Potato Review Group

 

Introduction to application of cationic fertilisers

  1. As well as release from minerals which provide cation exchange sites, elements may become fixed to them, e.g. soils rich in chlorites (iron / magnesium silicate minerals related to micas) preferentially fix Mg, releasing Ca and K and thereby potentially reducing the impact of Mg fertiliser added.
    • This effect has been observed in experiments with K at Rothamsted, where continuous excesses of K fertiliser input have not always increased exchangeable K as much as expected.
  2. Magnesium concentrations in soil tend to be much lower than calcium concentrations, in general. The exception is those with naturally high Mg concentrations, which are known as ‘serpentine’; also some chalky soils have high concentrations of Mg as well.
    • In addition to the higher likelihood of magnesium leaching (compared with Ca) Mg deficiency is therefore more likely than calcium deficiency is.
    • In particular, in situations where large amounts of potassium fertiliser are being applied, monitor magnesium status of crops.
  3. Figure 1 shows the potential fate of cations in soil after application as fertiliser. Note that all pathways will occur at the same time and the dominant effect will depend on precise soil chemistry and texture, including natural composition, historical fertiliser applications, and recent fertiliser applications.

 

The fate of cations in soil after application as fertiliser

 

Figure 1. The fate of cations in soil after application as fertiliser (from Potassium application to potatoes 2014).

 

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Further information

Cationic fertiliser notes

Potassium application model – background 2019 (Principles of the model)

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

Potassium for potatoes 2015 (Availability of potassium in soils of different types)

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application to potatoes 2014 (Assessing availability of potassium and potato requirements; application model now outdated)

 

Model

Potassium application model

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