How does soil develop

Over time, five major factors control how a soil forms. To identify, understand, and manage soils, soil scientists have developed a soil classification or taxonomy system. Like the classification systems for plants and animals, the soil classification system contains several levels of detail, from the most general to the most specific. The most general level of classification in the United States system is the soil order , of which there are Each order is based on one or two dominant physical, chemical, or biological properties that differentiate it clearly from the other orders.

Perhaps the easiest way to understand why certain properties were chosen over others is to consider how the soil i. That is, the property that will most affect land use is given precedence over one that has a relatively small impact.

The 12 soil orders all end in "sol" which is derived from the Latin word "solum" meaning soil or ground. Most of the orders also have roots that tell you something about that particular soil.

For example, "molisol" is from the Latin "mollis" meaning soft. Explore more about each soil order. Find your state soil! Texture - The particles that make up soil are categorized into three groups by size: sand, silt, and clay.

Sand particles are the largest and clay particles the smallest. Although a soil could be all sand, all clay, or all silt, that's rare. Instead most soils are a combination of the three. The relative percentages of sand, silt, and clay are what give soil its texture. A loamy texture soil, for example, has nearly equal parts of sand, silt, and clay. Structure - Soil structure is the arrangement of soil particles into small clumps, called "peds".

Basalt rock outcrops in Maui pastures. Volcanic ash has a non-crystalline, or amorphous, structure due to its rapid cooling. The soils that developed from volcanic ash contain mostly amorphous materials and minerals, such as allophane, imogolite, and ferrihydride.

Coral is a carbonaceous material that develops along coastal areas. In nutrient management, coral can serve as a valuable source of liming material to raise soil pH. In uplands of Maui, some soils have formed from the deposition and accumulation of plant residues, and other organic materials.

Often these soils are called peat or muck. In Maui, organic soils are not widely used in agriculture. Organic soils of the world can be very important and productive agricultural soils.

If you are interested in organic soils and would like to find out more, visit the websites listed below:. The soil develops in situ, or in place, as the parent material weathers. This is often the source of soil along mountain ridges. In the tropics, the soil formed tends to be highly leached and oxidic. The soil develops from rock fragments that fall from greater heights due to gravity.

These soils tend to be coarse and stony. This mode of formation dominates along mountain slopes. The soil develops along a stream or river systems in floodplains, alluvial plains, or delta deposits. Soils have a great degree of particle sorting, as finer particles are kept in suspension by flowing water. Alluvial soils are common on Maui. The soil develops from deposits of particles by the wind. Particles are highly sorted because wind carries finer particles the furthest.

Since volcanic ash is carried and deposited by wind, this method of formation is particularly important for volcanic ash soils. There are three categories of parent rock material. The first category is igneous rock. Igneous rock is cooled molten lava that originated from within the earth. There are two types of igneous rock: intrusive and extrusive igneous rock.

Intrusive rocks are large crystals that have slowly cooled within the crust of the earth. An example is granite, which is used as a common building material. While this type of parent rock is common on the continental United States, it is not common in Hawaii. Instead, the most common parent rock material in Hawaii is extrusive igneous rock, which is often associated with volcanic activity.

An example of extrusive igneous rock is basalt. Since extrusive rock materials, such as basalt, are finely textured, the soils weathered from this parent rock tend to be finely-textured.

And so, this is a reason why much of Hawaii soils are finely-textured. The next category of rock material is sedimentary rock. After long periods of compression, weathered products from older rocks will become cemented. As a result, another type of rock forms. Examples of this type are sandstone, shale, and limestone. The final category of parent rock is metamorphic rock.

Metamorphic rocks have formed as the result of extreme heat or pressure. As a result, a change in the form of igneous or sedimentary rock occurs. For example, under extreme heat or pressure, igneous rocks will form schist gneiss, limestone will form marble, and shale will form slate. Likewise necessary are organic substances, water, air and microorganisms. Soil is formed and develops over extremely lengthy periods and results from the interplay between myriad factors. The most important natural factors are rock, climate, plants, animals, terrain form and slope, and water availability.

Of great importance in this process are the development period and the scope of anthropogenic ground use, which in recent centuries has induced major changes in the soil. Soil develops through a process involving weathering, fracturing and comminution of rock into mineral soil particles. These processes evolve at varying speeds, depending on the intensity of the factors that are brought to bear.

But soil is far more than a mere mixture of exclusively mineral particles of varying sizes; for it is above all a mixture of decomposed organic substances, humus and mineral elements that are subjected to water and air, as well as myriad flora and fauna organisms. Soil is formed from surface rock that is fractured and comminuted by a vast array of weathering processes.

For example, when water penetrates rock cracks and crevices, causing the rock to expand by around nine per cent via the force exerted when the water freezes, the resulting pressure causes the rock to burst, fracture and comminute.

For dark rock in particular, the alternation of heating and cooling also exerts mechanical stress on the rock, causing it to fracture. In a virtually identical process known as hydratation, water molecules are uptaken in the crystal lattice of a mineral, and the consequent swelling causes the rock to lose its rigidity. The pressure exerted by growing roots can also cause rock to fracture and comminute — an explosive force that can be observed in the roots that surface on forest walking paths.

Rock fracturing and mineral particle comminution are accompanied by a host of other soil formation processes, such as humification, browning, and loaming.

Humification involves the formation of highly stable substances, known as humic substances, consequent upon plant residue decomposition that is mainly attributable to soil organisms. Browning is a process in which soil minerals such as olivine or biotite react with oxygen.

The resulting oxides and hydroxides lend the soil its characteristic reddish brown coloration. Loaming, which almost always occurs in conjunction with browning, results in clay minerals that are engendered by the comminution of the silicate minerals feldspar and mica.

Inasmuch as the particle size of clay minerals is the same as that of clay, originally sandy soil becomes more loamy and fertile through comminution.

All ions released into the soil during the comminution process, as well as the resulting clay minerals and humic substances, are ultimately transported downward with percolating soil water.