Clay is often used as an additive for potting soils. The aim is often to grow stronger plants which are more compact but…..what does clay actually do?
Physical effect

First of all there is the physical effect of adding clay: because clay is a heavy material (weight can be more than 1.000 kg per m3) it will compress the total mix.
For example: if we start with 10 m3 of peat and we mix 1 m3 clay, the end volume will be less than 10 m3; we lose volume!
So it is obvious that we lose air-porosity by adding clay in lighter mixes.
Because there is less air available, the plants will grow somewhat slower and the end result will be: shorter plants which are stronger.
For some cultures the growers ask for potting soils that are a bit sticky (like blocking compost) and for that real fine types of clay are sometimes used.
After adding clay to potting soils, often the water absorption will be improved; the effect depends on the structure of the clay and also the coarseness of the clay.
For cultures like Chrysanthemum (small pots with big plants), clay is often used for more heavy pots (so they don’t fall / tumble too easy).
Chemical effect 

Clay has a CEC (Cat ion Exchange Capacity) which means that clay has a negatively loaded complex to which positive elements (Cat ions) will be bounded (like Ca, K, Mg, Na, NH4, Fe, Mn, Al and H).
You can say that you find a buffer of these elements in the clay, which might bring a positive effect in longer cultures.
For potting soil producers it is also something to be aware of, because the base fertilizer will be partly “adsorbed” by the clay and will not be directly available for the plants.
The CEC is different with the different types of clay.
Different types

Not everything the market calls “clay” is really clay. 
We call the material “clay” if the particles are smaller than 2 µm.
Often coarser material is used and called clay but is actually getting close to being sand ( > 50 µm) which has the effect of compressing the potting soil, but does not have the other capacities we are looking for.
In the production of clay, there is always a strict selection; not only the types (fineness), the CEC but also practical things like the growth of weeds are checked and controlled.

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