Step 1: Define the terms.
(1) Phrenology: A pseudoscience involving the measurement of bumps on the skull to predict mental traits.
(2) Pedology: The branch of science concerned with the study of soil in its natural environment. It deals with soil formation, classification, and mapping. This is the correct term for the scientific study of soil.
(3) Petrology: The branch of geology that studies rocks and the conditions under which they form. While related, it focuses on rocks, not soil.
(4) Podology: An older term for Podiatry, the study and treatment of disorders of the foot, ankle, and lower extremity.
Conclusion: The correct term for the study of soil is Pedology.