Composition of Soil, Soil Profile, Soil Texture and Soil Structure

 

Composition of Soil

Soil is composed of both biotic things, like plants and insects and abiotic materials like minerals, water, and air.  The soil is a heterogeneous body composed of 5% organic matter, 45% minerals, 20-30% different gases and 20-30% water.

The solid fraction of the soil consists of mineral and organic matter, which is typically about 50% of the soil by volume.  Soil pores consist of the air and water filled fractions of the soil, and together they make up about 50% of the soil by volume.  Soil pores are typically classified based on size: Macropores ( >75µm) Mesopores ( 30-70µm) Micropores ( 5-30µm) Ultramicropores (0.1-5µm) Cryptoporus ( <0.1µm).  The air filled pores of the soil typically have a similar distribution of gases as the atmosphere above the soil, with slightly lower oxygen and slightly more CO2 due to the respiration of microorganisms.  The main source of soil water is rainfall and overland flow. The amount of water that enters the soil is a function of soil structure and texture. Water moves in soil through mass flow and capillary action.

https://www.ctahr.hawaii.edu/mauisoil/a_comp.aspx

Organic Matter

Plants and animals are the main sources of organic matter in the soil. Depending upon the decomposition stage, the organic matter may be Completely decomposed organic matter, Partially decomposed organic matter or Undecomposed organic matter.  Soil organic matter (SOM) consists of all of the organic components of a soil, including living biomass, decomposing tissue, and fully decomposed tissue called humus.

Minerals

Minerals are an important element of the soil and there are numerous types of minerals found in soil.  Minerals are formed by the breakdown of large rocks.  A Primary mineral comes directly from rock and these break down into different types of clay minerals. Some of the most common minerals found in soil are, Iron, Potassium, Magnesium, Calcium, Sulphur etc.  The minerals in soil are classified into two categories,

Primary Minerals - These are minerals which have not been chemically altered since deposition and are similar to their parent materials. These materials are bigger in size and irregular in shape and are usually found in sand and silt.

Secondary Minerals are formed as a result of weathering of Primary Minerals and are mainly found in fine silt or clay. The particle size of these minerals is much smaller, due to the weathering process and since they have a large surface area that helps them retain moisture.

Air or Gaseous Components

The air-filled pores of soil contain the gaseous components, mainly Nitrogen, oxygen and carbon dioxide.

Water

The soil dissolves the minerals and nutrients in water. The relative amounts of water and air in the soil constantly changes as the soil wets or dries.

Soil Moisture

Water in soil is known as soil moisture. Water absorption in the soil  is determined by various factors. It plays a major role in soil formation. As a result of precipitation, water arrives at the surface. The particle size distribution of soil determines its porous nature and causes downward movement of water vertically, known as infiltration. This penetration continues deep in the layers of soil until it reaches saturation. Further from here, water cannot seep vertically and thus moves sideways. Formation of puddles as a result of saturation is called surface ponding. Water that is available to plants is called Root zone moisture and surface soil moisture is the water available in the immediate upper region of soil.

Different types of water present in the soil are

Gravitational Water

The water that reaches the water table of the soil due to the gravitational force is referred to as gravitational water. This is not available to the plants.

Hygroscopic Water

This is a thin film of water tightly held by the soil particles and is also not available to the plants.

Chemically Combined Water

The chemical compounds present in the soil particles contain water. This is known as chemically combined water. This is also not available to the plants.

Capillary Water

This water exists between soil particles in small capillaries and is available to the plants.

Atmospheric Humidity

The hanging roots of the epiphytes absorb the moisture in the air due to the presence of hygroscopic hairs and spongy velamen tissues.

Importance of Soil Moisture

·         Soil water carries food nutrients for the growth of plants

·         Soil moisture content determines the yield of the crop in a region

·         Crucial in maintaining soil’s temperature

·         Soil moisture act as nutrients, is a primary need for photosynthesis in plants

·         Important for soil formation

·         Soil moisture catalyses biological activities of microbes in the soil

Measuring Soil Moisture

The soil moisture can be measured by various tools such as Tensiometer, electrical resistance blocks, time domain reflectometry, etc

Tensiometers

Moisture content in the soil can be measured using a device known as Tensiometer. They are water-filled tubes which are sealed with a porous ceramic tip towards the bottom and a gauge at the top. For taking measurement, it is penetrated into the soil till the root level and as water passes between the tip of the device and the ambient soil, an equilibrium will be reached.  The tension is recorded on the gauge that gives the measure of soil moisture in that region.

Electrical Resistance Blocks

These consist of two electrodes connected to lead wires extending to the soil surface. It is used to measure soil water tension.

Time Domain Reflectometry (TDR)

TDR – Time Domain Reflectometry is used to determine the soil moisture content. Steel rods are placed in the soil and electrical signals are sent through them. The returned signals are measured to determine soil water content.

Soil Profile

The soil is the topmost layer of the earth’s crust mainly composed of organic minerals and rock particles that support life. A soil profile is a vertical cross-section of the soil, made of layers running parallel to the surface and these layers are known as soil horizons.

These layers or horizons are known as the soil profile. It is the vertical section of the soil that is exposed by a soil pit. The layers of soil can easily be identified by the soil colour and size of soil particles. The different layers of soil are Topsoil, Subsoil and Parent rock.  Each layer of soil has distinct characteristics.  The soil profile is composed of a series of horizons or layers of soil stacked one on top of the other. These layers or horizons are represented by letters O, A, E, B, C and R.

The O-Horizon

O horizon is the upper layer of topsoil which is mainly composed of organic materials such as dried leaves, grasses, dead leaves, small rocks, twigs, surface organisms, fallen trees, and other decomposed organic matter. This horizon of soil is often black brown or dark brown in colour due to the presence of organic content.

The A-Horizon or Topsoil

This layer is rich in organic material and is known as the humus layer. This layer consists of both organic matter and other decomposed materials. The topsoil is soft and porous to hold enough air and water.  Seed germination takes place in this layer and new roots are produced which grows into a new plant. This layer consists of  living organisms such as earthworms, fungi, bacteria, etc.

The E-Horizon

This layer is composed of nutrients leached from the O and A horizons. This layer is more common in forested areas and has lower clay content.

The B-Horizon or Subsoil

It is the subsurface horizon, present just below the topsoil and above the bedrock. It is comparatively harder and compact than topsoil. It contains less humus, soluble minerals, and organic matter. It is a site of deposition of certain minerals and metal salts such as iron oxide.

This layer holds enough water than the topsoil and is lighter brown due to the presence of clay soil.

The C-Horizon or Saprolite

This layer is devoid of any organic matter and is made up of broken bedrock. This layer is also known as saprolite.

The R-Horizon

It is a compacted and cemented layer. Different types of rocks such as granite, basalt and limestone are found here.

Types of Soil

Soil is classified into four types based on its texture, proportions and different forms of organic and mineral compositions, they are Sandy soil, Silt Soil, Clay Soil and Loamy Soil.

                                                 https://www.javatpoint.com/types-of-soil

 

Sandy Soil

It consists of small particles of weathered rock. Sandy soil is usually formed by the breakdown or fragmentation of rocks like granite, limestone and quartz.  Sandy soils are one of the poorest types of soil for growing plants because it has very low nutrients and poor water holding capacity due to its larger particle size, which makes it hard for the plant’s roots to absorb water. This type of soil is very good for the drainage system.

Silt Soil

This have much smaller particles compared to sandy soil.  This is made up of rock and other mineral particles which are smaller than sand and larger than clay and holds water better than sand.  The silt soil is more fertile compared to the other three types of soil and easily transported by moving currents and thus mainly found near river, lake, etc.

Clay Soil

Clay has the smallest particle size and particles in this soil are tightly packed together with each other with very little or no airspace. This soil has very good water storage qualities and makes it hard for moisture and air to penetrate into it. It is very sticky to the touch when wet, but smooth when dried. Clay is the densest and heaviest type of soil which does not drain well and do not provide space for plant roots to flourish.

Loamy Soil

Loam is the fourth type of soil. It is a combination of sand, silt and clay. It retain moisture and nutrients and thus is more suitable for farming. This soil is also termed as agricultural soil as it includes all three types of soil materials and humus and has higher calcium and pH levels.

Importance of Soil

• Soil is an important element essential for the survival of living organisms. The importance of soil are
• Fertile soil helps in the growth and development of plants that provide food, clothing, furniture, and medicines.
• It supports many life forms including bacteria, fungi, algae, etc and thus help to maintain environmental balance.
• The topsoil supports life activities such as reproduction, hatching, nesting, breeding, etc. of a organisms.
• Soil is used for making cups, utensils, tiles, etc and for the construction of homes, roads, buildings, etc.
• Useful mineral medicines such as calcium, iron, and other substances such as petroleum jelly for cosmetics are extracted from the soil.
• The soil absorbs rainwater which is evaporated and released into the air during sunny days, making the atmosphere cooler.

Soil Texture and Soil Structure

Soil texture and soil structure are properties of the soil that have a significant effect on water holding capacity, nutrient retention and supply, drainage, and nutrient leaching of soils.

Soil Texture

Soil texture influences nutrient retention. Fine textured soils have greater ability to store soil nutrients. Soil particles are divided into the 3 separate size classes, sand, silt, and clay. The size of sand particles ranges between 2.0 - 0.05 mm, size of silt ranges from 0.05 mm - 0.002 mm and size of clay is less than 0.002 mm. This difference in size is due to the type of parent material from which soil particle is formed and also on the degree of weathering. Sand particles are generally primary minerals that have not undergone much weathering. Clay particles are secondary minerals that are the products of the weathering of primary minerals. As weathering continues, the soil particles break down and become smaller and smaller. 

Textural Triangle -  describes the relative proportions of sand, silt and clay in various types of soils. (http://soils.usda.gov/technical/manual/print_version/complete.html)

Soil texture is the relative proportions of sand, silt, or clay in a soil. The soil textural class is grouping of soils based upon these relative proportions. Soils with finest texture are called clay soils, while soils with coarsest texture are called sands. A soil that has a relatively even mixture of sand, silt, and clay and exhibits the properties of each separate is called a loam. There are different types of loams, based upon which soil separate is most abundantly present. To know and identify the texture of soil, the textural triangle is used, if the percentages of clay, silt, and sand are known.

Clay particles, as well as other particles of similar size, are important components of a soil. More amount of clay particles in soil contribute more surface area since they are having small size.  This increase in surface area is important since it provides more area for soil particles to retain and supply nutrients such as calcium, potassium, magnesium, phosphate and water for plant uptake.

Soil Structure

Soil structure is the arrangement of soil particles into groupings  called peds or aggregates.  These form distinctive shapes typically found within soil horizons. For example, granular soil particles are mainly observed in the surface horizon.

Types of Soil Structures (Source: http://www.cst.cmich.edu/users/Franc1M/esc334/lectures/physical.htm)

Soil Aggregates

Very small particles such as silicate clays, volcanic ash minerals, organic matter, and oxides form aggregates. There are various mechanisms of soil aggregation.

Mechanisms of soil aggregation

• Soil microorganisms excrete substances that act as cementing agents to bind soil particles together.
• Fungal hyphae extend into the soil and tie soil particles together.
• Roots excrete sugars into the soil that help bind minerals.
• Oxides act as glue and join particles together.
• Soil particles may naturally be attracted to one another through electrostatic forces.

Aggregate Stability

Stable soil aggregation is a very valuable property of productive soils. The stability of soil aggregation depend on the type of minerals present in the soil.

Highly weathered silicate clays, oxides, and amorphous volcanic materials form the most stable aggregates and the presence of organic matter with these materials improves the aggregate stability.  

Less weathered silicate clays form weak aggregates. Some silicate clays have a shrink-swell potential, the soil minerals expand or swell, when wet, causing the soil to become sticky and drain poorly and when dry, these soils shrink and form cracks.