Soil forms through the slow, combined action of physical and chemical weathering breaking down underlying parent rock into progressively smaller mineral particles, together with the gradual accumulation of decomposing organic matter from plants and other living organisms. The specific type of soil that ultimately develops at a given location isn't determined by any single one of these processes alone; it depends on how climate, parent material, living organisms, local terrain and the sheer amount of time available all interact together at that particular site.
Weathering supplies the mineral base soil is ultimately built from
Physical weathering breaks solid parent rock into progressively smaller fragments through mechanical forces like repeated freezing and thawing, while chemical weathering breaks rock down through chemical reactions that alter its mineral composition, often accelerated by water and naturally occurring acids. Together, these weathering processes gradually convert solid bedrock into the loose mineral particles that form soil's basic structural material, a process that typically unfolds over a very long timescale, often centuries or millennia, rather than happening quickly.
Climate, organisms and terrain shape which specific soil type actually develops
Beyond weathering alone, a location's climate strongly shapes soil formation, since temperature and rainfall directly affect both how quickly weathering proceeds and how much organic matter accumulates and decomposes at that location. Living organisms, from burrowing animals to microorganisms breaking down organic material, mix and enrich the developing soil in ways that vary considerably by local ecosystem, while a site's terrain, its slope and drainage in particular, determines how much weathered material and water actually stays in place at that location rather than being carried elsewhere. Because all of these factors vary independently from place to place, soil scientists classify soil into genuinely distinct types based on which particular combination of these formative factors has shaped a given location.
What we're still unsure about
The basic processes of weathering and organic accumulation behind soil formation, and the major factors shaping which soil type develops where, are well established, extensively studied soil science, confirmed through direct field observation across an enormous range of environments. What's more genuinely an ongoing area of active research is predicting exactly how quickly a specific degraded or disturbed soil can actually recover its original fertility and structure under a given restoration approach, since soil formation's inherently slow natural timescale makes controlled, direct experimental study of full soil recovery genuinely difficult — soil scientists continue studying accelerated restoration techniques and their limits, without full agreement yet on how reliably human intervention can meaningfully speed up a process that naturally unfolds over centuries.
This sits inside Soil Formation & Types, one of seven topics in Physical Geography, one of five domains in Geography, one of seventeen subjects the app can quiz you on.