Soil formation facts for kids

Soil created on a no-till farm in South Dakota, United States

Soil formation, also called pedogenesis, is the amazing natural process that creates soil. Soil is much more than just dirt; it's a living system! This process is shaped by the place, environment, and history of an area. Over time, these changes create different layers in the soil, called soil horizons. You can tell these layers apart by their color, structure, texture, and chemical makeup. Understanding how soil forms helps us learn about different soil types around the world.

Studying soil formation is part of pedology, which is the science of soil in its natural home. It helps us understand where different soils are found today and where they were in the past.

How Soil is Made

Soil starts its journey from parent material, which is usually rock. This rock slowly breaks down through a process called weathering. Tiny living things like bacteria and fungi help by eating simple nutrients from the weathering rock. They also create special acids and proteins that break down minerals even more. These microbes leave behind organic bits that turn into humus, a rich, dark part of soil. Plant roots, with their helpful mycorrhizal fungi, also pull nutrients from rocks.

New soil gets deeper as more rock weathers and new materials are added, like dust carried by wind. Slowly, the soil becomes strong enough to support plants and animals. It starts with simple pioneer species and grows into complex communities. The top layer of soil, called topsoil, gets deeper as dead plants and microbes add more humus. As soil gets older, it develops distinct soil horizons. This happens as organic matter builds up and minerals are weathered and leached (washed away).

What Shapes Soil: The CLORPT Factors

Soil formation is influenced by five main factors. Scientists often remember them with the acronym CLORPT:

• Climate
• Organisms
• Relief (or topography)
• Parent material
• Time

Let's explore each one!

Parent Material: The Starting Rock

The original mineral material that soil forms from is called the parent material. This can be igneous, sedimentary, or metamorphic rock. It's the source of almost all the minerals and nutrients in soil. As the parent rock breaks down and moves, it changes into soil.

Common minerals found in parent material include:

• Quartz
• Calcite
• Feldspar
• Mica

Soil, on an agricultural field in Germany, which has formed on loess parent material

Parent materials are grouped by how they got to where they are.

• Residual materials are rocks that broke down right where they were.
• Transported materials were moved by water, wind, ice, or gravity.
• Cumulose material is organic matter that grew and piled up in one spot, like peat.

Most soils come from transported materials. For example:

• Wind can carry tiny silt and fine sand particles for hundreds of miles, creating loess soils. These are common in places like the Midwestern United States.
• Water moves materials in rivers (alluvial), lakes (lacustrine), or ancient seas (marine deposits).
• Ice (glaciers) can push and drop huge amounts of rock and soil, forming moraines.
• Gravity pulls material down steep slopes, creating piles called colluvial material.

How Rocks Break Down: Weathering

Weathering is the process that breaks down parent material into smaller pieces and changes its chemistry. It happens in the top few meters of the Earth's surface.

• Physical weathering means breaking rocks into smaller bits without changing their chemical makeup.
  • Temperature changes cause rocks to expand and shrink, making them crack.
  • Water can get into cracks, freeze, and expand, splitting the rock.
  • Wind and water can rub materials together, making them smaller.
  • Plant roots can grow into cracks, pushing rocks apart.
  • Animals digging can also break up material.

• Chemical weathering changes the minerals inside the rock.
  • Water can dissolve salts, washing them away.
  • Water can react with minerals, changing them into new, more soluble substances.
  • Carbon dioxide in water forms carbonic acid, which can dissolve rocks like limestone.
  • Minerals can absorb water (hydration), making them swell and weaken.
  • Minerals can react with oxygen (oxidation), making them weaker and easier to break down. This is like rust forming on metal.
  • The opposite, reduction, happens when oxygen is scarce, like in waterlogged soil. This also makes minerals unstable.

These processes work together. Physical weathering creates more surface area, which helps chemical weathering happen faster. A special type of residual soil called saprolite forms when granite and other rocks turn into clay minerals through these weathering processes.

Climate: The Weather's Role

Climate is a huge factor in soil formation. The main things are how much precipitation (rain, snow) an area gets and the temperature. Both affect how fast chemical, physical, and biological processes happen in the soil.

• Warm temperatures and plenty of water speed up weathering, leaching, and plant growth.
• Humid climates often have lots of trees, while drier areas have grasses or shrubs.

Water is vital for most chemical weathering. The more water that soaks into the ground, the deeper the soil can develop. Water moving through the soil carries soluble materials and tiny particles from upper layers to lower layers. This helps create distinct soil horizons.

In dry regions, water is scarce. This means soluble salts aren't washed away and can build up, sometimes stopping plants from growing. These soils might also collect carbonates, forming hard layers. In tropical areas, if soil loses its plant cover, intense evaporation can pull water with dissolved iron and aluminium salts upwards. This can form a hard, unusable crust called laterite or bauxite.

Climate also affects:

• How much lime builds up in dry areas.
• The formation of acid soils in wet places.
• Erosion on steep hillsides.
• Intense chemical weathering in warm, humid regions.

Wind also plays a role, moving sand and dust, especially in dry areas with little plant cover.

Topography: The Shape of the Land

Topography refers to the shape of the land, including its slope, elevation, and direction it faces. This affects how much rain runs off and how much soaks in, which impacts erosion and soil development.

• Steep slopes often lose soil quickly due to erosion. Less water soaks in, so plants might not grow as well. Soils on steep slopes tend to be shallow and less developed.
• Flat areas or depressions (like valleys) collect more water, minerals, and organic matter. This can lead to deeper, more developed soils. However, too much water can make the soil waterlogged, slowing down some processes.

The direction a slope faces (its aspect) also matters. Slopes facing the sun will be drier than those facing away. This affects plant growth and soil moisture.

Organisms: Life in the Soil

Every soil has a unique mix of microbes, plants, animals, and even human influences. These living things are incredibly important for soil formation.

• Microbes (like fungi and bacteria) help break down minerals and change chemicals in the soil. Some bacteria can even pull nitrogen from the air into the soil, which plants need.
• Plants help prevent erosion with their roots. When they die, their leaves and stems add humus to the soil. Plant roots also release compounds that feed microbes and can even break down rocks.
• Animals like earthworms, termites, moles, and gophers mix the soil as they dig burrows. This mixing, called bioturbation, helps air and water move through the soil. Earthworms also improve soil by eating organic matter and creating stable soil aggregates.
• Humans have a big impact. Activities like tillage (plowing), removing natural vegetation, and using fire can lead to erosion or change how soil forms. For example, ancient people in the Amazon basin created very fertile soils called terra preta by adding charcoal and organic waste. Native Americans also used controlled fires to maintain grasslands, which influenced soil.

It's clear that life is essential for true soil formation. While some researchers have looked at places like Mars and called its surface material "soil," most agree that without living organisms, it's really just regolith (broken rock).

Time: The Slow Builder

Time is the final factor, and it interacts with all the others. It takes a long time for soil to develop its distinct features – from decades to many thousands of years!

• When new material is deposited, like after a flood, it takes time for soil layers to form.
• Soils are always changing, even on very old landscapes. Materials are added, removed, and altered.
• Scientists study how soil changes over time by looking at chronosequences (soils of different ages in similar environments) and paleosols (ancient soils buried underground).

How We Learned About Soil Formation

Early scientists helped us understand how soil forms.

Dokuchaev's Ideas

Vasily Dokuchaev, a Russian geologist, is often called the father of pedology. In the late 1800s, he figured out that soil forms over time because of climate, vegetation, topography, and parent material. He showed that soil is a function of these factors and time.

Jenny's State Equation

Later, in 1941, American soil scientist Hans Jenny created a famous equation to describe soil formation:

S = f(cl, o, r, p, t, ...)

This equation means:

• S is soil formation
• cl is climate
• o is organisms (all the living things in soil)
• r is relief (the shape of the land)
• p is parent material
• t is time

Jenny's equation is often remembered as CLORPT. It's a great way to think about all the different things that work together to create the soil beneath our feet.

An Example: The Makgadikgadi Pans

A cool example of soil evolution can be seen in the Makgadikgadi Pans in the Kalahari Desert. This area used to be a huge lake bed. Over thousands of years, changes in river courses led to a buildup of salt and the formation of hard layers called calcretes and silcretes in the soil.

See also

In Spanish: Pedogénesis para niños