Which relevance and characteristics does clay have as an additive in terracult substrate?

Clay helps plants store and release nutrients in accordance with their requirements during cultivation. In general, clay also improves the absorption of water and its distribution. It should not be too fine as it otherwise can be washed out and it can clog the pores up in the lower pot area. This then can lead to waterlogging.

An important quality criterion for clay is the lime concentration which should be as near to zero as possible. Otherwise, the slow release of lime can lead to an unintentional increase in pH during the course of cultivation.