Source: Board of examiners
The iron ocher is the result of a chemical reaction or a biological process, both of which can occur individually or simultaneously:
When the biological process is added to the chemical reaction, the oxidation effect is greatly increased.
In both cases, the result is the formation of slime on the walls of slotted drains. It is this deposit is called ocher.
The development of this material is more likely in a more aerated soil, such as sandy soil containing silt and less likely in a clay soil.
Chemical or biological reaction is also faster:
Under the action of iron ocher, even crushed gravel loses its filtering properties. Soils rich in iron and iron bacteria produce iron deposits indefinitely.