Soil

Soil, also called earth, is a special mixture that helps plants grow. It is made up of many things like tiny rocks, dead plants and animals, air, water, and tiny living creatures. All these parts work together to give plants the nutrients they need to live and grow strong.

Soil is not just dirt; it is a complex and living part of our world. It helps store and clean water, supports many kinds of life, and even affects the air around us. Scientists study soil to learn how it forms and how it helps our planet.

The soil on Earth is part of something called the pedosphere, which connects the land, water, air, and all living things. Understanding soil helps us take care of our planet and grow food for people and animals everywhere.

Processes

Soil is a key part of the Earth’s ecosystem. It helps control important processes like the carbon cycle and can be affected by changes in the climate. Soil provides many benefits, acting as a home for tiny living things, a system to recycle nutrients, and a way to clean water.

Soil supports plant life by giving them what they need to grow, such as air, water, and nutrients. It also helps keep the environment healthy by breaking down harmful substances. The tiny spaces in soil let air and water flow through, which is important for both plants and the many organisms that live there.

Composition

Soil is made up of solids, water, and air. About half of soil is solid material, which includes minerals and organic matter like decayed plants. The other half is space filled with water and air. These spaces let air and water move through the soil, which is important for plants and tiny creatures that live there.

Over time, soil can form layers called horizons. These layers differ in color, texture, and other properties. The top layer usually has the most living things, while deeper layers change more with minerals. Soil gives plants nutrients to grow, and these nutrients come from minerals, tiny living organisms, and the decaying matter in the soil. The amount of water in soil and the types of nutrients available depend on many factors, including the type of rocks the soil comes from and the living things in it.

Formation

Soil forms when organic material gathers and tiny particles move down, creating layers like the B horizon made of clay, humus, iron oxide, carbonate, and gypsum. Water and animals help move these particles around, forming different layers in the soil. Even without organic material, soil can form, like on Mars or in dry desert areas on Earth.

Soil starts from rocks that break down over time, called parent material. In warm, rainy places, plants like lichens and tiny blue-green cyanobacteria grow quickly on new rock, helping to break it apart. These plants get help from special fungi that live on their roots, which also assist in breaking down the rock. Over time, this creates richer soil layers. The main things that affect how soil forms are climate (CL), organisms (O), the shape of the land or relief (R), the type of parent material, and time. These factors are often remembered by the acronym CLORPT.

Physical properties

Main article: Physical properties of soil

For the academic discipline, see Soil physics.

Soil has many important features that help plants grow and support life. Some key features include its texture, which depends on the mix of tiny particles like sand, silt, and clay. These particles can clump together to form larger structures, helping the soil stay stable. The space between soil particles holds air and water, which are important for plant roots. Other features, like how tightly the soil is packed, its color, and its temperature, also play big roles in how well plants can grow and how water moves through the soil. All these properties change with depth, creating different layers in the soil.

Soil moisture

Soil water content tells us how much water is in the ground. We can measure it by how much space the water takes up or by its weight. There are different levels of soil moisture, from very wet to very dry. When soil is just right, plants can grow well. If there is too much water, it can drain away, but if there is too little, plants may not get enough.

Plants need water from the soil to grow. As the soil gets drier, it gets harder for plants to pull up water. Good watering schedules help keep soil at just the right moisture level so plants stay healthy. Water moves through soil from wet areas to dry areas, which helps plants get water even if they are not right next to a water source. Measuring soil moisture helps us understand how alive and active the soil is.

Soil gas

The air found in soil, called soil gas, is quite different from the air we breathe. Tiny living things in the soil, like microbes and plant roots, use up oxygen and release carbon dioxide. This makes there less oxygen and more carbon dioxide in the soil air than in the air above us.

Soil also holds water vapor, and the way gases move in and out of soil depends on things like soil texture, structure, and how much water is present. When soil is dug up, we sometimes notice a special smell — this is because certain gases are released from the soil. These gases help soil creatures communicate and affect how the soil ecosystem works.

Solid phase (soil matrix)

Soil is made up of tiny particles that come in different sizes and types. These particles are important because they help decide how much water the soil can hold and how easily water can move through it. The tiniest particles, called clay, play a big role in these properties.

Soil biodiversity

Soil is full of life! Many tiny microbes, small animals, plants, and fungi live in the dirt. Because most of these creatures are very small, it’s hard to know just how many different types of living things are in soil. Some scientists think soil might be home to more than half of all the species on Earth.

Some animals, like potworms called Enchytraeidae, spend most of their lives in soil. Many types of insects and almost half of all arachnids also live underground. Even some vertebrates, such as moles, live their whole lives in soil. These animals have adapted to life underground, which might have helped them survive during tough times in Earth’s past.

Chemistry

The chemistry of soil helps decide how well it can give plants the nutrients they need. It also affects how the soil behaves and stays healthy. The chemistry of soil depends on tiny particles called colloids, which can hold onto different substances.

These colloids can attract and hold certain particles, which helps keep important nutrients available for plants. This process changes how acidic or basic the soil is, which in turn affects what plants can grow there and how well the soil can clean water that passes through it.

Nutrients

Plants need 17 important elements to grow and make seeds. These include carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), iron (Fe), boron (B), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), nickel (Ni), and chlorine (Cl). Plants get some of these from air and water, but most come from the soil.

Plants can only use these elements when they are in the right form, often as tiny particles in water around the soil. Even though minerals in the soil hold most nutrients, they break down slowly. Plants help by releasing natural substances that speed up this process. The nutrients stuck to tiny bits of soil and to decaying plant material are easier for plants to use. This helps keep the soil rich and healthy for growing plants.

Soil organic matter

Soil is made up of tiny bits of plants, animals, and tiny living things mixed with minerals. These living things include worms, fungi, bacteria, and other small creatures. Together, they help break down dead plants and animals, turning them into food for other plants.

The main part of soil’s living material is called humus. This is a mix of different tiny parts that help keep soil healthy. It helps soil hold water and air, which plants need to grow. In grasslands, most of the soil’s living material comes from deep grass roots, while in forests, it mostly comes from fallen leaves.

Horizons

A horizontal layer of soil, called a soil horizon, has unique features and is different from the layers above and below it. These layers are named based on what they are made of, which shows how the soil was formed.

Over time, plants add organic material to the soil, like fallen leaves and roots. This creates a top layer called the O horizon, where many small creatures live and break down the plant material. This helps make the soil richer and better for growing plants. As time passes, some of this rich material moves down into a layer called the A horizon, mixing with minerals and helping plants grow even better.

Classification

One of the first ways to group different types of soil was created by a scientist from Russia, Vasily Dokuchaev, in the late 1800s. Later, scientists from America and Europe changed and improved this system. By the 1960s, a new way to group soils started, focusing more on how the soil looks and feels rather than what it was made from. Today, there are many ways to classify soil, and one big goal is to create a universal system that works for scientists all over the world.

In the United States, a special system called Soil Taxonomy is used. This system was created and is managed by the United States Department of Agriculture. The latest version of this system was published in 1999.

Uses

Soil is very important for growing plants, as it gives them support and nutrients. The type of soil and how much water it has affect what kinds of plants can grow. While soil is often used in farming, there are also ways to grow plants without soil, such as using special water-based systems.

Soil is also used in building and mining. It helps support structures and can be moved to make roads or dams. Soil plays a big role in keeping our environment healthy. It helps control water, preventing too much from flooding or too little from drying out. Many tiny animals and plants live in soil, and they help keep the land healthy and full of life. Soil also helps trap carbon, which can be important for fighting climate change.

Degradation

Land degradation happens when the land can no longer work well, either because of humans or natural causes. Soil degradation includes problems like making the soil too acidic, adding harmful substances, turning dry areas into deserts, washing away soil, or making the soil too salty.

When soil becomes too acidic, it can hurt plants and make the soil weaker. Sometimes, using certain fertilizers or cutting down trees can make this problem worse. Soil can also get harmful substances from things like waste or chemicals. In some places, soils are damaged so badly that they can't be used for growing plants anymore.

Another big problem is when dry areas turn into deserts, often because people take too many plants or animals from the land. Erosion happens when wind, water, or other forces wash soil away from where it belongs. This can be made worse by things like farming without covering the soil or cutting down trees. In the past, big areas of land in places like America and Canada lost their soil because of poor farming practices. In China, huge amounts of soil are washed into rivers every year. Finally, soil can become too salty, especially in dry places, which makes it hard for plants to grow. This can happen when too much water is used to irrigate crops.

Reclamation

Some soils are very fertile because they contain certain types of clay, like smectites. However, using too many fertilizers and chemicals can harm these soils. Farmers in some areas have found ways to improve their soil by adding special kinds of clay, like bentonite. This helps the soil hold water and nutrients, which can increase crop yields and allow farmers to grow different crops, such as vegetables, improving their income.

Adding organic matter, like wood chips or compost, can also help improve soil that lacks nutrients. In some places, people have used charcoal or biochar to enrich poor tropical soils, which can make them more fertile. However, it's important to use these materials carefully to protect both the soil and human health.

History of studies and research

The history of studying soil is closely linked to humans’ need to grow food for themselves and their animals. Throughout history, the success or failure of civilizations often depended on the quality and productivity of their soils.

Early thinkers like the Greek historian Xenophon suggested that turning plants into the ground could enrich the soil. Later, Columella recommended using certain plants to improve soil quality. Over time, people learned how different farming methods affected the land. As chemistry advanced, scientists began to understand what elements in soil helped plants grow. Important discoveries showed that plants get nutrients from both the soil and the air. These findings led to the development of fertilizers to help keep soil rich and productive. Researchers also learned that tiny living things in the soil play a big role in helping plants get the nutrients they need.