Soil formation, function, classification, and conservation.

Created March 2020, Offline version here
Video by Paul Anderson, also on his website Bozeman Science.

    Soils start out as rock, which is then transformed into smaller particles and different materials that form soils by a process called , which can be divided into two main types. The two types of weathering are , which decreases the surface area of rock and can be caused by water, ice-wedging, or plant roots, and , which would include oxidation or formation of rust, or the reaction of granite with acid to form clay. As the soil forms from these processes, it becomes layered into , which each have distinctive characteristics. At the surface, dead and dying plant material form the . Below that, we have the A horizon, which is a mix of mineral and organic material, also known as . The subsoil layer, which has little organic matter, has collected minerals washed down from above, and lies directly below the A horizon is called the . At the bottom of the soil profile, we find the , which is composed of parent material. Sometimes, a soil will also have an E horizon, which is formed by a process called , where water moving downward carries away minerals and nutrients, leaving a layer of just sand and silt particles. Soil-forming processes also produce different sizes of particles, which we group into 3 size classes: sand, , and clay. Sand particles are the largest and have a diameter of . Clay particles are the smallest and have a diameter of . The different sizes of these particles give different properties to soils. For example, if we poured water onto a cylinder of soil, would drain the most quickly, probably in minutes, whereas might take years to fully drain. The proportions of each of these particle sizes are used to classify soils with different properties. For example, a soil with 20% clay, 40% silt, and 40% sand would be classified as a , which has ideal properties for growing crops. In addition to drainage, particle sizes also affect soil nutrients. For example, clay particles tend to have a high , or ability to attract and hold onto nutrients, making them available to plants. Soils provide many important functions to society, but they are also vulnerable to degradation by several different processes. One example is means the washing off of the topsoil layer. Another type of degradation occurs when heavy machinery drives on soils, especially when they are too wet. This is called . The regeneration time of soils makes them a resource.