‘Craniology’ is related to ‘Skull’ and ‘Oology’ is related to ‘Eggs’…

‘Craniology’ is related to ‘Skull’ and ‘Oology’ is related to ‘Eggs’ in the same way as ‘Pedology’ is related to _____.

The logic followed here is :

'Craniology’ - it is science that study of skull

and

‘Oology’ - it is science that study of Eggs

Similarly

‘Pedology’ - - it is science that study of Soil