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Earth's Crust and Inner Cores
Earth is composed of a thin outer crust atop tectonic plates. The tectonic plates in turn float on a mantle of semi-liquid magma; underneath is an outer core of liquid magma and a solid inner core of iron. The most abundant elements found in Earth's planetary rock are oxygen, silicon, aluminum, iron and calcium. The combination of these and other elements determine the composition of soils on the surface.
Mineral Composition of Rocks
Rocks found on Earth are categorized as igneous, sedimentary and metamorphic. Igneous rock is formed by lava cooling and solidifying. Silicate minerals (granite) and other elements make up most igneous rock. Metamorphic rocks were once either igneous or sedimentary rocks which have been subjected to high pressure and heat; this results in rocks such as marble or quartzite. Sedimentary rocks are formed by the breakdown of igneous, metamorphic or other sedimentary deposits. This happens through weathering and actions by microbes and plants. Sedimentary rock can be made up of sandstone, limestone or shale.
Formation of Soil
Soil is formed by the weathering and chemical breakdown of rocks. Soils are enriched by organic material, in the form of decomposing plant matter. Bacteria and fungi decompose organic material into simpler compounds. The particular minerals which form local soils also affect its acidity level, which is important in determining what kind of plant life it can support. In addition to plant life, soils also support animal life. Various organisms such as insects and worms help aerate and enrich soil further.
Types of Soils
Soils have varied mineral compositions, and are affected by the kind of rocks which they were formed from. This is due to topography, such as proximity to water, mountains or other geological features. The texture of the soil is also dependent on climate, as this determines which organic mechanisms (plant and animal) further act on the soil. There are three main types of soil: sandy soil (mostly sand), loam (half sand, half silt and some clay) and clay, which consists of minerals ground to a fine consistency to make the soil thick and paste-like.
Soil Layers
Most soils have a topsoil, a subsoil and parent material. The topsoil has more of the organic matter than the other layers, and high amounts of bacterial and fungal activity. This is the layer where plants derive most of their nourishment. The subsoil is usually 1 to 2 feet below the surface, and has thicker roots growing in it. It is lighter in color than the higher topsoil layer. The parent material has higher amounts of larger soil granules and unweathered rock. There are fewer nutrients and less microbial activity in the parent material.