Question: How are different kinds of soil formed on Earth? Is it the same on other planets?

James Taylor answered on 20 Jun 2014:

The type of soil formed at a location is dependent on 1) the parent material (bedrock), 2) the climate 3) the topography (shape of the land), 4) organisms and 5) time. Soil starts off as rock and there is chemical and physical weathering of the hard rock into smaller pieces. Overtime the smallest soil particles tend to move downwards – either down a slope or down the soil profile. For this reason the top of a slope tends to have sandier shallower soils while the bottom of a slope has deeper more clay soils (clay particles are 1000s of times smaller than sand particles). Likewise, topsoil tends to be more sandy (lighter texture) than the subsoil (more clayey).
If it is a hot climate, then the organic material (old leaves grass etc) breaksdown very fast so there is not a lot of organic matter in the soil. In cold climates, there is often more growth (more water) and the breakdown of organic matter is slower so there is a net build up of organic matter in the soil. Wet soils also have even slower decays – which is why we have peat bogs thta are just full of organic matter.
Theoreticlally soil formation would be similar on other planets – except that the climate effects are more extreme and there are no water effects, so the time scales would be very very different. There is rudimentary soil on Mars, possibly because we think at one time there was water there as well.