Soil Definition: Importance, Components & Soil Profile

Understanding the essence of soil is pivotal for the world of agriculture. Soil, in its broadest definition, refers to the natural occuring medium in which plants can grow & its composition plays a vital role in agricultural success. Soil is a dynamic mixture of organic matter, minerals, water, and air, constituting its diverse components.

As farmers & agriculturalists goes deeper into understanding soil science, they unlock the potential to optimize yields, conserve resources, and promote sustainable farming practices, ensuring a bountiful harvest for generations to come. So through this article, let’s get to know soil definition, components, and soil profile.

Soil Definition

Definition of soil varies among professionals, soils for soil scientists may be regarded as fragmented rocks for a geologist. So in every field of study be it agriculture, geology, engineering, etc. definition of soil can vary. But, as we are an agriculturist, so we will define soil from our perspective.

There are two approaches in studying soils, pedologist approach & edaphologist approach. The Pedologist approach includes the study of origin of the soil, classification and description. While the edaphologist approach is based on the study of soil in relation to growth, nutrition & yield.

Generally, we can define soil as an accumulation of natural bodies which has been synthesized in a profile form, from a diversified mixture of disintegrated and weathered minerals and decaying organic matter which covers the surface of our planet and support or provide optimum amount of air, water, sustenance & mechanical support to the plants.

Here are some definitions given by popular soil scientists.

Soils are applied solely to those superficial or nearly superficial horizons of rocks, that have been more or less modified naturally by the interaction of water, air and various kinds of organisms, either living or dead; this being reflected in a certain manner in composition, structure and colour of such formations. Where this conditions are absent, there are no natural soils, but either artificial mixtures or rocks

Dokuchaev (Father Of Soil Science)

Soil is a natural body developed by natural forces acting on natural materials. It is usually differentiated into horizons from mineral and organic constituents of variable depth which differ from the parent material below in morphology, physical properties and constituents, chemical properties and composition and biological characteristics.

Joffe and Marbut

Soil is the more or less loose and crumby part of the outer earth crust in which, by means of their roots, plants may or do find foot hold and nourishment as well as all other conditions essential to their growth.

Hilgard

Soil may be defined as “A dynamic natural body on the surface of the earth in which plants grow, composed of mineral and organic materials and living forms.

Buckman and Brady

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Components Of Soil

Soil consist of four major components that are mineral matter, organic matter, water & air. These components are very intimately mixed with each other and can not be separated easily. We can generally divide them in soil solids & soil pore.

Soild space consist of mineral matter & organic matter while the pore space consist of water and air. We will understand later about volume composition of the soil based on this understanding.

Mineral Matter In The Soil

The size and composition of mineral matter vary in the soil due to the nature of the parent rock from which soil formation took place. Normally, you can find very fine broken rock pieces and minerals. These minerals are very coarse and vary in size. You will even need to see some of the particles with the help of an electron microscope.

The most commonly found minerals in the soil are quartz, biotite, muscovite, etc. Secondary minerals such as silicate clays and hydroxide clays of iron and aluminium, etc. are present as very finer fraction.

Organic Matter In The Soil

Soil organic matter is a mix of partly decayed and partially made substances from plants and animals. Microbes in the soil break down these organic residues, and because of this constant change, we need to add more to keep the soil healthy. The amount of organic matter in the soil is small, usually around 3 to 5% (by weight) in the top layer. Besides decayed plant and animal residues, it also contains living and dead microbial cells and compounds produced by microbes.

Organic matter is like a storage of nutrients in the soil. It helps create good soil structure on the surface and increases the size of pores, which holds water better and improves soil aeration. It is a significant source of nitrogen, about 5 to 60% of phosphorus & maybe around 80% of sulphur. It can also supply other important trace elements like boron and molybdenum that plants need to grow.

Soil microorganisms rely on organic matter as their main source of energy. Organic matter acts like a chelate, forming a bond with metals and creating a ring-like structure. This helps make metallic elements more available to plants, improving their mobility in the soil.

Organic matter also contributes to cation-exchange capacity, which is important for nutrient uptake by plants. Moreover, it plays a role in reducing soil erosion, providing shade to the soil, and keeping it cooler during hot weather and warmer in winter.

Soil Water

Soil water is crucial for the relationship between soil and plant growth. The water is held in the soil’s tiny spaces, and how strongly it’s held depends on how much water there is. When there’s more water in the soil, it’s not held as strongly, and when there’s less water, it’s held more tightly.

The movement and retention of water in the soil are mainly affected by the soil’s characteristics like texture, structure, the nature of inorganic and organic particles, the type and amount of certain elements, and the size and total amount of empty spaces in the soil. But, all the water present in the soil is not always available to plants. For example, water held very tightly by the soil isn’t easily accessible to plants.

Soil Air

In soil, there are spaces called air spaces or pore spaces that are like tiny holes or gaps not filled with solid materials. These spaces are usually filled with either air or water. When there’s more water in the soil, there is less room for air, and vice versa.

The balance between air and water in these spaces changes all the time. During the rainy season, water takes up the pore spaces, but when the water goes away through evaporation or other processes, air comes back.

The air in the soil contains different gases such as carbon dioxide, a small amount of oxygen, and nitrogen. It is not the same as the air we breathe outside. Soil air has more carbon dioxide and less oxygen compared to the air in the atmosphere. This happens because of the activities of tiny living things in the soil that release carbon dioxide and use up oxygen.

When the soil has good structure and large pore spaces, it allows plant roots to grow easily & new shoots to emerge without forming hard crusts. To have good air circulation, the soil needs to be well-drained, meaning it lets water pass through and has about 10% of its volume as pore spaces.

Different farming practices can affect soil aeration and plant growth by changing physical properties like density, porosity, and aggregation. Soil air is always changing & it plays an important role in influencing plant growth and the activities of helpful microorganisms in the soil.

Volume Composition Of The Soil

Soil is made up of two main parts: solid space and pore space. The solid space and pore space each make up half of the soil. The solid space is further divided into mineral matter (about 45%) and organic matter (about 5%). The pore space is filled with air and water, each occupying 50% of the pore space.

This means there’s about 25% air and 25% water in the soil. The amount of air and water can change depending on the weather and the environment. It’s important to know that these components, solid and pore spaces, along with mineral and organic matter, are all mixed together in the soil.

This helps different reactions to happen within and between these components, creating the best conditions for crops to grow.

The Soil Profile

In simple terms, the soil profile can be defined as a vertical slice through the ground that shows us different layers of soil. These layers are called horizons, and they have different properties but are connected to each other. The soil profile is like a history book, telling us about all the processes that formed the soil over time.

It’s an essential thing to study when we want to understand soils better. Soil scientists use the soil profile to classify and manage soils. By looking at the profile, they can figure out how fertile the soil is and what nutrients it has.

The soil profile is divided into five main horizons (Master Horizons): organic horizons (like OL1 and OL2), A horizon, B horizon, C horizon, and E horizon. Each horizon has specific features, and soil scientists use capital letters like O, A, B, C, and E (Elurial horizon) to identify the main horizons and lowercase letters for their sub-divisions. The soil profile extends from the soil surface down to the parent rock material below.

Organic Horizons

At the very surface, we find the organic horizons (group ‘O’), which come from decayed plants and animals. This layer is commonly found in forests where there are plenty of plants. It can be divided into two parts:

1. OL1: In this layer, you can easily see the original shapes of plants and animals with your eyes.

2. OL2: In this layer, you cannot see the original shapes of plants and animals with your eyes.

A (Eluvial) Horizons

‘A’ horizon is found at or near the surface and is known for “washing out” or maximum leaching. There are 3 parts of A horizon:

1. A1: This is the uppermost layer of mineral soil and has lots of organic matter, which makes it darker than the lower layers.

2. A2: This layer is where clay, iron, and aluminum oxides get washed out or moved away, and resistant minerals like quartz accumulate. It looks lighter than the upper layer (A₁).

3. A3: This is a transition layer between A and B horizons. It has properties that are similar to either A or A2 or B horizons. Sometimes, it may not be present.

B (Illuvial) Horizons

These horizons are a layer in the soil where materials like iron, aluminum oxides, and clay particles are washed down from above or deposited from the lower layers. This happens in dry areas, and sometimes salts like calcium carbonate and calcium sulfate are left behind when the water evaporates.

These horizons are also called “Sub-soil,” but they are not considered the top layer where plants grow. B Horizons can be divided into three specific layers:

1. B1: This layer is between the A and B horizons and has properties more similar to B than A. But this layer doesn’t need to present in every soil profile or section.

2. B2: The B2 horizon is where clays and hydrous oxides accumulate the most. There is usually more organic matter in this layer compared to the A horizon. It may also have a blocky or prismatic structure.

3. B3: This layer is a transition zone between the B and C horizons, and its properties are more like those of the B horizon than the C horizon.

C Horizon

C Horizon is the material found beneath the solum (A + B horizon) that has not been compacted together. It might or might not be the same as the original material from which the solum formed. It is considered outside the areas where biological activities occur in abundance and is hardly affected by the processes that form the solum.

The upper part of the C horizon might sometimes be seen as the solum since weathering and erosion continue to happen there. In soil science, the solum (plural-sola) comprises the top layers of the soil and the sub-soil that have been affected by soil-forming conditions. The base of the solum contains relatively untouched parent material. It’s important to note that the terms “solum” and “soils” are not the same.

The main components of the solum are the A and B horizons. It’s worth mentioning that the solum of a soil is not necessarily limited to the area of significant biological activity, and it can vary in thickness without having a specific maximum or minimum thickness.

Regolith

Regolith is a layer of broken rock and minerals that sits on top of the solid rock below. It’s made by rocks getting weathered over time. Regolith covers most of the land surface and includes all the weathered material in the area.

There are two main parts in regolith: the most weathered part called the solum, and the less weathered part known as saprolite. Saprolite is found right above the solid rock, but below the rest of the regolith. It consists of three parts called A, B, and C horizons.