What Is Soil? Scientific Soil Definition and 28 Common Q&A

Soil, according to the NRCS, is the natural body of minerals, organic matter, liquid, and gases on the surface of the land. Earth provides just the right combination of materials for plants to grow and the perfect home for worms, ants, and other bugs that live in the ground.

Here is everything you need to know including, what is soil, and how important is this nutrient-rich dirt for life on Earth?

What is soil

What Is Soil? Scientific Definition

According to the United States Department of Agriculture (USDA), the soil definition is:

  • The unconsolidated mineral or organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants.
  • The unconsolidated mineral or organic matter on the surface of the Earth that has been subjected to and shows effects of genetic and environmental factors of climate, and macro and microorganisms, conditioned by relief, acting on parent material over a period of time.

Importance of Soil

Soil supports all life. It stands as the foundation, literally, for the Earth’s entire ecosystem. The importance of it comes down to the nutrients the earth gives to plants as food, which allows them to grow as food for animals and humans.

The ground itself also filters and regulates rainwater to prevent flooding, stores organic carbon, and buffers against pollutants that would normally taint groundwater. The functions of soil make self-sustainability possible. Plants interact with each other to form a formidable structure inside the ground layers and provide a solid ground for mammals to walk on.

Uses of Soil

Scientific definition, composition, formation, sand, silt, clay, physical properties, horizons profile, texture, nutrients.

There are many uses of soil. All of which are equally important for different aspects of agriculture.

The main functions of soil include the following:

  • Food production
  • Storage, filtration, and transformation of the environment
  • Gene pool and home for organisms
  • Resource for raw building materials
  • Cultural heritage

Without the soil, we would not be able to grow fruit trees, vegetable gardens, indoor herbs potting, or fields of crops, which are absolutely essential for getting the nutrients our bodies require to live. And although we could begin to eat only meats from animals, they too would die off eventually without plants.

It would also be nearly impossible to build homes without wood from trees, which grow in the soil. Rocks may suffice, but even those make up ground layers deep beneath the ground.

Every function of soil is essential for sustaining life.

How Do We Protect Soil?

Since soil is so important, we should protect it. But how can we do that?

Forest protection efforts are one way to protect the earth. Agencies and people who run organizations that protect national forests make sure that there is not an over-harvesting of trees and that those forest ecosystems are not harmed in any way. When they do this, the ground thrives and maintains all of its life-giving nutrients, which it will give to plants.

What Is Soil Erosion?

Soil erosion is a naturally occurring event. When it rains, storms, or even when the wind blows hard one day, the ground gets picked up, moved, and compacted. Soil erosion changes the earth physical structure and can work against the growth of crops.

What Is Soil Made Of?

Soil is a type of dirt made up of rocks of all sizes, decaying plant matter, animal secretions and feces, minerals, moisture, and sometimes also sand and clay. It is a mixture of everything decayed into the ground from the surrounding area. And so, all regions of the world have different types of ground.

Soil Composition

The exact composition of soil depends on five formation factors:

  1. Parent material – the original material from which ground horizons form
  2. Topography – arrangement of physical features in an area
  3. Climate – temperature, humidity, or dryness specific to a region
  4. Organisms – living creatures in the area
  5. Time – how the earth has been compressed or untouched

Although every soil type has a different composition, an average sample, has 45% minerals, 25% water, 25% air, and 5% organic matter in the ground.

How Is Ground Formed?

Soils form gradually over time. Several environmental factors play a role in this formation and, depending on the climate of the region, the ground can form into one of many different types.

For instance, glacial till happens when glaciers move ground materials, which in turn, helps to form soils using loamy sand and Precambrian rock fragments.

Besides shifting ice, there are five total soil-forming factors. Each factor contributes to the formation of soils, and some factors can have a more significant impact than others. The formation also can determine the uses of soils.

5 Soil Forming Factors

Parent material

The original ground material that starts the formation of soils (rock fragments from glacial till, gravel, and sand from river sediment, etc.)


The temperature, humidity, and other aspects of the weather in a particular area. Climate can contribute to the dryness, biological activity, and presence of vegetation in the land.


This refers to the slope of the ground. Steep hills can cause an excess of erosion of the land, resulting in loss of topsoil and the creation of deposits.

Biological factors

Microorganisms shape soils when they burrow into it; humans tend to compact the ground from walking and building, and animals introduce nitrogen and other minerals and nutrients to the ground when they defecate.


The last soil-forming factor is time. Over time, the land changes and deposits form due to a constant buildup of organic materials.

What Is Humus?

What Is Soil

Soil humus is the part of the ground composition that comes from the decayed remains of plants and animals that lived on and within the surface of the ground. Erth humus is usually dark and contains leaves along with other remnants from the decayed organisms, including mushrooms.

What Is Soil Texture?

Ground texture is a way to describe the composition of a particular type of land. Examples of ground texture include sand, silt, and clay, which make up the land by combining at different sized portions. Texture refers directly to the size of the mineral pieces in the ground. These are called soil “separates.”

The sizes for separates are as follows:

Sand: <2 to 0.05 mm

Silt: 0.05 to 0.002 mm

Clay: <0.002 mm

Physical Properties of Soil

The physical properties of soils include color, texture, structure, consistency, and bulk density.

These physical properties of soils are indicative of the ground’s mineral contents.

For example, with the soil color, it could indicate a surplus of a certain mineral within the ground. Redness indicates a ground composed of an abundance of iron oxide; darkness indicates decayed organic matter, and yellow shows hydrated iron oxides and hydroxide.

The texture shows the percentage of sand, silt, and loamy clay within.

The structure can let you know how well the land will grow crops.

Consistency demonstrates the level of moisture. Wet ground will be sticky or plastic, moist ground is loose or friable, and dry ground is generally hard.

What Is Loam?

When referring to soils as loam, it means that there are equal parts of sand and silt within the ground and some clay. Loamy ground is thicker than regular soils and can be perfect for raising crops because the ground is sturdy. The loamy ground is also great for planting trees and flowers.

What Is Soil Profile?

What Is Soil

Although it seems like it from the surface, soils are not just one big concoction of evenly mixed soil. Soils have many layers beneath the earth, each of a different composition. These layers are called horizons, and they make up the ground profile when you extract a long piece from the depths of the ground. If you dug a hole deep enough and used a shovel to get a piece that contains every range of depth, you would see these distinct layers.

What Are Soil Horizons?

The horizons that show in the profile of soils are organized by the letters O, A, E, B, C, and R. There are three major horizons, which are A, B, and C; horizon E is the master horizon, and horizon R concerns the bedrock. Let’s take a closer look.

Horizon O

Horizon O is the organic horizon, which is usually on the surface or buried 0”-2” deep. It has a high percentage of organic matter from objects such as leaves, twigs, and partially decomposed plant and animal matter.

Horizon A

Horizon A is just below horizon O at about 2”-10” deep. It is also called the topsoil, which holds minerals for the roots that grow from plants on the surface.

Horizon E

Horizon E, the eluviated horizon, is the master horizon that acts as a zone where the upper horizons can move dissolved materials downward into the ground.

Horizon B

Horizon B is the subsoil that is between 10” and 30” deep.

Horizon C

Horizon C is called the substratum. It comes below horizon B and holds consolidated weathered rock fragments near the bottom of the ground.

Horizon R

Horizon R is the hard bedrock that sits below the soils. Once you reach ground composed of nothing but rock, you have hit the R horizon.

What Is Fertility Of Soils?

According to the FAO (Food and Agriculture Organization of the United Nations), “soil fertility is the capacity to receive, store, and transmit energy to support plant growth.”

The fertility of soils can be broken down into three aspects or components – physical, chemical, and biological. Interactions between those aspects are what determines whether the land is fertile or not. You can test for fertility using a fertility meter.

What Are Nutrients and Micronutrients?

Scientific definition, composition, formation, sand, silt, clay, physical properties, horizons profile, texture, nutrients.

Formation of soils happens from the ground obtaining nutrients from decomposed animal waste and dead plant matter. Some of these nutrients include nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. It also has micronutrients, which are boron, chlorine, copper, iron, manganese, molybdenum, and zinc.

What Are Soil Tests? And How Are They Done?

Soil tests are products you can buy to measure the level of certain nutrients in the ground like nitrogen and magnesium. Some tests also assess the pH level, temperature, resistivity, or humidity of the ground.

What Is Soil Humidity?

Soil humidity is the ratio of air and water in the ground – the level of moisture. Certain soils hold more moisture than others, so that is why when you use one type of ground, it grows plants faster compared to a different kind because of the rate it allows the plants soak up the water.

What Is ground Temperature?

The soil temperature is a measurement in Fahrenheit or Celsius of the temperature within the ground. Some gardeners watch the ground temperature to know when certain garden plants should move indoors. Ground temperature is below the surface and does not always relate to the air or surface temperature.

What Is Soil Acidity?

Soil acidity is the level of pH in soils. So, what is soil pH? Well, when you go to plant crops, vegetables, or even potting herbs, a bad ground pH could stunt the growth of the next harvest. You can test and adjust the acidity with ground conditioner products, special types of pre-mixed soils, or home-made compost.

What Is Soil Resistivity?

Resistivity is the conductivity of soils. With earth resistivity testing, you can see how well the ground conducts electricity. This factor matters most with electrical grounding design.

The Nitrogen Cycle

Nitrogen is one of the top 3 ground nutrients. Nitrogen is vital because it helps with plants’ structures, how the plant processes food, and helps to create chlorophyll. There are five stages of the nitrogen cycle.

Nitrogen Cycle Stages:

  1. Nitrogen fixation

Nitrogen exists in the atmosphere and gets deposited into the ground through precipitation. Some nitrogen becomes fixed through high energy fixation and turns into ammonia and nitrates.

2. Nitrification

Nitrogen converts from ammonia into nitrite and then nitrate. Nitrification is possible because of bacteria.

3. Assimilation

The nitrates, nitrites, ammonia, and ammonium go into plants from the ground and form plant and animal proteins.

4. Ammonification

Animal and plant waste gives off ammonia when they are broken down by decomposers (microorganisms), and nitrogen gets reintroduced to the ground.

5. Denitrification

Nitrogen in the ground turns back into a gas and releases into the atmosphere to start the nitrogen cycle over again.

All Types of Lands and The Differences Between Them

Scientific definition, composition, formation, sand, silt, clay, physical properties, horizons profile, texture, nutrients.

Each type of soil has unique levels of nutrients, type of texture, and overall composition. The composition of different types of land depends on factors of the region like climate (temperature, humidity), plant and animal species, and naturally-occurring minerals.

For example, desert soils have mostly sand at around 90-95% and have low levels of nitrogen and organic matter in the earth. Red soils have a lot of iron, but not a lot of nitrogen, humus, phosphoric acid, magnesium, or lime. Tropical rainforest ground has low fertility rates.

What Is Soil Amendment?

A question you may have come across when conducting your research is: what is soil amendment, and how does it work?

Soil amendments are products that you can buy and use to change the pH of land by either raising or lowering it according to the desired acidity.

What Is topsoil Fertilizer?

Soil fertilizer introduces nutrients and minerals to the ground that it needs to grow plants better.

What Is Soil Science?

what is soil scientific definition, composition, formation, sand, silt, clay, physical properties, horizons profile, texture, nutrients.

Soil is a lot more complicated than most people may think. The composition of ground can break down to a science – that is called soil science. Science of soils consists of tests on the soil in a laboratory and getting a hands-on analysis of the physical properties of the soil.

What Soil Scientist Do?

A soil scientist, also called a pedologist, conducts scientific tests and records their observations of earth profiles and ground horizons, in order to classify them.

What Are Typical Soil Experiments?

A soil scientist may map earth types, expose types of soil to certain conditions, or study the physical properties of soil. These experiments provide useful information that can help to develop new fertilizers or solve issues with plant growth in a specific region.

What Is Soil Mechanics?

Soil mechanics is the way soil scientists study soil. It describes the way the scientist analyzes the physical behavior of soil.

More Interesting Facts About Soil

Here are a few other facts about soil that you might find interesting

  • 0.01% of the earth’s water is inside the ground

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