Facts About Soil: What It is Made of and How It Forms

At its broadest, the term soil can refer to any kind of loose sediment. In geology, soil is the end result of rock weathering. And in soil science, it is weathered rock combined with decayed organic material that can support plant growth.

Soil is made of sand, silt, clay, and humus (decayed organic material). How much of each is in the soil depends on the type of rock it weathered from, how long weathering has been occurring, the local climate (wet or dry, hot or cold), the abundance of plant life to decay into organics, and the ecosystem of living things that decomposes the dead plants.

Sand is important for keeping the soil loose, aerated, and well-drained. Clay minerals hold water and nutrients in the soil just loosely enough to allow plant roots to take them. And humus provides the bulk of the soil's fertility.

It begins with solid rock. For soil-forming purposes, rocks come in two broad categories, silicate and carbonate, and are subject to two types of weathering, mechanical and chemical.

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Mechanical weathering does the same thing to all rocks: breaks them down into small pieces, which eventually become the size of sand, silt, and clay. Sand, silt, and clay are primarily measures of grain size. Sand grains have a diameter from 1/16th to 2 mm. Silt is between 1/256th and 1/16th mm, and clay is anything less than 1/256th mm. Sand looks like individual grains and feels gritty to the fingers. Silt looks like flour and feels gritty between the teeth. And clay isn't gritty at all.

Beyond grain size, sand and clay typically have very different chemical compositions, especially after long-term weathering - to the point where the minerals making up the end result clay-sized particles are called clay minerals.

Chemical weathering does very different things to carbonate vs. silicate rocks. Carbonate rocks, such as limestone and marble, are composed mainly of the mineral calcite. Calcite completely dissolves under chemical weathering, eventually leaving nothing solid behind. In the context of soil, carbonate rocks are a natural source of lime.

Silicate rocks include all types of igneous rocks (the most common are granite and basalt), sandstone, shale, and most types of metamorphic rocks (slate, phyllite, schist, and gneiss). For soil-forming purposes, they are made of two types of minerals: quartz and everything else.

Quartz, or SiO₂, has a very sturdy 3D tetrahedral framework structure that isn't affected at all by chemical weathering. Through mechanical weathering, quartz eventually becomes sand. After long-term weathering, all of the sand in the soil will be quartz.

Silicate rocks that are basically all quartz, like sandstone and metamorphic quartzite, will only produce sand. Silicate rocks that don't have any quartz at all, such as mafic igneous rocks (basalt), might not produce any sand. In between, how sandy the soil is depends on how much quartz the rock has and how long weathering has gone on to break down non-quartz sand-sized particles.

Everything else includes the ferromagnesian minerals (olivine, augite, hornblende, and biotite), garnet, feldspars, and micas. They have a variety of crystalline structures, from single and double chains of tetrahedrons, to 3D frameworks similar to quartz, to sheets. Early in the development of a soil, some of these will be sand-sized particles, especially feldspar. Under chemical weathering in the long-term, all of these become clay minerals (kaolinite, montmorillonite, talc, and many others), which have a sheet structure that occur as tiny flakes.

Clay minerals have a negative electrical charge on the flat parts of the flakes. This allows them to attract and hold water molecules and positive ions like Ca⁺⁺ and K⁺, which are important plant nutrients.