Podzolisation is a soil forming process. It is characterised by the deterioration of organic clays which is a sign that the soils is becoming acidic.
This lowering of the pH causes compounds between organic molecules and metals (iron, aluminium) to form which are then leached by water.
Podzolisation therefore eventually leads to acid soil (podzol) which is poor in minerals and not very fertile.
This process generally occurs in cold, wet environments with acid soils such as taigas and temperate conifer forests.
Pedological section of a podzol. © Ailith Stewart, Geograph CC by-sa 2.0