Physical processes include freezing and thawing of water, plant root growth, physical transport of material)

Chemical processes are caused by decomposition of rock by slightly acidic rainwater. Over time, rock is turned into clay, and chemical are leached away. The higher the acidity of inflowing water, the more rapidly this will happen. The major sources of water acidity are from rain itself, and from organic acids found in decaying organic matter.

Soils will be richer and thicker at the bottom of a hill

Soils will become more leached and thick as precipitation increases

Soils will become more leached and thick as time increases
 

Soils developed from acidic igneous rock (rhyolite, granite), sandstone, or shale, have the lowest ability to retain nutrients

Soils developed into basic igneous rock (basalt, gabbro) have higher ability to hold onto nutrients

Soils which have lots of organic matter have the highest ability to retain soil nutrients. However, some organic rich soils (peat bogs) have low nutrient levels, as none have been leached into system.
 

A-Horizon lies below O, and is made of highly decomposed mineral material mixed with highly decomposed organic material (humus). Often, some leaching occurs in this zone.

B-Horizon has no organic material, and is made of highly decomposed parent material. In many cases, the materials leached from the A-Horizon (primarily clays and iron oxide) are deposited here

C-Horizon is made up of partially-decomposed parent material

With increasing depth, soil generally becomes less leached and less developed.

Your book also refers to an E-Horizon (which sets between A and B), and from which leaching occurs. Not all soils will have this zone.

Your book also calls the parent material the R-Horizon.

Geology will determine the type of parent material, what it will decompose into, and the ability of soil to retain nutrients.

Climate (ratio of precipitation to evaporation) will determine the degree and intensity of leaching

Vegetation will also determine the leaching rate (if dead leaves are acidic, leaching rates will be high), the rate of physical breakdown of parent material, and the ability of soils to retain nutrients.

Thus, each major climate and life zone on Earth will have different soils.

(1) Seasonal climates with more rainfall than evaporation and with deciduous tree cover will form the 'typical' soil profile (called an alfisol)

(2) Cold or seasonal climate with acidic parent material, much more precipitation than evaporation, and coniferous tree cover will form a spodosol. As conifer leaf litter is very acidic, leaching rates will be very high, leading to a very strongly leached lower A-Horizon and a dark upper B-Horizon where the leached material is deposited. These soils are characteristic from Green Bay north to Hudson's Bay where Precambrian granites are exposed.

(3) Climate which are warm year-round, and in which precipitation is very much greater than evaporation form oxisols. In these soils, leaching is so severe that only iron and aluminum oxides are not leached away, forming a brick-like substance called laterite. As essentially no nutrients are found in these soils, nutrients must be stored in the vegetation. This is why tropical forests will not recover after they are cut down and the vegetation taken away.

(4) Climates which have slightly more evaporation than precipitation, and grassland vegetation, will form a mollisol. Because more evaporation than precipitation, no leaching occurs. Nutrients are drawn up from parent material and retained in A-Horizon, which becomes very thick. These soils created the rich farmlands of the central U.S., the Ukraine, Argentina.