Potato Review Group

Contents

Principles

Competition within soil

Competition at point of uptake

pH: a special case of cation competition

Further information

Cation competition notes

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Principles

Cation competition can occur in soil, at the point of uptake, and within the plant.

 

Competition within soil

  1. Cation exchange capacity (CEC) is the capacity of a soil to reversibly bind cations. It depends on the number and type of negatively charged binding sites on the surface of soil particles: high CEC is the result of lots of negative binding sites, usually due to high concentrations of organic matter and/or clay.
  2. Reversible binding means that individual cations constantly attach then detach from the soil surface, moving between being attached to the solid and in solution. Overall there will be an average concentration of any given cation in solution and an average amount attached to the soil particles. This is known as ‘dynamic equilibrium’ because although individual particles are moving, overall there appears to be no change in concentrations.
  3. When fertilisers are applied to soil, they may be applied in solution or as a solid which dissolves. Either way, cationic nutrients initially join the solution ‘pool’ and therefore start attaching/detaching in the same way as the cations previously present.
    • If a large amount of any one cation is added, this disrupts the equilibrium and on average, the cations which spend more time attached to the soil surface are those with the strongest affinity for the negative charge.
    • Cations which have lower charge density are more easily displaced from the binding sites and as a result a greater proportion of these move into solution. This is the reason why magnesium tends to leach more easily than, say, calcium
    • Figure 1 shows the effect of cation competition: when larger amounts of potassium fertiliser were applied to soil, the concentration in solution of magnesium was increased more readily than the solution concentration of potassium, until the point (between 0.75 and 1.5 mmol/100 g soil) where binding sites became saturated.
    • Leaching is therefore a greater issue for a) cations with lower charge density (e.g. magnesium), and b) in soils with fewer binding sites, i.e. lower CEC (e.g. sands).
    • See also Cations for soil structure for more information on this concept.

 

Displacement of magnesium by applied potassium in fertiliser

Figure 1. Displacement of magnesium by applied potassium in fertiliser

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Competition at point of uptake

  • Roots use both specific and non-specific (which don’t differentiate between Ca2+ / Mg2+ / K+) mechanisms for uptake of nutrient; non- specific mechanisms tend to result in greater uptake of whichever nutrient is at highest concentration in solution.
    • However, incoming charge must be balanced, therefore a plant can take up two ions of K+ for every one of Ca2+ or Mg2+, charge-wise.
    • Also K+ is smaller than (hydrated) Ca and Mg ions so enters roots more easily.
    • These reasons tend to mean that potassium antagonism against calcium and/or magnesium tend to be more of a problem, in terms of competition at the point of uptake.

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pH: a special case of cation competition

  • By definition, more acidic solutions (lower pH) contain a higher ‘activation’ (a sort of measure of concentration) of H+ Therefore these H+ ions also take part in cation competition.
  • This is one of the reasons that cationic metals including iron, manganese, copper and zinc tend to be present at higher concentrations in solution in low pH soils: more H+ ions in solution means that the metals, which have lower charge density, are more likely to be displaced from the CEC sites and thus move into solution.
  • Remember that high pH means low H+ concentration, and although this is often associated with high Ca2+ concentration (particularly due to the use of lime), the two do not necessarily go together, i.e. it is possible to have high pH without high Ca2+ concentration and vice versa.

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Further information

Cation competition notes

Calcium and magnesium 2018 (Roles of calcium and magnesium in soil structure and availability in different fertilisers)

Potassium application to potatoes 2014 (Assessing availability of potassium and potato requirements; application model now outdated)

 

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