Properties of Soil: Physical, Chemical & Biological – UPSC

In this article, You will read Soil Characteristics i.e. Properties of Soil – Physical, Chemical, and Biological for UPSC (Biogeography).

Soil is the upper weathered layer of the Earth’s crust affected by plants and animals. A vertical section through this zone constitutes a soil profile; in each soil profile, there are usually several distinguishable layers or horizons, which enable different types of soil to be recognized.

Soil contains matter in all three states: solid, liquid, and gaseous. The solid portion is partly organic and partly inorganic. The inorganic, or mineral, part of the soil is made up of particles derived from the parent material, the rocks which weather to form the soil. The organic portion consists of living and decayed plant and animal materials such as roots and worms. The end-product of decay is humus, black amorphous organic matter. Soil water is a dilute but complex chemical solution derived from direct precipitation and from run-off, seepage, and groundwater. The soil atmosphere fills the pore spaces of the soil when these are not occupied by water.

The texture of soil refers to the sizes of the solid particles composing the soil. The sizes range from gravel to clay. The proportions of the different sizes present vary from soil to soil and from layer to layer. Texture largely determines the water-retention properties of the soil. In sandy soil, pore spaces are large and water drains rapidly: in clay soil, the individual pore spaces are too small for adequate drainage. Generally speaking, loam textures are best for plant growth.

Soil acidity is a property related to the proportion of exchangeable hydrogen ion present in the soil in relation to other elements. The degree of acidity is measured on the logarithmic pH scale which ranges from 0 (extreme acidity) to 14 (extreme alkalinity). Few soils reach these limits; a pH value of about 6.5 is normally regarded as the most favorable for the growth of cereal crops.

Colour varies considerably in soils and can tell us much about how a soil is formed and what it is made up of. In recently formed soils, the colour will largely reflect that of the parent material, but in many other cases, the colour is different from the underlying rock. Soils can range from white to black, usually depending on the amount of humus.

In cool humid areas, most soils contain relatively high humus content and are generally black or dark brown, whereas in desert or semi-desert areas, little humus is present and soils are light brown or grey. Reddish colors in soils are associated with the presence of ferric compounds, particularly the oxides and hydroxides, and usually indicate that the soil is well-drained, although locally the colour may be derived from a red-coloured parent material.

Properties of Soil

All soils contain mineral particles, organic matter, water, and air. The combinations of these determine the soil’s properties – its texture, structure, porosity, chemistry, and colour.

Knowing a soil’s water, mineral, and organic components and their proportions can help us determine its productivity and what the best use for that soil may be. Several soil properties that can be readily tested or examined are used to describe and differentiate soil types.

Physical Properties of soil

It depends upon the amount, size, shape, arrangement, and mineral composition of soil particles. It also depends on the organic matter content and pore spaces.

Soil Texture:

Soil texture defines the proportion in which the soil separates to make the mineral component of the soil. These separates can be classified as sand, clay, and silt. Sand and silt are of no importance to the soil as they don’t contribute to the soil’s ability to restore water or nutrients. Clay is an active part of soil texture as clay has a small size and it has a large amount of surface area per unit mass and it helps in storing ions and water.

The soil texture refers to the coarseness/fineness of the mineral matter in the soil. It is determined by the proportion of the sand, silt, and clay particles. The equal proportion of all three of them is known as loam. Soil texture affects the water holding capacity, nutrient retention, nutrient fixation, drainage, compressibility, and aeration of the soil.

• Clay: Particle Size – diameters less than 0.002 millimeter
• Silt: Particle Size – diameters between 0.002 millimeters to 0.05 millimeters.
• Sand: Particle Size – diameters between 0.05 and 2 millimeters.

Rocks larger than 2 millimeters are regarded as pebbles, gravel, or rock fragments and technically are not soil particles.

Loamy Soil: Loamy soil is the one in which none of the three (sand/silt/clay) dominates the other two. In particular, loamy soil has about 40% sand, 40%silt, and 20% clay.

Soil Structure:

It is the arrangement of soil particles into certain patterns like- plate like structure, block like structure, prism-like structure, etc.

Soil structure describes the way the sand, silt, and clay particles are clumped together. Organic matter (decaying plants and animals) and soil organisms like earthworms and bacteria influence soil structure.

Clays, organic matter, and materials excreted by soil organisms bind the soil particles together to form aggregates. Soil structure is important for plant growth, regulating the movement of air and water, influencing root development, and affecting nutrient availability.

Good quality soils are friable (crumbly) and have fine aggregates so the soil breaks up easily if you squeeze it. Poor soil structure has coarse, very firm clods or no structure at all.

Some structural characteristics of soil:

• Permeability – The ease with which liquids/gases can pass through rocks or a layer of soil is called permeability. It depends on the size, shape, and packing of particles. It is usually greatest in sandy soils and poor in clayey soils.
• Porosity – The volume of water which can be held within the soil is called its porosity. It is expressed as a ratio of the volume of voids (pores) to the total volume of the material.

There are 5 basic types of structural units:

1. Platy: Plate-like aggregates that form parallel to the horizons like pages in a book.
   • This type of structure may reduce air, water, and root movement.
   • a common structure in an E horizon and usually not seen in other horizons.
2. Blocky: Two types–angular blocky and subangular blocky
   • These types of structures are commonly seen in the B horizon.
   • Angular is cube-like with sharp corners while subangular blocky has rounded corners.
3. Prismatic: Vertical axis is longer than the horizontal axis. If the top is flat, it is referred to as prismatic.If the top is rounded, it is called columnar.
4. Granular: Peds are round and porous, spheroidal. This is usually the structure of A horizons.
5. Structureless: No observable aggregation or structural units.
   • Single grain-sand
   • Massive-solid mass without aggregates

Soil Colour:

Basically soil color (brown, yellow, red) depends on oxidized or ferric iron compounds. Darker the color of the soil, the more organic content it contains. The higher the organic content, the higher soil temperature as they absorb more heat due to the darker color.

Soils rich in humus tend to be dark because decomposed organic matter is black or brown. Soils with high humus content are usually very fertile, so dark brown or black soils are often referred to as ‘rich’.

Red or yellow soils typically indicate the presence of iron.

Soil colour is described by the parameters called hue, value and chroma. Hue represents the dominant wave length or colour of the light; value refers to the lightness of the colour; chroma, relative purity or strength of the colour.

The colour of the soil in terms of the above parameters could be quickly determined by comparison of the sample with a standard set of colour chips mounted in a note-book called MUNSELL SOIL COLOUR CHARTS.

In these charts, the right-hand top corner represents the Hue; the vertical axis, the value; and the horizontal axis, the chroma.

Soil permeability:

Soil permeability is a broad term used to define the ability of the soil for transmitting water. It is important to understand the water dynamics and the water balance of the soil and it must be known for accurate management of irrigation. It is determined partly by texture, with sandy soils having high permeability as compared to clay soils and it can be altered by soil management.

• Most porous rocks are permeable with the exception of clay in which pore spaces are so small that they are often sealed with groundwater held by surface tension. Another exception – granite is non-porous but permeable. It is a crystalline rock and hence non-porous. Its individual crystals absorb little or no water but the rock may have numerous joints/ cracks through which the water can pass rendering it permeable.
• A soil with high organic content also tends to have high porosity.

Soil Horizon:

1. The soil is divided vertically into different horizons from top to bottom namely:

• A-Horizon: This is the uppermost layer of soil and also called topsoil. This layer is rich in humus and minerals and holds most of the water as compared to other layers. This layer consists of sand, silt, and clay. It is also home to many living organisms like snakes, earthworms, etc.
• B-Horizon: This is the second layer from the top and is a little rich in humus and it supports moisture. This layer consists of silt, clay, weathered rocks, and some nutrients. Minerals are more in this layer as compared to the top layer.
• C-Horizon: This layer consists of small pieces of rocks broken down due to weathering.
• BedRock: This layer is the last layer and consists of layers of solid unweathered rock.

Chemical properties of soil

Chemical properties of soils depend on the following factors:

• Inorganic matter present in the Soil: The mineral content of the soil is the major factor that differentiates various types of soil. It is so because of its abundance in the soil.
• Organic matter present in Soil: Though these matters present in very small quantities but they play important role in deciding the fertility of the soil.
• Colloidal properties of Soil: Colloids are mainly of two types:
  • Clay Colloids: they are important for the adsorption of a large quantity of water.
  • Organic Colloids: these help in increasing the moisture and nutrient retention capacity of the soil.
• the pH of Soil: The measure of the chemical reaction which a soil shows is expressed by its pH value. The pH value of soil determines its acidic or basic nature.

Acidity & Alkalinity:

An important aspect of soil chemistry is acidity, alkalinity (baseness), or neutrality.

Low pH values indicate acidic soil, and a high pH indicates alkaline conditions. Most complex plants grow only in the soils with levels between pH 4 and pH 10 but optimum pH varies with the plant species.

• In arid and semi-arid regions, soils tend to be alkaline and soils in humid regions tend to be acidic.
• To correct soil alkalinity and to make the soil more productive, the soil can be flushed with irrigation water.
• Strongly acidic soils are also detrimental to plant growth, but soil acidity can generally be corrected by adding lime to the soil.

The most important effect of pH in the soil is on ion solubility, which in turn affects microbial and plant growth. A pH range of 6.0 to 6.8 is ideal for most crops because it coincides with optimum solubility of the most important plant nutrients. Some minor elements (e.g., iron) and most heavy metals are more soluble at lower pH. This makes pH management important in controlling the movement of heavy metals (and potential groundwater contamination) in soil.

Lime requirement, or the amount of liming material needed to raise the soil pH to a certain level, increases with CEC(cation exchange capacity). To decrease the soil pH, sulfur can be added, which produces sulfuric acid.

Soil Colloids:

Soil colloids are the most active constituent of the soil and they are important because their surfaces attract soil nutrients dissolved in soil, water as positively charged mineral ions, or cations. 

Some cations are needed for plant growth, including calcium (Ca++), Magnesium (Mg ++), Potassium (K+), and sodium (Na+). They need to be dissolved in a soil-water solution to be available to plants when they are in close contact with root membranes.

The fertility of the soil-water solution for plants is based on the capability of the soil to hold and exchange cations; this is referred to as the cation-exchange capacity. Without soil colloids, most vital nutrients would be leached out of the soil by percolating water and carried away in streams.

Biological Properties of Soil

Organic matter in the soil improves soil structure and increasing the nutrient and water holding capacity of the soil. Organic matter also provides a food supply for soil biology. Soils with low organic matter can have a ‘poor’ structure, hold little water, and erode or leach nutrients easily. The exception is cracking clay soils where clay minerals have the main effect on the structure. Soils with high organic matter levels have a ‘good’ structure, good water-holding capacity, and reduced erosion and nutrient leaching.

Biological properties include:

• organic matter
• soil organisms
• the presence of disease-causing organisms.

The total role of biologic processes in soil formation includes the presence and activities of living plants and animals as well as their non-living organic products. Living plants contribute to soil formation in two basic ways.

(i) Biomass i.e. the production of organic matter the biomass both above the soil as stems and leaves and within the soil as roots. It provides the raw material of organic matter in the O horizon and in lower horizons. The decomposer organisms process this raw material, reducing it to humus and ultimately to its initial components carbon dioxide and water.

(ii) Nutrient Recycling: It involves the cycling of nutrients from the soil in dead plant tissues. Nutrient recycling is a mechanism by which nutrients are prevented from escaping through the leaching action of surplus soil water moving downward through the soil.

Animals living in the soil play an important role in biologic processes of soil. e.g. earthworms rework the soil not only by burrowing but also bypassing the soil through their intestinal tracts.

Some of the important factors which decide the biological behavior of soil are:

• Respiration rate: CO2 evolution under standard laboratory conditions or at the field.
• Potential N/C mineralization: Increase in mineral Nitrogen or Carbon content under standard laboratory conditions.
• Earthworms: Density of earthworms.
• Bacterial biomass: Total bacterial biomass for a given soil mass.
• Bacterial diversity: It can be determined by functional groups, or describing genetic diversity.
• Presence of pathogens: By different pathology techniques, from cultures to DNA profiling.