Why are time, topography and parent material considered as passive control factors in soil formation?
Parent material is considered a passive control factor because it provides the basic input for soil formation in the form of weathered rock debris or transported deposits. The texture, structure, and mineral composition of parent material influence soil formation, but it does not actively drive the process.
Topography is a passive factor as it influences soil formation through environmental conditions like exposure to sunlight and surface and sub-surface drainage. It creates the setting for soil development, with thin soils on steep slopes and thick soils over flat upland areas, but does not actively cause soil formation.
Time is passive because it determines the duration for which soil-forming processes operate, leading to maturation and profile development. It allows processes to act sufficiently long but is not an active agent of change itself.
Explanation
The textbook explicitly states that parent material and topography are passive control factors. Parent material provides the raw material (weathered debris) that processes act upon. Topography creates environmental conditions (sunlight exposure, drainage) that influence soil formation passively. Time, while called 'the third important controlling factor,' is grouped with these passive factors because it determines duration of processes rather than actively causing change. The passive factors provide conditions or materials, while active factors like climate and biological activity drive the actual soil-forming processes.