots via that soil water.

It isn’t as simple or as straightforward as implied here, as some ions will dislodge more readily than others, and other factors such as which ions are present and in what concentrations will also determine which other ions will exchange or stay put. The soil type and the amount of colloids present also come into play. But if you refer back to The Importance of Soil pH to Plant Health, it might be clearer now how pH at least (related to the concentration of H⁺) has such an influence on the availability of nutrients?

The stability of soil crumbs is heavily dependent on exchangeable cations and the types they are. H⁺, Al³⁺ and Ca²⁺ cations are associated with stable crumbs, but a concentration of exchangeable Na+ (sodium) ions above about 6% will lead to increasing crumb instability when wet. Soils with this much sodium present as cations are called sodic soils, and the higher the exchangeable sodium, the less stable the soil becomes when wet.

Higher concentrations of sodium cations means higher concentrations attached to the clay particles in the soil, but sodium ions weaken the bonds between clay minerals, and hence compromise ped structure. Clay minerals swell when a soil is wetted, but as they are less firmly held together in a sodic soil, the already-weakened peds collapse and the individual clay particles disperse into solution. Sodic soils can be improved with time by replacing the sodium ions with calcium ones.

[Please don’t confuse sodicity for salinity, though both are characterised by too much salt [usually sodium chloride (table salt)]. Sodicity is caused by sodium ions attaching to clay and causing structural problems. Salinity is caused by too much dissolved salt in the soil restricting water access to plants. Some soils can be both sodic and saline. I’ll work in an article or two specifically on sodicity and salinity eventually!]

Speaking of calcium ions (Ca²⁺), these and magnesium ions (Mg²⁺) behave similarly in soils, though Mg²⁺ adheres less tightly to colloid surfaces. However, high concentrations of exchangeable Mg²⁺ relative to Ca²⁺ can cause clay dispersion and surface sealing. Such soils can be ‘hard’ and difficult to cultivate. Soils high in magnesium and sodium have particularly poor structures. Addition of Ca²⁺ can improve these.

The subject of cations and cation exchange can be quite complex and involved, but the main take-home point here is that the two together are very important drivers of chemical reactions in soil. Healthy plants and good soil structure both rely on good cation activity, and for a soil to have good cation activity it needs good amounts of humus and clay to provide the surface area for that activity to launch from.

But where there are cations there are also anions, and these too have a role in soil. This will be the topic of our next post!