Soil

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

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

The soil on Earth is part of something called the pedosphere. Understanding soil helps us take care of our planet and grow food for people and animals.

Processes

Soil is a key part of the Earth’s ecosystem. It helps with important processes like the carbon cycle. Soil provides many benefits. It is a home for tiny living things. It helps recycle nutrients and clean water.

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

Composition

Soil is made up of solids, water, and air. Half of soil is solid material. This includes minerals and bits of dead plants. The other half is space filled with water and air. These spaces let air and water move through the soil. This is important for plants and tiny creatures that live there.

Over time, soil can form layers called horizons. These layers look different in color and texture. The top layer usually has the most living things. Deeper layers have more minerals. Soil gives plants nutrients to grow. These nutrients come from minerals, tiny living things, and decaying matter in the soil. The water in soil and the nutrients depend on many things, like the rocks the soil comes from and the living things in it.

Formation

Soil forms when tiny pieces of plants and rocks break down and mix together. This creates layers in the ground, such as the B horizon made of clay, humus, iron oxide, carbonate, and gypsum. Water and animals help move these pieces around, forming different layers.

Soil can form even without plants, like on Mars or in dry desert areas on Earth.

Soil starts from rocks that slowly break down over time, called parent material. In warm, rainy places, tiny plants and blue-green cyanobacteria grow on new rock, helping to break it apart. These plants work with special fungi on their roots, which also help break 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 remembered by the acronym CLORPT.

Physical properties

Main article: Physical properties of soil

For the academic discipline, see Soil physics.

Soil has many features that help plants grow and support life. One key feature is its texture, which comes from the mix of tiny particles like sand, silt, and clay. These particles can stick together to form bigger structures, helping the soil stay stable. The spaces between soil particles hold air and water, which are important for plant roots. Other features, like how tightly the soil is packed, its color, and its temperature, also matter for plant growth and how water moves through the soil. All these properties change with depth, creating different layers in the soil.

Soil moisture

Soil water content shows how much water is in the ground. We can measure it by how much space the water takes up or by its weight. Soil moisture can range from very wet to very dry. When soil has just the right amount of water, plants grow well. If there is too much water, it can drain away. If there is too little water, plants may not get enough.

Plants need water from the soil to grow. As soil gets drier, it becomes harder for plants to get water. Good watering schedules help keep soil at the right moisture level so plants stay healthy. Water moves through soil from wet areas to dry areas, helping plants get water even if they are not near a water source. Measuring soil moisture helps us learn how healthy the soil is.

Soil gas

The air found in soil, called soil gas, is 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 means there is less oxygen and more carbon dioxide in soil air than in the air above us.

Soil also holds water vapor. The way gases move in and out of soil depends on things like soil texture, structure, and how much water is there. When soil is dug up, we sometimes notice a special smell. This happens 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 of very small pieces in different sizes and kinds. These pieces are important because they help decide how much water the soil can hold and how easily water can move through it. The smallest pieces, called clay, are very important for these qualities.

Soil biodiversity

Soil is full of life! Many tiny microbes, small animals, plants, and fungi live in the dirt. Because these creatures are very small, it’s hard to know how many different types of living things are in soil.

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.

Chemistry

The chemistry of soil helps plants get the nutrients they need. It also helps soil stay healthy. Soil chemistry depends on tiny particles called colloids. These colloids can hold onto different substances.

These colloids attract and hold particles, which keeps nutrients available for plants. This also changes how acidic or basic the soil is. This affects what plants can grow there and how well the soil can clean water.

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. 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 from tiny pieces of plants, animals, and small living things mixed with minerals. These living things include worms, fungi, bacteria, and other tiny creatures. They help break down dead plants and animals, turning them into food for new plants.

The main part of soil’s living material is called humus. Humus is a mix of different small parts that 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. 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.

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. 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.

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.

Uses

Soil is very important for growing plants. It gives plants support and nutrients. The type of soil and how much water it has affect what kinds of plants can grow. Soil is often used in farming. But plants can also be grown without soil, using special water-based systems.

Soil is also used in building and mining. It helps support buildings and can be moved to make roads or dams. Soil helps keep our environment healthy. It controls water, stopping too much from flooding or too little from drying out. Many tiny animals and plants live in soil. They help keep the land healthy and full of life. Soil also helps trap carbon, which is important for our climate.

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. 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.

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. In the past, big areas of land 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.

Reclamation

Some soils are very fertile because they contain certain types of clay. 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. It can also help farmers grow more crops, such as vegetables, which can improve their income.

Adding organic matter, like wood chips or compost, can also help improve soil. In some places, people have used charcoal or biochar to make poor tropical soils more fertile. It is important to use these materials carefully to protect both the soil and human health.

History of studies and research

The study of soil is tied to humans growing food. The success of many places depended on how good their soil was for growing.

Early thinkers like the Greek historian Xenophon thought putting plants in the ground could make soil better. Later, Columella suggested using some plants to improve soil. Over time, people saw how farming changed the land. As chemistry grew, scientists learned which parts of soil helped plants. They found that plants get food from both soil and air. This led to making fertilizers to keep soil rich. Scientists also found that small living things in soil help plants get the food they need.